LM2743 LM2743 Low Voltage N-Channel MOSFET Synchronous Buck Regulator Controller Literature Number: SNVS276E LM2743 Low Voltage N-Channel MOSFET Synchronous Buck Regulator Controller General Description Features The LM2743 is a high-speed synchronous buck regulator controller with an accurate feedback voltage accuracy of 2%. It can provide simple down conversion to output voltages as low as 0.6V. Though the control sections of the IC are rated for 3 to 6V, the driver sections are designed to accept input supply rails as high as 16V. The use of adaptive non-overlapping MOSFET gate drivers helps avoid potential shoot-through problems while maintaining high efficiency. The IC is designed for the more cost-effective option of driving only N-channel MOSFETs in both the high-side and low-side positions. It senses the low-side switch voltage drop for providing a simple, adjustable current limit. The fixed-frequency voltage-mode PWM control architecture is adjustable from 50 kHz to 1 MHz with one external resistor. This wide range of switching frequency gives the power supply designer the flexibility to make better tradeoffs between component size, cost and efficiency. Features include soft-start, input under-voltage lockout (UVLO) and Power Good (based on both under-voltage and overvoltage detection). In addition, the shutdown pin of the IC can be used for providing startup delay, and the soft-start pin can be used for implementing precise tracking, for the purpose of sequencing with respect to an external rail. Power stage input voltage from 1V to 16V Control stage input voltage from 3V to 6V Output voltage adjustable down to 0.6V Power good flag and shutdown Output over-voltage and under-voltage detection 2% feedback voltage accuracy over temperature Low-side adjustable current sensing Adjustable soft-start Tracking and sequencing with shutdown and soft-start pins Switching frequency from 50 kHz to 1 MHz TSSOP-14 package Applications 3.3V Buck Regulation Cable Modem, DSL and ADSL Laser Jet and Ink Jet Printers Low Voltage Power Modules DSP, ASIC, Core and I/O Typical Application 20095201 (c) 2011 National Semiconductor Corporation 200952 www.national.com LM2743 Low Voltage N-Channel MOSFET Synchronous Buck Regulator Controller January 6, 2011 LM2743 Connection Diagram 20095202 14-Lead Plastic TSSOP JA = 155C/W Ordering Information Order Number Package Type NSC Package Drawing Supplied As LM2743MTC TSSOP-14 MTC14 94 Units, Raill LM2743MTCX TSSOP-14 MTC14 2500 Units on Tape and Reel EAO (Pin 8) - Output of the error amplifier. The voltage level on this pin is compared with an internally generated ramp signal to determine the duty cycle. This pin is necessary for compensating the control loop. SS/TRACK (Pin 9) - Soft-start and tracking pin. This pin is internally connected to the non-inverting input of the error amplifier during soft-start, and in fact any time the SS/TRACK pin voltage happens to be below the internal reference voltage. For the basic soft-start function, a capacitor of minimum value 1 nF is connected from this pin to ground. To track the rising ramp of another power supply's output, connect a resistor divider from the output of that supply to this pin as described in Application Information. FB (Pin 10) - Feedback pin. This is the inverting input of the error amplifier, which is used for sensing the output voltage and compensating the control loop. FREQ (Pin 11) - Frequency adjust pin. The switching frequency is set by connecting a resistor of suitable value between this pin and ground. The equation for calculating the exact value is provided in Application Information, but some typical values (rounded up to the nearest standard values) are 324 k for 100 kHz, 97.6 k for 300 kHz, 56.2 k for 500 kHz, 24.9 k for 1 MHz. SD (Pin 12) - IC shutdown pin. Pull this pin to VCC to ensure the IC is enabled. Connect to ground to disable the IC. Under shutdown, both high-side and low-side drives are off. This pin also features a precision threshold for power supply sequencing purposes, as well as a low threshold to ensure minimal quiescent current. HG (Pin 14) - High-gate drive pin. This is the gate drive for the high-side N-channel MOSFET. This signal is interlocked with LG (Pin 2) to avoid shoot-through. Pin Descriptions BOOT (Pin 1) - Bootstrap pin. This is the supply rail for the gate drivers. When the high-side MOSFET turns on, the voltage on this pin should be at least one gate threshold above the regulator input voltage VIN to properly turn on the MOSFET. See MOSFET Gate Drivers in the Application Information section for more details on how to select MOSFETs. LG (Pin 2) - Low-gate drive pin. This is the gate drive for the low-side N-channel MOSFET. This signal is interlocked with the high-side gate drive HG (Pin 14), so as to avoid shootthrough. PGND (Pins 3, 13) - Power ground. This is also the ground for the low-side MOSFET driver. Both the pins must be connected together on the PCB and form a ground plane, which is usually also the system ground. SGND (Pin 4) - Signal ground. It should be connected appropriately to the ground plane with due regard to good layout practices in switching power regulator circuits. VCC (Pin 5) Supply rail for the control sections of the IC. PWGD (Pin 6) - Power Good pin. This is an open drain output, which is typically meant to be connected to VCC or any other low voltage source through a pull-up resistor. Choose the pullup resistor so that the current going into this pin is kept below 1 mA. For most applications a recommended value for the pull-up resistor is 100 k. The voltage on this pin is thus pulled low under output under-voltage or over-voltage fault conditions and also under input UVLO. ISEN (Pin 7) - Current limit threshold setting pin. This sources a fixed 40 A current. A resistor of appropriate value should be connected between this pin and the drain of the low-side MOSFET (switch node). www.national.com 2 If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. VCC BOOT Voltage ISEN -0.3 to 7V -0.3 to 21V -0.3 to 9.5V -0.3 to VCC + 0.3V 150C -65C to 150C All other pins Junction Temperature Storage Temperature 260C 235C 2 kV Operating Ratings Supply Voltage Range (VCC) Junction Temperature Range (TJ) 3V to 6V -40C to +125C 155C/W Thermal