LT1795 Dual 500mA/50MHz Current Feedback Line Driver Amplifier DESCRIPTIO U FEATURES The LT(R)1795 is a dual current feedback amplifier with high output current and excellent large signal characteristics. The combination of high slew rate, 500mA output drive and up to 15V operation enables the device to deliver significant power at frequencies in the 1MHz to 2MHz range. Short-circuit protection and thermal shutdown insure the device's ruggedness. The LT1795 is stable with large capacitive loads and can easily supply the large currents required by the capacitive loading. A shutdown feature switches the device into a high impedance, low current mode, reducing power dissipation when the device is not in use. For lower bandwidth applications, the supply current can be reduced with a single external resistor. 500mA Output Drive Current 50MHz Bandwidth, AV = 2, RL = 25 900V/s Slew Rate, AV = 2, RL = 25 Low Distortion: -75dBc at 1MHz High Input Impedance, 10M Wide Supply Range, 5V to 15V Full Rate, Downstream ADSL Supported Low Power Shutdown Mode Power Saving Adjustable Supply Current Stable with CL = 10,000pF Power Enhanced Small Footprint Packages TSSOP-20, S0-20 Wide Available in a 20-Lead TSSOP Package U APPLICATIO S The LT1795 comes in the very small, thermally enhanced, 20-lead TSSOP package for maximum port density in line driver applications. ADSL HDSL2, G.lite Drivers Buffers Test Equipment Amplifiers Video Amplifiers Cable Drivers , LTC and LT are registered trademarks of Linear Technology Corporation. U TYPICAL APPLICATION Low Loss, High Power Central Office ADSL Line Driver V+ +IN + 1/2 LT1795 12.5 - 1k 1:2* 165 100 1k - 1/2 LT1795 -IN 12.5 + V- * MIDCOM 50215 OR EQUIVALENT 1795 TA01 1795fa 1 LT1795 W W U W ABSOLUTE AXI U RATI GS (Note 1) Supply Voltage ...................................................... 18V Input Current ...................................................... 15mA Output Short-Circuit Duration (Note 2) ............ Indefinite Operating Temperature Range ................ - 40C to 85C Specified Temperature Range (Note 3) ... - 40C to 85C Junction Temperature ........................................... 150C Storage Temperature Range ................. - 65C to 150C Lead Temperature (Soldering, 10 sec).................. 300C U W U PACKAGE/ORDER I FOR ATIO TOP VIEW V- 1 20 V- NC 2 19 NC -IN 3 18 OUT +IN 4 17 ORDER PART NUMBER V+ LT1795CFE LT1795IFE COMP 1 V+ 2 OUT 3 V- 4 20 COMP 19 V + LT1795CSW LT1795ISW 18 OUT 17 V- SHDN 5 16 COMP V- 5 16 V - SHDNREF 6 15 COMP V- 6 15 V - +IN 7 14 V + V- 7 14 V - -IN 8 13 OUT -IN 8 13 -IN NC 9 12 NC +IN 9 12 +IN V - 10 11 V - SHDN 10 FE PACKAGE 20-LEAD PLASTIC TSSOP ORDER PART NUMBER TOP VIEW 11 SHDNREF S PACKAGE 20-LEAD PLASTIC SW TJMAX = 150 C, JA 40C/W (Note 4) TJMAX = 150 C, JA = 40C/W (Note 4) UNDERSIDE METAL INTERNALLY CONNECTED TO V- (PCB CONNECTION OPTIONAL) Consult LTC Marketing for parts specified with wider operating temperature ranges. ELECTRICAL CHARACTERISTICS The denotes the specifications which apply over the full specified temperature