Resistance (JA) Electrical Characteristics VCC = 3.3V unless otherwise indicated. Typicals and limits appearing in plain type apply for TA= TJ= 25C. Limits appearing in boldface type apply over full Operating Temperature Range. Datasheet min/max specification limits are guaranteed by design, test, or statistical analysis. Symbol Parameter Conditions VFB FB Pin Voltage VCC = 3V to 6V VON UVLO Thresholds Rising Falling IQ_VCC Operating VCC Current Min 0.588 Typ Max Units 0.6 0.612 V 2.76 2.42 V VCC = 3.3V, VSD = 3.3V Fsw = 600kHz 1.0 1.5 2.1 VCC = 5V, VSD = 3.3V Fsw = 600kHz 1.0 1.7 2.1 110 185 mA Shutdown VCC Current VCC = 3.3V, VSD = 0V tPWGD1 PWGD Pin Response Time VFB Rising 6 s tPWGD2 PWGD Pin Response Time VFB Falling 6 s ISS-ON SS Pin Source Current VSS = 0V ISS-OC SS Pin Sink Current During Over Current VSS = 2.5V ISEN-TH ISEN Pin Source Current Trip Point 7 10 14 90 25 40 A A A 55 A ERROR AMPLIFIER GBW G Error Amplifier Unity Gain Bandwidth 9 MHz Error Amplifier DC Gain 106 dB SR Error Amplifier Slew Rate 3.2 V/s IEAO EAO Pin Current Sourcing and Sinking Capability VEAO = 1.5, FB = 0.55V VEAO = 1.5, FB = 0.65V 2.6 9.2 mA VEA Error Amplifier Output Voltage Minimum 1 V Maximum 2 V 3 www.national.com LM2743 Soldering Information Lead Temperature (soldering, 10sec) Infrared or Convection (20sec) ESD Rating (Note 3) Absolute Maximum Ratings (Note 1) LM2743 Symbol Parameter Conditions Min Typ Max Units 18 90 A GATE DRIVE IQ-BOOT BOOT Pin Quiescent Current VBOOT = 12V, VSD = 0 RHG_UP High-Side MOSFET Driver Pull-Up VBOOT = 5V @ 350mA Sourcing ON resistance 3 RHG_DN High-Side MOSFET Driver PullDown ON resistance 2 RLG_UP Low-Side MOSFET Driver Pull-Up VBOOT = 5V @ 350mA Sourcing ON resistance 3 RLG_DN Low-Side MOSFET Driver PullDown ON resistance 2 HG = 5V @ 350mA Sourcing LG = 5V @ 350mA Sourcing OSCILLATOR fSW PWM Frequency RFADJ = 702.1 k 50 RFADJ = 98.74 k 300 RFADJ = 45.74 k 475 Max High-Side Duty Cycle 725 kHz 1000 RFADJ = 24.91 k D 600 fSW = 300kHz fSW = 600kHz fSW = 1MHz 80 76 73 % LOGIC INPUTS AND OUTPUTS V STBY-IH Standby High Trip Point VFB = 0.575V, VBOOT = 3.3V, VSD Rising V STBY-IL Standby Low Trip Point VFB = 0.575V, VBOOT = 3.3V, VSD Falling V SD-IH SD Pin Logic High Trip Point VSD Rising V SD-IL 1.1 0.232 V V 1.3 V SD Pin Logic Low Trip Point VSD Falling 0.8 VPWGD-TH-LO PWGD Pin Trip Points FB Falling 0.408 0.434 0.457 V VPWGD-TH-HI PWGD Pin Trip Points FB Rising 0.677 0.710 0.742 V VPWGD-HYS PWGD Hysteresis FB Falling FB Rising V 60 90 mV Note 1: Absolute maximum ratings indicate limits beyond which damage to the device may occur. Operating ratings indicate conditions for which the device operates correctly. Operating Ratings do not imply guaranteed performance limits. Note 2: The power MOSFETs can run on a separate 1V to 16V rail (Input voltage, VIN). Practical lower limit of VIN depends on selection of the external MOSFET. Note 3: The human body model is a 100 pF capacitor discharged through a 1.5 k resistor into each pin. www.national.com 4 LM2743 Typical Performance Characteristics Efficiency (VOUT = 1.2V) VCC = 3.3V, fSW = 300 kHz Efficiency (VOUT = 2.5V) VCC = 3.3V, fSW = 300 kHz 20095240 20095257 Efficiency (VOUT = 3.3V) VCC = 5V, fSW = 300 kHz VCC Operating Current plus BOOT Current vs Frequency FDS6898A FET (TA = 25C) 20095245 20095241 BOOT Pin Current vs Temperature for BOOT Voltage = 3.3V fSW = 300 kHz, FDS6898A FET, No-Load BOOT Pin Current vs Temperature for BOOT Voltage = 5V fSW = 300 kHz, FDS6898A FET, No-Load 20095242 20095243 5 www.national.com LM2743 BOOT Pin Current vs Temperature for BOOT Voltage = 12V fSW = 300 kHz, FDS6898A FET, No-Load Internal Reference Voltage vs Temperature 20095258 20095244 Frequency vs Temperature Output Voltage vs Output Current 20095260 20095256 Switch Waveforms (HG Rising) VCC = 3.3V, VIN = 5V, VOUT = 1.2V IOUT = 4A, CSS = 12nF, fSW = 300 kHz Switch Waveforms (HG Falling) VCC = 3.3V, VIN = 5V, VOUT = 1.2V IOUT = 4A, CSS = 12nF, fSW = 300 kHz 20095247 20095246 www.national.com 6 LM2743 Start-Up (No-Load) VCC = 3.3V, VIN = 5V, VOUT = 1.2V CSS = 12nF, fSW = 300 kHz Start-Up (Full-Load) VCC = 3.3V, VIN = 5V, VOUT = 1.2V IOUT = 4A, CSS = 12nF, fSW = 300 kHz 20095248 20095249 Shutdown (Full-Load) VCC = 3.3V, VIN = 5V, VOUT = 1.2V IOUT = 4A, CSS = 12nF, fSW = 300 kHz Load Transient Response (IOUT = 0A to 4A) VCC = 3.3V, VIN = 5V, VOUT = 1.2V CSS = 12nF, fSW = 300 kHz 20095251 20095250 Load Transient Response (IOUT = 4A to 0A) VCC = 3.3V, VIN = 5V, VOUT = 1.2V CSS = 12nF, fSW = 300 kHz Load Transient Response VCC = 3.3V, VIN = 5V, VOUT = 1.2V CSS = 12nF, fSW = 300 kHz 20095253 20095252 7 www.national.com LM2743 Line Transient Response (VIN = 3V to 9V) VCC = 3.3V, VOUT = 1.2V IOUT = 2A, fSW = 300 kHz Line Transient Response (VIN = 9V to 3V) VCC = 3.3V, VOUT = 1.2V IOUT = 2A, fSW = 300 kHz 20095254 www.national.com 20095255 8 Block Diagram 20095203 LM2743 9 www.national.com LM2743 require precise sequencing. When the tracking function is used no soft-start capacitor should be connected to the SS/ TRACK pin. Otherwise, a CSS value of at least 1 nF between the soft-start pin and ground should be used. Application Information THEORY OF OPERATION The LM2743 is a voltage-mode, high-speed synchronous buck regulator with a PWM control scheme. It is designed for use in set-top boxes, thin clients, DSL/Cable modems, and other applications that require high efficiency buck converters. It has output shutdown (SD), input under-voltage lock-out (UVLO) mode and power good (PWGD) flag (based on overvoltage and under-voltage detection). The over-voltage and under-voltage signals are logically OR'ed to drive the power good signal and provide a logic signal to the system if the output voltage goes out of regulation. Current limit is achieved by sensing the voltage VDS across the low side MOSFET. START UP/SOFT-START When VCC exceeds 2.76V and the shutdown pin (SD) sees a logic high, the soft-start period begins. Then an internal, fixed 10 A source begins charging the soft-start capacitor. During soft-start the voltage on the soft-start capacitor CSS is connected internally to the non-inverting input of the error amplifier. The soft-start period lasts until the voltage on the softstart capacitor exceeds the LM2743 reference voltage of 0.6V. At this point the reference voltage takes over at the noninverting error amplifier input. The capacitance of CSS determines the length of the soft-start period, and can be approximated by: 20095207 FIGURE 1. Tracking Circuit One way to use the tracking feature is to design the tracking resistor divider so that the master supply's output voltage (VOUT1) and the LM2743's output voltage (represented symbolically in Figure 1 as VOUT2, i.e. without explicitly showing the power components) both rise together and reach their target values at the same time. For this case, the equation governing the values of the tracking divider resistors RT1 and RT2 is: Where CSS is in F and tSS is in ms. During soft start the Power Good flag is forced low and it is released when the FB pin voltage reaches 70% of 0.6V. At this point the chip enters normal operation mode, and the output overvoltage and undervoltage monitoring starts. NORMAL OPERATION While in normal operation mode, the LM2743 regulates the output voltage by controlling the duty cycle of the high side and low side MOSFETs (see Typical Application Circuit). The equation governing output voltage is: The current through RT1 should be about 3 mA to 4 mA for precise tracking. The final voltage of the SS/TRACK pin should be set higher than the feedback voltage of 0.6V (say about 0.65V as in the above equation). If the master supply voltage was 5V and the LM2743 output voltage was 1.8V, for example, then the value of RT1 needed to give the two supplies identical soft-start times would be 150. A timing diagram for the equal soft-start time case is shown in Figure 2. The PWM frequency is adjustable between 50 kHz and 1 MHz and is set by an external resistor, RFADJ, between the FREQ pin and ground. The resistance needed for a desired frequency is approximately: Where fSW is in Hz and RFADJ is in k. TRACKING A VOLTAGE LEVEL The LM2743 can track the output of a master power supply during soft-start by connecting a resistor divider to the SS/ TRACK pin. In this way, the output voltage slew rate of the LM2743 will be controlled by the master supply for loads that www.national.com 10 LM2743 20095208 FIGURE 2. Tracking with Equal Soft-Start Time 20095214 TRACKING A VOLTAGE SLEW RATE The tracking feature can alternatively be used not to make both rails reach regulation at the same time but rather to have similar rise rates (in terms of output dV/dt). This method ensures that the output voltage of the LM2743 always reaches regulation before the output voltage of the master supply. Because the output of the master supply is divided down, in order to track properly the output voltage of the LM2743 must be lower than the voltage of the master supply. In this case, the tracking resistors can be determined based on the following equation: FIGURE 4. Sequencing Circuit A desired delay time tDELAY between the startup of the master supply output voltage and the LM2743 output voltage can be set based on the SD pin low-to-high threshold VSD-IH and the slew rate of the voltage at the SD pin, SRSD: tDELAY = VSD-IH / SRSD Note again, that in Figure 4, the LM2743's output voltage has been represented symbolically as VOUT2, i.e. without explicitly showing the power components. VSD-IH is typically 1.08V and SRSD is the slew rate of the SD pin voltage. The values of the sequencing divider resistors RS1 and RS2 set the SRSD based on the master supply output voltage slew rate, SROUT1, using the following equation: For the example case of VOUT1 = 5V and VOUT2 = 1.8V, with RT1 set to 150 as before, RT2 is calculated from the above equation to be 265. A timing diagram for the case of equal slew rates is shown in Figure 3. For example, if the master supply output voltage slew rate was 1V/ms and the desired delay time between the startup of the master supply and LM2743 output voltage was 5ms, then the desired SD pin slew rate would be (1.08V/5 ms) = 0.216V/ ms. Due to the internal impedance of the SD pin, the maximum recommended value for RS2 is 1k. To achieve the desired slew rate, RS1 would then be 274. A timing diagram for this example is shown in Figure 5. 20095210 FIGURE 3. Tracking with Equal Slew Rates SEQUENCING The start up/soft-start of the LM2743 can be delayed for the purpose of sequencing by connecting a resistor divider from the output of a master power supply to the SD pin, as shown in Figure 4. 11 www.national.com LM2743 goes high. This bootstrap voltage is usually supplied from a local charge pump structure. But looking at the Typical Application schematic, this also means that the difference voltage VCC - VD1, which is the voltage the bootstrap capacitor charges up to, must be always greater than the maximum tolerance limit of the threshold voltage of the upper MOSFET. Here VD1 is the forward voltage drop across the bootstrap diode D1. This therefore may place restrictions on the minimum input voltage and/or type of MOSFET used. The most basic charge bootstrap pump circuit can be built using one Schottky diode and a small capacitor, as shown in Figure 7. The capacitor CBOOT serves to maintain enough voltage between the top MOSFET gate and source to control the device even when the top MOSFET is on and its source has risen up to the input voltage level. The charge pump circuitry is fed from VCC, which can operate over a range from 3.0V to 6.0V. Using this basic method the voltage applied to the gates of both high-side and low-side MOSFETs is VCC VD. This method works well when VCC is 5V10%, because the gate drives will get at least 4.0V of drive voltage during the worst case of VCC-MIN = 4.5V and VD-MAX = 0.5V. Logic level MOSFETs generally specify their on-resistance at VGS = 4.5V. When VCC = 3.3V10%, the gate drive at worst case could go as low as 2.5V. Logic level MOSFETs are not guaranteed to turn on, or may have much higher on-resistance at 2.5V. Sub-logic level MOSFETs, usually specified at VGS = 2.5V, will work, but are more expensive, and tend to have higher on-resistance. The circuit in Figure 7 works well for input voltages ranging from 1V up to 16V and VCC = 5V 10%, because the drive voltage depends only on VCC. 20095211 FIGURE 5. Delay for Sequencing SD PIN IMPEDANCE When connecting a resistor divider to the SD pin of the LM2743 some care has to be taken. Once the SD voltage goes above VSD-IH, a 17 A pull-up current is activated as shown in Figure 6. This current is used to create the internal hysteresis (170 mV); however, high external impedances will affect the SD pin logic thresholds as well. The external impedance used for the sequencing divider network should preferably be a small fraction of the impedance of the SD pin for good performance (around 1k). 