range, otherwise specifications are at TA = 25C. VCM = 0V, 5V VS 15V, pulse tested, VSHDN = 2.5V, VSHDNREF = 0V unless otherwise noted. (Note 3) SYMBOL PARAMETER VOS Input Offset Voltage CONDITIONS MIN TYP MAX UNITS 3 4.5 13 17 mV mV 1 1.5 3.5 5.0 mV mV 10 2 8 5 20 A A 0.5 1.5 2 7 A A 10 20 70 100 A A 10 20 30 50 A A Input Offset Voltage Matching Input Offset Voltage Drift IIN + Noninverting Input Current Noninverting Input Current Matching IIN- Inverting Input Current Inverting Input Current Matching V/C en Input Noise Voltage Density f = 10kHz, RF =1k, RG = 10, RS = 0 3.6 nV/Hz + in Input Noise Current Density f = 10kHz, RF =1k, RG = 10, RS = 10k 2 pA/Hz - in Input Noise Current Density f = 10kHz, RF =1k, RG = 10, RS = 10k 30 pA/Hz 1795fa 2 LT1795 ELECTRICAL CHARACTERISTICS The denotes the specifications which apply over the full specified temperature range, otherwise specifications are at TA = 25C. VCM = 0V, 5V VS 15V, pulse tested, VSHDN = 2.5V, VSHDNREF = 0V unless otherwise noted. (Note 3) SYMBOL RIN + PARAMETER CONDITIONS Input Resistance VIN = 12V, VS = 15V V = 2V, VS = 5V MIN TYP MAX UNITS 1.5 0.5 10 5 M M Input Capacitance VIN = 15V 2 pF Input Voltage Range (Note 5) VS = 15V VS = 5V 12 2 13.5 3.5 V V Common Mode Rejection Ratio VS = 15V, VCM = 12V VS = 5V, VCM = 2V 55 50 62 60 dB dB Inverting Input Current Common Mode Rejection VS = 15V, VCM = 12V VS = 5V, VCM = 2V Power Supply Rejection Ratio VS = 5V to 15V Noninverting Input Current Power Supply Rejection VS = 5V to 15V 30 500 nA/V Inverting Input Current Power Supply Rejection VS = 5V to 15V 1 5 A/V AV Large-Signal Voltage Gain VS = 15V, VOUT = 10V, RL = 25 VS = 5V, VOUT = 2V, RL = 12 55 55 68 68 dB dB ROL Transresistance, VOUT/IIN - VS = 15V, VOUT = 10V, RL = 25 VS = 5V, VOUT = 2V, RL = 12 75 75 200 200 k k VOUT Maximum Output Voltage Swing VS = 15V, RL = 25 11.5 10.0 12.5 11.5 V V 2.5 2.0 3 3 V V 0.5 CIN+ CMRR PSRR VS = 5V, RL = 12 IOUT Maximum Output Current VS = 15V, RL = 1 IS Supply Current Per Amplifier VS = 15V, VSHDN = 2.5V Supply Current Per Amplifier, RSHDN = 51k, (Note 6) VS = 15V 1 1 60 77 VS = 15V, VSHDN = 0.4V VS = 15V, VSHDN = 0.4V dB 1 A 34 42 mA mA 15 20 25 mA mA Output Leakage Current, Shutdown A/V A/V 29 Positive Supply Current, Shutdown 10 10 1 200 A 1 200 A Channel Separation VS = 15V, VOUT = 10V, RL = 25 HD2, HD3 2nd and 3rd Harmonic Distortion Differential Mode f = 1MHz, VO = 20VP-P, RL = 50, AV = 2 SR Slew Rate (Note 7) AV = 4, RL = 400 Slew Rate AV = 4, RL = 25 900 V/s Small-Signal BW AV = 2, VS = 15V, Peaking 1.5dB RF = RG = 910, RL = 100 65 MHz AV = 2, VS = 15V, Peaking 1.5dB RF = RG = 820, RL = 25 50 MHz BW Note 1: Absolute Maximum Ratings are those values beyond which the life of a device may be impaired. Note 2: Applies to short-circuits to ground only. A short-circuit between the output and either supply may permanently damage the part when operated on supplies greater than 10V. Note 3: The LT1795C is guaranteed to meet specified performance from 0C to 70C and is designed, characterized and expected to meet these extended temperature limits, but is not tested at - 40C and 85C. The LT1795I is guaranteed to meet the extended temperature limits. 