20095212 FIGURE 7. Basic Charge Pump (Bootstrap) 20095206 Note that the LM2743 can be paired with a low cost linear regulator like the LM78L05 to run from a single input rail between 6.0 and 14V. The 5V output of the linear regulator powers both the VCC and the bootstrap circuit, providing efficient drive for logic level MOSFETs. An example of this circuit is shown in Figure 8. FIGURE 6. SD Pin Logic MOSFET GATE DRIVERS The LM2743 has two gate drivers designed for driving Nchannel MOSFETs in a synchronous mode. Note that unlike most other synchronous controllers, the bootstrap capacitor of the LM2743 provides power not only to the driver of the upper MOSFET, but the lower MOSFET driver too (both drivers are ground referenced, i.e. no floating driver). To fully turn the top MOSFET on, the BOOT voltage must be at least one gate threshold greater than VIN when the high-side drive www.national.com 12 UVLO The 2.76V turn-on threshold on VCC has a built in hysteresis of about 300 mV. If VCC drops below 2.42V, the chip enters UVLO mode. UVLO consists of turning off the top and bottom MOSFETS and remaining in that condition until VCC rises above 2.76V. As with shutdown, the soft-start capacitor is discharged through an internal MOSFET, ensuring that the next start-up will be controlled by the soft-start circuitry. 20095213 CURRENT LIMIT Current limit is realized by sensing the voltage across the lowside MOSFET while it is on. The RDS(ON) of the MOSFET is a known value; hence the current through the MOSFET can be determined as: FIGURE 8. LM78L05 Feeding Basic Charge Pump Figure 9 shows a second possibility for bootstrapping the MOSFET drives using a doubler. This circuit provides an equal voltage drive of VCC - 3VD + VIN to both the high-side and low-side MOSFET drives. This method should only be used in circuits that use 3.3V for both VCC and VIN. Even with VIN = VCC = 3.0V (10% lower tolerance on 3.3V) and VD = 0.5V both high-side and low-side gates will have at least 4.5V of drive. The power dissipation of the gate drive circuitry is directly proportional to gate drive voltage, hence the thermal limits of the LM2743 IC will quickly be reached if this circuit is used with VCC or VIN voltages over 5V. VDS = IOUT x RDS(ON) The current through the low-side MOSFET while it is on is also the falling portion of the inductor current. The current limit threshold is determined by an external resistor, RCS, connected between the switching node and the ISEN pin. A constant current of 40 A is forced through RCS, causing a fixed voltage drop. This fixed voltage is compared against VDS and if the latter is higher, the current limit of the chip has been reached. To obtain a more accurate value for RCS you must consider the operating values of RDS(ON) and ISEN-TH at their operating temperatures in your application and the effect of slight parameter differences from part to part. RCS can be found by using the following equation using the RDS(ON) value of the low side MOSFET at it's expected hot temperature and the absolute minimum value expected over the full temperature range for the for the ISEN-TH which is 25 A: RCS = RDSON-HOT x ILIM / 40 A For example, a conservative 15A current limit in a 10A design with a minimum RDS(ON) of 10 m would require a 6 k resistor. To prevent the ISEN pin from sinking too much current when the switch node goes above 9.5V, the value of the current limit setting resistor RCS should not be too low. The criterion is as follows, 20095219 FIGURE 9. Charge Pump with Added Gate Drive All the gate drive circuits shown in the above figures typically use 100 nF ceramic capacitors in the bootstrap locations. where the 10 mA is the maximum current ISEN pin is allowed to sink. For example if VIN = 13.2V, the minimum value of RCS is 370. Because current sensing is done across the lowside MOSFET, no minimum high-side on-time is necessary. The LM2743 enters current limit mode if the inductor current exceeds the current limit threshold at the point where the highside MOSFET turns off and the low-side MOSFET turns on. (The point of peak inductor current, see Figure 10). Note that in normal operation mode the high-side MOSFET always POWER GOOD SIGNAL The open drain output on the Power Good pin needs a pullup resistor to a low voltage source. The pull-up resistor should be chosen so that the current going into the Power Good pin is less than 1 mA. A 100 k resistor is recommended for most applications. 13 www.national.com LM2743 The Power Good signal is an OR-gated flag which takes into account both output over-voltage and under-voltage conditions. If the feedback pin (FB) voltage is 18% above its nominal value (118% x VFB = 0.708V) or falls 28% below that value (72 %x VFB = 0.42V) the Power Good flag goes low. The Power Good flag can be used to signal other circuits that the output voltage has fallen out of regulation, however the switching of the LM2743 continues regardless of the state of the Power Good signal. The Power Good flag will return to logic high whenever the feedback pin voltage is between 72% and 118% of 0.6V. LM2743 turns on at the beginning of a clock cycle. In current limit mode, by contrast, the high-side MOSFET on-pulse is skipped. This causes inductor current to fall. Unlike a normal operation switching cycle, however, in a current limit mode switching cycle the high-side MOSFET will turn on as soon as inductor current has fallen to the current limit threshold. The LM2743 will continue to skip high-side MOSFET pulses until the inductor current peak is below the current limit threshold, at which point the system resumes normal operation. smooth, controlled ramp of output voltage when the current limit condition is cleared. FOLDBACK CURRENT LIMIT In the case where extra protection is used to help an output short condition, a current foldback resistor (RCLF) should be considered, see Figure 11. First select the percentage of current limit foldback (PLIM): PLIM = ILIM x P where P is a ratio between 0 and 1. 20095295 FIGURE 11. Foldback Current Limit Circuit Obtain the RCS with the following equation: 20095288 FIGURE 10. Current Limit Threshold Unlike a high-side MOSFET current sensing scheme, which limits the peaks of inductor current, low-side current sensing is only allowed to limit the current during the converter offtime, when inductor current is falling. Therefore in a typical current limit plot the valleys are normally well defined, but the peaks are variable, according to the duty cycle. The PWM error amplifier and comparator control the off-pulse of the high-side MOSFET, even during current limit mode, meaning that peak inductor current can exceed the current limit threshold. Assuming that the output inductor does not saturate, the maximum peak inductor current during current limit mode can be calculated with the following equation: where ISEN = 40 A. If the switch node goes above 9.5V the following criterion must be satisfied: The equation for calculating the foldback resistance value is: SHUTDOWN If the shutdown pin is pulled low, (below 0.8V) the LM2743 enters shutdown mode, and discharges the soft-start capacitor through a MOSFET switch. The high and low-side MOSFETs are turned off. The LM2743 remains in this state as long as VSD sees a logic low (see the Electrical Characteristics table). To assure proper IC start-up the shutdown pin should not be left floating. For normal operation this pin should be connected directly to VCC or to another voltage between 1.3V to VCC (see the Electrical Characteristics table). Where TSW is the inverse of switching frequency fSW. The 200 ns term represents the minimum off-time of the duty cycle, which ensures enough time for correct operation of the current sensing circuitry. In order to minimize the time period in which peak inductor current exceeds the current limit threshold, the IC also discharges the soft-start capacitor through a fixed 90 A sink. The output of the LM2743 internal error amplifier is limited by the voltage on the soft-start capacitor. Hence, discharging the soft-start capacitor reduces the maximum duty cycle D of the controller. During severe current limit this reduction in duty cycle will reduce the output voltage if the current limit conditions last for an extended time. Output inductor current will be reduced in turn to a flat level equal to the current limit threshold. The third benefit of the soft-start capacitor discharge is a www.national.com DESIGN CONSIDERATIONS The following is a design procedure for all the components needed to create the Typical Application Circuit shown on the front page. This design converts 3.3V (VIN) to 1.2V (VOUT) at a maximum load of 4A with an efficiency of 89% and a switching frequency of 300 kHz. The same procedures can be 14 LM2743 followed to create many other designs with varying input voltages, output voltages, and load currents. Duty Cycle Calculation The complete duty cycle for a buck converter is defined with the following equation: where n is the number of capacitors, and ESR is the equivalent series resistance of each capacitor. The equation above indicates that power loss in each capacitor decreases rapidly as the number of input capacitors increases. The worst-case ripple for a Buck converter occurs during full load and when the duty cycle (D) is 0.5. For this 3.3V to 1.2V design the duty cycle is 0.364. For a 4A maximum load the ripple current is 1.92A. where VSWL and VSWH are the respective forward voltage drops that develop across the low side and high side MOSFETs. Assuming the inductor ripple current is 20% to 30% of the output current, therefore: Output Inductor The output inductor forms the first half of the power stage in a Buck converter. It is responsible for smoothing the square wave created by the switching action and for controlling the output current ripple (IOUT). The inductance is chosen by selecting between tradeoffs in efficiency and response time. The smaller the output inductor, the more quickly the converter can respond to transients in the load current. However, as shown in the efficiency calculations, a smaller inductor requires a higher switching frequency to maintain the same level of output current ripple. An increase in frequency can mean increasing loss in the MOSFETs due to the charging and discharging of the gates. Generally the switching frequency is chosen so that conduction loss outweighs switching loss. The equation for output inductor selection is: VSWL = IOUT x RDS(ON)LOW (Low-Side MOSFET) VSWH = IOUT x RDS(ON)HIGH (High-Side MOSFET) To calculate the maximum duty cycle use the estimated 'hot' RDS(on) value of the MOSFETs, the minimum input voltage, and maximum load. As shown in Figure 12, the worst case maximum duty cycles of the LM2743 occurs at 125C junction temperature vs VCC (IC control section voltage). Ensure that the operating duty cycle is below the curve in Figure 12, if this condition is not satisfied, the system will be unable to develop the required duty cycle to derive the necessary system power and so the output voltage will fall out of regulation. L = 1.6H Here we have plugged in the values for output current ripple, input voltage, output voltage, switching frequency, and assumed a 40% peak-to-peak output current ripple. This yields an inductance of 1.6 H. The output inductor must be rated to handle the peak current (also equal to the peak switch current), which is (IOUT + (0.5 x IOUT)) = 4.8A, for a 4A design. The Coilcraft DO3316P-222P is 2.2 H, is rated to 7.4A peak, and has a direct current resistance (DCR) of 12 m. After selecting an output inductor, inductor current ripple should be re-calculated with the new inductance value, as this information is needed to select the output capacitor. Re-arranging the equation used to select inductance yields the following: 20095291 FIGURE 12. Maximum Duty Cycle vs VCC TJ = 125C Input Capacitor The input capacitors in a Buck converter are subjected to high stress due to the input current trapezoidal waveform. Input capacitors are selected for their ripple current capability and their ability to withstand the heat generated since that ripple