80 400 110 dB -75 dBc 900 V/s Note 4: Thermal resistance varies depending upon the amount of PC board metal attached to the device. If the maximum dissipation of the package is exceeded, the device will go into thermal shutdown and be protected. Note 5: Guaranteed by the CMRR tests. Note 6: RSHDN is connected between the SHDN pin and V +. Note 7: Slew rate is measured at 5V on a 10V output signal while operating on 15V supplies with RF = 1k, RG = 333 (AV = +4) and RL = 400. 1795fa 3 LT1795 W SMALL-SIGNAL BANDWIDTH U U RSD = 0, IS = 30mA per Amplifer, VS = 15V, Peaking 1dB, RL = 25 RSD = 51k, IS = 15mA per Amplifer, VS = 15V, Peaking 1dB, RL = 25 AV RF RG -3dB BW (MHz) AV RF RG -3dB BW (MHz) -1 976 976 44 -1 976 976 30 1 1.15k -- 53 1 1.15k -- 32 2 976 976 48 2 976 976 32 10 649 72 46 10 649 72 27 U W TYPICAL PERFOR A CE CHARACTERISTICS V+ 35 OUTPUT SATURATION VOLTAGE (V) VS = 15V AV = 1 RL = RSD = 0 30 25 20 15 RSD = 51k 10 5 0 -50 -25 50 25 75 0 TEMPERATURE (C) 100 VS = 15V -1 2.0 RL = 2k -2 RL = 25 -3 -4 4 3 RL = 25 2 1 RL = 2k V- -50 -25 125 25 50 0 75 TEMPERATURE (C) LT1795 G01 DISTORTION (dBc) CURRENT INTO SHDN PIN (mA) -50 0.3 0.2 SOURCING 1.2 SINKING 1.0 0.8 1 2 3 4 VOLTAGE APPLIED AT SHDN PIN (V) 50 25 75 0 TEMPERATURE (C) 100 -60 5 1795 G04 125 LT1795 G03 Third Harmonic Distortion vs Frequency -40 AV = 2 DIFFERENTIAL VOUT = 20VP-P VS = 15V RLOAD = 50 IQ PER AMPLIFIER -50 IQ = 5mA -70 IQ = 10mA -80 -90 0.1 0 1.4 0.6 -50 -25 125 -40 0.4 0 1.6 Second Harmonic Distortion vs Frequency VS = 15V VSHDNREF = 0V 0.5 100 VS = 15V 1.8 LT1795 G02 SHDN Pin Current vs Voltage 0.6 Output Short-Circuit Current vs Junction Temperature DISTORTION (dBc) 40 SUPPLY CURRENT PER AMPLIFIER (mA) Output Saturation Voltage vs Junction Temperature OUTPUT SHORT-CIRCUIT CURRENT (A) Supply Current vs Ambient Temperature -100 10k IQ = 15mA -70 -80 IQ = 5mA IQ = 10mA IQ = 20mA -90 IQ = 20mA 100k FREQUENCY (Hz) -60 AV = 2 DIFFERENTIAL VOUT = 20VP-P VS = 15V RLOAD = 50 IQ PER AMPLIFIER 1M LT1795 G05 IQ = 15mA -100 10k 100k FREQUENCY (Hz) 1M LT1795 G06 1795fa 4 LT1795 U W TYPICAL PERFOR A CE CHARACTERISTICS Second Harmonic Distortion vs Frequency Third Harmonic Distortion vs Frequency -40 -60 AV = 10 DIFFERENTIAL VOUT = 20VP-P VS = 15V RLOAD = 50 IQ PER AMPLIFIER -70 IQ = 5mA -50 IQ = 20mA -70 -80 IQ = 10mA -50 DISTORTION (dBc) -60 -40 -40 AV = 10 DIFFERENTIAL VOUT = 20VP-P VS = 15V RLOAD = 50 IQ PER AMPLIFIER DISTORTION (dBc) DISTORTION (dBc) -50 Second Harmonic Distortion vs Frequency IQ = 10mA -80 IQ = 15mA -90 IQ = 5mA IQ = 20mA -90 -60 AV = 2 DIFFERENTIAL VOUT = 20VP-P VS = 12V RLOAD = 50 IQ PER AMPLIFIER IQ = 5mA -70 IQ = 10mA -80 IQ = 15mA -90 IQ = 15mA -100 10k 100k FREQUENCY (Hz) -100 10k 1M 100k FREQUENCY (Hz) 1M LT1795 G07 Third Harmonic Distortion vs Frequency -40 -40 IQ = 5mA -70 IQ = 20mA -80 -60 AV = 10 DIFFERENTIAL VOUT = 20VP-P VS = 12V RLOAD = 50 IQ PER AMPLIFIER -70 AV = 10 DIFFERENTIAL VOUT = 