current passes through their ESR. Input rms ripple current is approximately: VIN(MAX) is assumed to be 10% above the steady state input voltage, or 3.6V. The actual current ripple will then be 1.2A. Peak inductor/switch current will be 4.6A. Output Capacitor The output capacitor forms the second half of the power stage of a Buck switching converter. It is used to control the output The power dissipated by each input capacitor is: 15 www.national.com LM2743 and switching losses in that the actual dissipation occurs in the LM2743, and not in the MOSFET itself. Switching loss occurs during the brief transition period as the high-side MOSFET turns on and off, during which both current and voltage are present in the channel of the MOSFET. It can be approximated as: voltage ripple (VOUT) and to supply load current during fast load transients. In this example the output current is 4A and the expected type of capacitor is an aluminum electrolytic, as with the input capacitors. Other possibilities include ceramic, tantalum, and solid electrolyte capacitors, however the ceramic type often do not have the large capacitance needed to supply current for load transients, and tantalums tend to be more expensive than aluminum electrolytic. Aluminum capacitors tend to have very high capacitance and fairly low ESR, meaning that the ESR zero, which affects system stability, will be much lower than the switching frequency. The large capacitance means that at the switching frequency, the ESR is dominant, hence the type and number of output capacitors is selected on the basis of ESR. One simple formula to find the maximum ESR based on the desired output voltage ripple, VOUT and the designed output current ripple, IOUT, is: PSW = 0.5 x VIN x IO x (tr + tf) x fSW where tR and tF are the rise and fall times of the MOSFET. Switching loss occurs in the high-side MOSFET only. For this example, the maximum drain-to-source voltage applied to either MOSFET is 3.6V. The maximum drive voltage at the gate of the high-side MOSFET is 3.1V, and the maximum drive voltage for the low-side MOSFET is 3.3V. Due to the low drive voltages in this example, a MOSFET that turns on fully with 3.1V of gate drive is needed. For designs of 5A and under, dual MOSFETs in SO-8 package provide a good trade-off between size, cost, and efficiency. Support Components CIN2 - A small value (0.1 F to 1 F) ceramic capacitor should be placed as close as possible to the drain of the high-side MOSFET and source of the low-side MOSFET (dual MOSFETs make this easy). This capacitor should be X5R type dielectric or better. RCC, CCC- These are standard filter components designed to ensure smooth DC voltage for the chip supply. RCC should be 1 to 10. CCC should 1 F, X5R type or better. CBOOT- Bootstrap capacitor, typically 100 nF. RPULL-UP - This is a standard pull-up resistor for the opendrain power good signal (PWGD). The recommended value is 10 k connected to VCC. If this feature is not necessary, the resistor can be omitted. D1 - A small Schottky diode should be used for the bootstrap. It allows for a minimum drop for both high and low-side drivers. The MBR0520 or BAT54 work well in most designs. RCS - Resistor used to set the current limit. Since the design calls for a peak current magnitude (IOUT+ (0.5 x IOUT)) of 4.8A, a safe setting would be 6A. (This is below the saturation current of the output inductor, which is 7A.) Following the equation from the Current Limit section, a 1.3 k resistor should be used. RFADJ - This resistor is used to set the switching frequency of the chip. The resistor value is calculated from equation in Normal Operation section. For 300 kHz operation, a 97.6 k resistor should be used. CSS - The soft-start capacitor depends on the user requirements and is calculated based on the equation given in the section titled START UP/SOFT-START. Therefore, for a 700 s delay, a 12 nF capacitor is suitable. In this example, in order to maintain a 2% peak-to-peak output voltage ripple and a 40% peak-to-peak inductor current ripple, the required maximum ESR is 20 m. The Sanyo 4SP560M electrolytic capacitor will give an equivalent ESR of 14 m. The capacitance of 560 F is enough to supply energy even to meet severe load transient demands. MOSFETs Selection of the power MOSFETs is governed by a tradeoff between cost, size, and efficiency. One method is to determine the maximum cost that can be endured, and then select the most efficient device that fits that price. Breaking down the losses in the high-side and low-side MOSFETs and then creating spreadsheets is one way to determine relative efficiencies between different MOSFETs. Good correlation between the prediction and the bench result is not guaranteed, however. Single-channel buck regulators that use a controller IC and discrete MOSFETs tend to be most efficient for output currents of 2A to 10A. Losses in the high-side MOSFET can be broken down into conduction loss, gate charging loss, and switching loss. Conduction loss, or I2R loss, is approximately: PC = D ((IO)2 x RDSON-HI x 1.3) (High-Side MOSFET) PC = (1 - D) x ((IO)2 x RDSON-LO x 1.3) (Low-Side MOSFET) In the above equations the factor 1.3 accounts for the increase in MOSFET RDSON due to heating. Alternatively, the 1.3 can be ignored and the RDSON of the MOSFET estimated using the RDSON Vs. Temperature curves in the MOSFET datasheets. Gate charging loss results from the current driving the gate capacitance of the power MOSFETs, and is approximated as: Control Loop Compensation The LM2743 uses voltage-mode (`VM') PWM control to correct changes in output voltage due to line and load transients. One of the attractive advantages of voltage mode control is its relative immunity to noise and layout. However VM requires careful small signal compensation of the control loop for achieving high bandwidth and good phase margin. The control loop is comprised of two parts. The first is the power stage, which consists of the duty cycle modulator, output inductor, output capacitor, and load. The second part is the error amplifier, which for the LM2743 is a 9 MHz op-amp used in the classic inverting configuration. Figure 13 shows the regulator and control loop components. PGC = n x (VDD) x QG x fSW where `n' is the number of MOSFETs (if multiple devices have been placed in parallel), VDD is the driving voltage (see MOSFET Gate Drivers section) and QGS is the gate charge of the MOSFET. If different types of MOSFETs are used, the `n' term can be ignored and their gate charges simply summed to form a cumulative QG. Gate charge loss differs from conduction www.national.com 16 LM2743 b = L + CO(RORL + RORC + RCRL) c = RO + RL 20095269 20095264 FIGURE 13. Power Stage and Error Amp One popular method for selecting the compensation components is to create Bode plots of gain and phase for the power stage and error amplifier. Combined, they make the overall bandwidth and phase margin of the regulator easy to see. Software tools such as Excel, MathCAD, and Matlab are useful for showing how changes in compensation or the power stage affect system gain and phase. The power stage modulator provides a DC gain ADC that is equal to the input voltage divided by the peak-to-peak value of the PWM ramp. This ramp is 1.0VP-P for the LM2743. The inductor and output capacitor create a double pole at frequency fDP, and the capacitor ESR and capacitance create a single zero at frequency fESR. For this example, with VIN = 3.3V, these quantities are: 20095270 FIGURE 14. Power Stage Gain and Phase The double pole at 4.5 kHz causes the phase to drop to approximately -130 at around 10 kHz. The ESR zero, at 20.3 kHz, provides a +90 boost that prevents the phase from dropping to -180. If this loop were left uncompensated, the bandwidth would be approximately 10 kHz and the phase margin 53. In theory, the loop would be stable, but would suffer from poor DC regulation (due to the low DC gain) and would be slow to respond to load transients (due to the low bandwidth.) In practice, the loop could easily become unstable due to tolerances in the output inductor, capacitor, or changes in output current, or input voltage. Therefore, the loop is compensated using the error amplifier and a few passive components. For this example, a Type III, or three-pole-two-zero approach gives optimal bandwidth and phase. In most voltage mode compensation schemes, including Type III, a single pole is placed at the origin to boost DC gain as high as possible. Two zeroes fZ1 and fZ2 are placed at the double pole frequency to cancel the double pole phase lag. Then, a pole, fP1 is placed at the frequency of the ESR zero. A final pole fP2 is placed at one-half of the switching frequency. The gain of the error amplifier transfer function is selected to give the best bandwidth possible without violating the Nyquist stability criteria. In practice, a good crossover point is one-fifth In the equation for fDP, the variable RL is the power stage resistance, and represents the inductor DCR plus the on resistance of the top power MOSFET. RO is the output voltage divided by output current. The power stage transfer function GPS is given by the following equation, and Figure 14 shows Bode plots of the phase and gain in this example. a = LCO(RO + RC) 17 www.national.com LM2743 of the switching frequency, or 60 kHz for this example. The generic equation for the error amplifier transfer function is: In this equation the variable AEA is a ratio of the values of the capacitance and resistance of the compensation components, arranged as shown in Figure 13. AEA is selected to provide the desired bandwidth. A starting value of 80,000 for AEA should give a conservative bandwidth. Increasing the value will increase the bandwidth, but will also decrease phase margin. Designs with 45 to 60 are usually best because they represent a good trade-off between bandwidth and phase margin. In general, phase margin is lowest and gain highest (worst-case) for maximum input voltage and minimum output current. One method to select AEA is to use an iterative process beginning with these worst-case conditions. 1. Increase AEA 2. Check overall bandwidth and phase margin 3. Change VIN to minimum and recheck overall bandwidth and phase margin 4. Change IO to maximum and recheck overall bandwidth and phase margin The process ends when the both bandwidth and the phase margin are sufficiently high. For this example input voltage can vary from 3.0 to 3.6V and output current can vary from 0 to 4A, and after a few iterations a moderate gain factor of 101dB is used. The error amplifier of the LM2743 has a unity-gain bandwidth of 9 MHz. In order to model the effect of this limitation, the open-loop gain can be calculated as: 20095274 20095275 FIGURE 15. Error Amp. Gain and Phase In VM regulators, the top feedback resistor RFB2 forms a part of the compensation. Setting RFB2 to 10 k, 1% usually gives values for the other compensation resistors and capacitors that fall within a reasonable range. (Capacitances > 1pF, resistances < 1M) CC1, CC2, CC3, RC1, and RC2 are selected to provide the poles and zeroes at the desired frequencies, using the following equations: The new error amplifier transfer function that takes into account unity-gain bandwidth is: The gain and phase of the error amplifier are shown in Figure 15. www.national.com 18 LM2743 In practice, a good trade off between phase margin and bandwidth can be obtained by selecting the closest 10% capacitor values above what are suggested for CC1 and CC2, the closest 10% capacitor value below the suggestion for CC3, and the closest 1% resistor values below the suggestions for RC1, RC2. Note that if the suggested value for RC2 is less than 100, it should be replaced by a short circuit. Following this guideline, the compensation components will be: CC1 = 27pF 10% CC2 = 820pF 10% CC3 = 2.7nF 10% RC1 = 39.2k 1% RC2 = 2.55k 1% 20095285 The transfer function of the compensation block can be derived by considering the compensation components as impedance blocks ZF and ZI around an inverting op-amp: 20095286 FIGURE 16. Overall Loop Gain and Phase The bandwidth of this example circuit is 59 kHz, with a phase margin of 60. EFFICIENCY CALCULATIONS The following is a sample calculation. A reasonable estimation of the efficiency of a switching buck controller can be obtained by adding together the Output Power (POUT) loss and the Total Power (PTOTAL) loss: As with the generic equation, GEA-ACTUAL must be modified to take into account the limited bandwidth of the error amplifier. The