20VP-P VS = 12V RLOAD = 50 IQ PER AMPLIFIER -50 IQ = 20mA -80 IQ = 10mA DISTORTION (dBc) -50 DISTORTION (dBc) DISTORTION (dBc) -60 -60 IQ = 5mA -70 -80 IQ = 15mA -90 -90 IQ = 15mA IQ = 10mA -90 IQ = 5mA IQ = 10mA -100 10k 100k FREQUENCY (Hz) -100 10k 1M 100k FREQUENCY (Hz) LT1795 G10 IQ = 5mA -80 -90 -40 IQ = 10mA IQ = 20mA IQ = 15mA -60 AV = 2 DIFFERENTIAL VOUT = 4VP-P VS = 12V RLOAD = 50 IQ PER AMPLIFIER -50 IQ = 5mA DISTORTION (dBc) -50 -70 1M Second Harmonic Distortion vs Frequency -40 DISTORTION (dBc) DISTORTION (dBc) -60 100k FREQUENCY (Hz) LT1795 G12 Third Harmonic Distortion vs Frequency -40 -50 IQ = 15mA -100 10k 1M IQ = 20mA LT1795 G11 Second Harmonic Distortion vs Frequency AV = 2 DIFFERENTIAL VOUT = 4VP-P VS = 12V RLOAD = 50 IQ PER AMPLIFIER 1M LT1795 G09 Second Harmonic Distortion vs Frequency -40 -50 100k FREQUENCY (Hz) LT1795 G08 Third Harmonic Distortion vs Frequency AV = 2 DIFFERENTIAL VOUT = 20VP-P VS = 12V RLOAD = 50 IQ PER AMPLIFIER IQ = 20mA -100 10k IQ = 10mA -70 -80 IQ = 15mA -90 -60 AV = 10 DIFFERENTIAL VOUT = 4VP-P VS = 12V RLOAD = 50 IQ PER AMPLIFIER IQ = 10mA -70 -80 -90 IQ = 5mA IQ = 15mA IQ = 20mA IQ = 20mA -100 10k 100k FREQUENCY (Hz) 1M LT1795 G13 -100 10k 100k FREQUENCY (Hz) 1M LT1795 G14 -100 10k 100k FREQUENCY (Hz) 1M LT1795 G15 1795fa 5 LT1795 U W TYPICAL PERFOR A CE CHARACTERISTICS Third Harmonic Distortion vs Frequency -40 IQ = 5mA IQ = 10mA -70 IQ = 15mA -80 -90 -60 IQ = 5mA IQ = 10mA -80 IQ = 20mA IQ = 20mA -90 100k FREQUENCY (Hz) 1M -50 -70 -100 -110 10k -40 AV = 2 DIFFERENTIAL VOUT = 4VP-P VS = 5V RLOAD = 50 IQ PER AMPLIFIER -50 DISTORTION (dBc) DISTORTION (dBc) -60 AV = 10 DIFFERENTIAL VOUT = 4VP-P VS = 12V RLOAD = 50 IQ PER AMPLIFIER -100 10k 100k FREQUENCY (Hz) 1M -70 -50 IQ = 15mA IQ = 10mA -80 IQ = 5mA -90 -100 10k -100 10k -60 1M AV = 10 DIFFERENTIAL VOUT = 4VP-P VS = 5V RLOAD = 50 IQ PER AMPLIFIER IQ = 5mA IQ = 10mA -70 IQ = 15mA -80 -90 100k FREQUENCY (Hz) IQ = 20mA -100 10k 1M 100k FREQUENCY (Hz) 1M LT1795 G20 -3dB Bandwidth vs Supply Current Slew Rate vs Supply Current 50 1200 1000 800 FALLING 600 VS = 15V TA =25C AV = 4 RLOAD = 25 RF = 1k 25 15 7.5 10 20 30 SUPPLY CURRENT PER AMPLIFIER (mA) 1795 * G21 -3dB BANDWIDTH (MHz) 45 RISING SLEW RATE (V/s) 100k FREQUENCY (Hz) LT1795 G18 LT1795 G19 0 IQ = 20mA -40 AV = 10 DIFFERENTIAL VOUT = 4VP-P VS = 5V RLOAD = 50 IQ PER AMPLIFIER IQ = 20mA 200 IQ = 15mA -80 Third Harmonic Distortion vs Frequency DISTORTION (dBc) DISTORTION (dBc) IQ = 10mA -70 LT1795 G17 -40 400 IQ = 5mA -90 Second Harmonic Distortion vs Frequency -60 -60 AV = 2 DIFFERENTIAL VOUT = 4VP-P VS = 5V RLOAD = 50 IQ PER AMPLIFIER IQ = 15mA LT1795 G16 -50 DISTORTION (dBc) -40 -50 Third Harmonic Distortion vs Frequency Second Harmonic Distortion vs Frequency 40 35 30 VS = 15V TA =25C AV = 4 RLOAD = 25 RF = 1k 25 25 15 20 30 7.5 10 SUPPLY CURRENT PER AMPLIFIER (mA) 1795 * G22 1795fa 6 LT1795 U U W U APPLICATIO S I FOR ATIO The LT1795 is a dual current feedback amplifier with high output current drive capability. The amplifier is designed to drive low impedance