result is: The total control loop transfer function H is equal to the power stage transfer function multiplied by the error amplifier transfer function. The Output Power (POUT) for theTypical Application Circuit design is (1.2V x 4A) = 4.8W. The Total Power (PTOTAL), with an efficiency calculation to complement the design, is shown below. The majority of the power losses are due to low and high side of MOSFET's losses. The losses in any MOSFET are group of switching (PSW) and conduction losses(PCND). H = GPS x HEA The bandwidth and phase margin can be read graphically from Bode plots of HEA are shown in Figure 16. PFET = PSW + PCND = 61.38 mW + 270.42 mW PFET = 331.8 mW FET Switching Loss (PSW) PSW = PSW(ON) + PSW(OFF) 19 www.national.com LM2743 PSW = 0.5 x VIN x IOUT x (tr + tf) x fSW PSW = 0.5 x 3.3V x 4A x 300 kHz x 31 ns PSW = 61.38 mW where, The FDS6898A has a typical turn-on rise time tr and turn-off fall time tf of 15 ns and 16 ns, respectively. The switching losses for this type of dual N-Channel MOSFETs are 0.061W. FET Conduction Loss (PCND) Here n is the number of paralleled capacitors, ESR is the equivalent series resistance of each, and PCAP is the dissipation in each. So for example if we use only one input capacitor of 24 m. PCND = PCND1 + PCND2 PCND1 = (IOUT)2 x RDS(ON) x k x D PCND2 = (IOUT)2 x RDS(ON) x k x (1-D) RDS(ON) = 13 m and the factor is a constant value (k = 1.3) to account for the increasing RDS(ON) of a FET due to heating. PCAP = 88.8 mW PCND1 = (4A)2 x 13 m x 1.3 x 0.364 PCND2 = (4A)2 x 13 m x 1.3 x (1 - 0.364) Output Inductor Loss (PIND) PCND = 98.42 mW + 172 mW = 270.42 mW PIND = I2OUT x DCR There are few additional losses that are taken into account: IC Operating Loss (PIC) where DCR is the DC resistance. Therefore, for example PIND = (4A)2 x 11 m PIC = IQ_VCC x VCC, PIND = 176 mW where IQ-VCC is the typical operating VCC current PIC= 1.5 mA x 3.3V = 4.95 mW Total System Efficiency PTOTAL = PFET + PIC + PGATE + PCAP + PIND FET Gate Charging Loss (PGATE) PGATE = n x VCC x QGS x fSW PGATE = 2 x 3.3V x 3 nC x 300 kHz PGATE = 5.94 mW The value n is the total number of FETs used and QGS is the typical gate-source charge value, which is 3 nC. For the FDS6898A the gate charging loss is 5.94 mW. Input Capacitor Loss (PCAP) www.national.com 20 LM2743 Example Circuits 20095232 FIGURE 17. 3.3V to 1.8V @ 2A, fSW = 300kHz PART PART NUMBER TYPE PACKAGE U1 LM2743 Synchronous Controller TSSOP-14 Q1 FDS6898A Dual N-MOSFET SO-8 D1 MBR0520LTI Schottky Diode SOD-123 L1 DO3316P-472 Inductor CIN1 16SP100M Aluminum Electrolytic CO1 6SP220M CCC, CBOOT, CIN2, CO2 DESCRIPTION VENDOR NSC 20V, 10m@ 4.5V, 16nC Fairchild 4.7H, 4.8Arms 18m Coilcraft 10mm x 6mm 100F, 16V, 2.89Arms Sanyo Aluminum Electrolytic 10mm x 6mm 220F, 6.3V 3.1Arms Sanyo VJ1206Y104KXXA Capacitor 1206 0.1F, 10% Vishay CC3 VJ0805Y332KXXA Capacitor 805 3300pF, 10% Vishay CSS VJ0805A123KXAA Capacitor 805 12nF, 10% Vishay CC2 VJ0805A821KXAA Capacitor 805 820pF 10% Vishay CC1 VJ0805A220KXAA Capacitor 805 22pF, 10% Vishay RFB2 CRCW08051002F Resistor 805 10.0k 1% Vishay RFB1 CRCW08054991F Resistor 805 4.99k1% Vishay RFADJ CRCW08051103F Resistor 805 110k 1% Vishay RC2 CRCW08052101F Resistor 805 2.1k 1% Vishay RCS CRCW08052101F Resistor 805 2.1 k 1% Vishay RCC CRCW080510R0F Resistor 805 10.0 1% Vishay RC1 CRCW08055492F Resistor 805 54.9k 1% Vishay RPULL-UP CRCW08051003J Resistor 805 100k 5% Vishay 21 www.national.com LM2743 20095233 FIGURE 18. 5V to 2.5V @ 2A, fSW = 300kHz PART PART NUMBER TYPE PACKAGE U1 LM2743 Synchronous Controller TSSOP-14 Q1 FDS6898A Dual N-MOSFET SO-8 D1 MBR0520LTI Schottky Diode SOD-123 L1 DO3316P-682 Inductor CIN1 16SP100M Aluminum Electrolytic CO1 10SP56M CCC, CBOOT, CIN2, CO2 DESCRIPTION VENDOR NSC 20V, 10m@ 4.5V, 16nC Fairchild 6.8H, 4.4Arms, 27m Coilcraft 10mm x 6mm 100F, 16V, 2.89Arms Sanyo Aluminum Electrolytic 6.3mm x 6mm 56F, 10V 1.7Arms Sanyo VJ1206Y104KXXA Capacitor 1206 0.1F, 10% Vishay CC3 VJ0805Y182KXXA Capacitor 805 1800pF, 10% Vishay CSS VJ0805A123KXAA Capacitor 805 12nF, 10% Vishay CC2 VJ0805A821KXAA Capacitor 805 820pF 10% Vishay CC1 VJ0805A330KXAA Capacitor 805 33pF, 10% Vishay RFB2 CRCW08051002F Resistor 805 10.0k 1% Vishay RFB1 CRCW08053161F Resistor 805 3.16k 1% Vishay RFADJ CRCW08051103F Resistor 805 110k 1% Vishay RC2 CRCW08051301F Resistor 805 1.3k 1% Vishay RCS CRCW08052101F Resistor 805 2.1 k 1% Vishay RCC CRCW080510R0F Resistor 805 10.0 1% Vishay RC1 CRCW08053322F Resistor 805 33.2k 1% Vishay RPULL-UP CRCW08051003J Resistor 805 100k 5% Vishay www.national.com 22 LM2743 20095234 FIGURE 19. 12V to 3.3V @ 4A, fSW = 300kHz PART PART NUMBER TYPE PACKAGE U1 LM2743 Synchronous Controller TSSOP-14 Q1 FDS6898A Dual N-MOSFET SO-8 D1 MBR0520LTI Schottky Diode SOD-123 L1 DO3316P-332 Inductor CIN1 16SP100M Aluminum Electrolytic 10mm x 6mm CO1 6SP220M Aluminum Electrolytic CCC, CBOOT, CIN2, CO2 VJ1206Y104KXXA CC3 DESCRIPTION VENDOR NSC 20V, 10m@ 4.5V, 16nC Fairchild 3.3H, 5.4Arms 15m Coilcraft 100F, 16V, 2.89Arms Sanyo 10mm x 6mm 220F, 6.3V 3.1Arms Sanyo Capacitor 1206 0.1F, 10% Vishay VJ0805Y222KXXA Capacitor 805 2200pF, 10% Vishay CSS VJ0805A123KXAA Capacitor 805 12nF, 10% Vishay CC2 VJ0805Y332KXXA Capacitor 805 3300pF 10% Vishay CC1 VJ0805A820KXAA Capacitor 805 82pF, 10% Vishay RFB2 CRCW08051002F Resistor 805 10.0k 1% Vishay RFB1 CRCW08052211F Resistor 805 2.21k 1% Vishay RFADJ CRCW08051103F Resistor 805 110k 1% Vishay RC2 CRCW08052611F Resistor 805 2.61k 1% Vishay RCS CRCW08054121F Resistor 805 4.12 k 1% Vishay RCC CRCW080510R0F Resistor 805 10.0 1% Vishay RC1 CRCW08051272F Resistor 805 12.7k 1% Vishay RPULL-UP CRCW08051003J Resistor 805 100k 5% Vishay 23 www.national.com LM2743 Physical Dimensions inches (millimeters) unless otherwise noted TSSOP-14 Pin Package NS Package Number MTC14 www.national.com 24 LM2743 Notes 25 www.national.com LM2743 Low Voltage N-Channel MOSFET Synchronous Buck Regulator Controller Notes For more National Semiconductor product information and proven design tools, visit the following Web sites at: www.national.com Products Design Support Amplifiers www.national.com/amplifiers WEBENCH(R) Tools www.national.com/webench Audio www.national.com/audio App Notes www.national.com/appnotes Clock and Timing 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