loads such as twisted-pair transmission lines with excellent linearity. When VSHDN = VSHDNREF, the device is shut down. The device will interface directly with 3V or 5V CMOS logic when SHDNREF is grounded and the control signal is applied to the SHDN pin. Switching time between the active and shutdown states is about 1.5s. SHUTDOWN/CURRENT SET Figures 1 to 4 illustrate how the SHDN and SHDNREF pins can be used to reduce the amplifier quiescent current. In both cases, an external resistor is used to set the current. The two approaches are equivalent, however the required resistor values are different. The quiescent current will be approximately 115 times the current in the SHDN pin and 230 times the current in the SHDNREF pin. The voltage across the resistor in either condition is V + - 1.5V. For example, a 50k resistor between V + and SHDN will set the If the shutdown/current set feature is not used, connect SHDN to V + and SHDNREF to ground. The SHDN and SHDNREF pins control the biasing of the two amplifiers. The pins can be used to either turn off the amplifiers completely, reducing the quiescent current to less then 200A, or to control the quiescent current in normal operation. V+ V+ 10 SHDN RSHDN 11 SHDNREF 10 SHDN RSHDNREF 11 SHDNREF 1795 F03 1795 F01 Figure 1. RSHDN Connected Between V + and SHDN (Pin 10); SHDNREF (Pin 11) = GND. See Figure 2 Figure 3. RSHDNREF Connected Between SHDNREF (Pin 11) and GND; SHDN (Pin 10) = V +. See Figure 4 80 80 TA = 25C VS = 15V 70 AMPLIFIER SUPPLY CURRENT, ISY - mA (BOTH AMPLIFIERS) AMPLIFIER SUPPLY CURRENT, ISY - mA (BOTH AMPLIFIERS) 70 60 50 40 30 20 10 TA = 25C VS = 15V 60 50 40 30 20 10 0 0 0 25 50 75 100 125 150 175 200 225 RSHDN (k) 1795 F02 Figure 2. LT1795 Amplifier Supply Current vs RSHDN. RSHDN Connected Between V+ and SHDN, SHDNREF = GND (See Figure 1) 50 100 150 200 250 300 350 400 450 500 RSHDNREF (k) 1795 F04 Figure 4. LT1795 Amplifier Supply Current vs RSHDNREF. RSHDNREF Connected Between SHDNREF and GND, SHDN = V+ (See Figure 3) 1795fa 7 LT1795 U U W U APPLICATIO S I FOR ATIO quiescent current to 33mA with VS = 15V. If ON/OFF control is desired in addition to reduced quiescent current, then the circuits in Figures 5 to 7 can be employed. Figure 8 illustrates a partial shutdown with direct logic control. By keeping the output stage slightly biased on, the output impedance remains low, preserving the line termination. The design equations are: V+ R1 = RSHDN RB 10k OFF 10 SHDN Q1 ON INTERNAL LOGIC THRESHOLD ~1.4V 11 SHDNREF 115 * VH (IS )ON - (IS )OFF (0V) ( (3.3V/5V) Q1: 2N3904 OR EQUIVALENT R2 = 1795 F05 Figure 5. Setting Amplifier Supply Current Level with ON/OFF Control, Version 1 RPULLUP >500k RSHDN2 OFF (0V) (3.3V/5V) Q1A Q1B OFF (0V) (3.3V/5V) VCC R2 1795 F06 Q1A, Q1B: ROHM IMX1 or FMG4A (W/INTERNAL RB) R1 10 SHDN INTERNAL LOGIC THRESHOLD ISY ~ 1.4V CONTROL 11 SHDNREF 1795 F08 Figure 8. Partial Shutdown REXT OFF (0V) IPROG (3.3V/5V) IPROG 0.5mA FOR REXT = 0 (SEE SHDN PIN CURRENT vs VOLTAGE CHARACTERISTIC) ON OFF (0V) (3.3V/5V) Figure 6. Setting Multiple Amplifier Supply Current Levels with ON/OFF Control, Version 2 ON VSHDN = Shutdown Pin Voltage 1.4V VCC = Positive Supply Voltage RB2 10k ON (IS)ON = Supply Current Fully On (IS)OFF = Supply Current Partially On 11 SHDNREF RB1 10k (VSHDN / VH ) * (IS )ON - (IS )OFF + (IS )OFF VH = Logic High Level 10 SHDN ON ) where V+ RSHDN1 115 * VCC - VSHDN SHDN 10 ISY CONTROL INTERNAL LOGIC THRESHOLD ~ 1.4V SHDNREF 11 1795 F07 Figure 7. Setting Amplifier Supply Current Level with ON/OFF Control, Version 3 THERMAL CONSIDERATIONS The LT1795 contains a thermal shutdown feature that protects against excessive internal (junction) temperature. If the junction temperature of the device exceeds the protection threshold, the device will begin cycling between normal operation and an off state. The cycling is not harmful to the part. The thermal cycling occurs at a slow rate, typically 10ms to several seconds, which depends on the power dissipation and the thermal time constants of the package and heat sinking. Raising the ambient tempera1795fa 8 LT1795 U W U U APPLICATIO S I FOR ATIO ture until the device begins thermal shutdown gives a good indication of how much margin there is in the thermal design. For surface mount devices, heat sinking is accomplished by using the heat spreading capabilities of the PC board and its copper traces. For the TSSOP package, power is dissipated through the exposed heatsink. For the SO package, power is dissipated from the package primarily through the V - pins (4 to 7 and 14 to 17). These pins should have a good thermal connection to a copper plane, either by direct contact or by plated through holes. The copper plane may be an internal or external layer. The thermal resistance, junction-to-ambient will depend on the total copper area connected to the device. For example, the thermal resistance of the LT1795 connected to a 2 x 2 inch, double sided 2 oz copper plane is 40C/W. ing 0.5A current peaks into the load, a 1 power supply impedance will cause a droop of 0.5V, reducing the available output swing by that amount. Surface mount tantalum and ceramic capacitors make excellent low ESR bypass elements when placed close to the chip. For frequencies above 100kHz, use 1F and 100nF ceramic capacitors. If significant power must be delivered below 100kHz, capacitive reactance becomes the limiting factor. Larger ceramic or tantalum capacitors, such as 4.7F, are recommended in place of the 1F unit mentioned above. Inadequate bypassing is evidenced by reduced output swing and "distorted" clipping effects when the output is driven to the rails. If this is observed, check the supply pins of the device for ripple directly related to the output waveform. Significant supply modulation indicates poor bypassing. CALCULATING JUNCTION TEMPERATURE Capacitance on the Inverting Input The junction temperature can be calculated from the equation: Current feedback amplifiers require resistive feedback from the output to the inverting input for stable operation. Take care to minimize the stray capacitance between the output and the inverting input. Capacitance on the inverting input to ground will cause peaking in the frequency response (and overshoot in the transient response), but it does not degrade the stability of the amplifier. TJ = (PD)(JA) + TA where TJ = Junction Temperature TA = Ambient Temperature PD = Device Dissipation JA = Thermal Resistance (Junction-to-Ambient) Differential Input Signal Swing The differential input swing is limited to about 5V by an ESD protection device connected between the inputs. In normal operation, the differential voltage between the input pins is small, so this clamp has no effect. However, in the shutdown mode, the differential swing can be the same as the input swing. The clamp voltage will then set the maximum allowable input voltage. Feedback Resistor Selection The optimum value for the feedback resistors is a function of the operating conditions of the device, the load impedance and the desired flatness of response. The Typical AC Performance tables give the values which result in less than 1dB of peaking for various resistive loads and operating conditions. If this level of flatness is not required, a higher bandwidth can be obtained by use of a lower feedback resistor. For resistive loads, the COMP pin should be left open (see Capacitive Loads section). Capacitive Loads POWER SUPPLY BYPASSING To obtain the maximum output and the minimum distortion from the LT1795, the power supply rails should be well bypassed. For example, with the output stage supply- The LT1795 includes an optional compensation network for driving capacitive loads. This network eliminates most of the output stage peaking associated with capacitive loads, allowing the frequency response to be flattened. 1795fa 9 LT1795 U U W U APPLICATIONS INFORMATION Figure 9 shows the effect of the network on a 200pF load. Without the optional compensation, there is a 6dB peak at 85MHz caused by the effect of the capacitance on the output stage. Adding a 0.01F bypass capacitor between the output and the COMP pins connects the compensation 14 VS = 15V CL = 200pF 12 RF = 3.4k NO COMPENSATION VOLTAGE GAIN (dB) 10 RF = 1k COMPENSATION 8 6 4 2 and greatly reduces the peaking. A lower value feedback resistor can now be used, resulting in a response which is flat to 1dB to 45MHz. The network has the greatest effect for CL in the range of 0pF to 1000pF. Although the optional compensation works well with capacitive loads, it simply reduces the bandwidth when it is connected with resistive loads. For instance, with a 25 load, the bandwidth drops from 48MHz to 32MHz when the compensation is connected. Hence, the compensation was made optional. To disconnect the optional compensation, leave the COMP pin open. 0 -2 -6 DEMO BOARD RF = 3.4k COMPENSATION -4 1 10 FREQUENCY (MHz) A demo board (DC261A) is available for evaluating the performence of the LT1795. The board is configured as a differential line driver/receiver suitable for xDSL applications. For details, consult your local sales representative. 100 1795 F09 Figure 9 U PACKAGE DESCRIPTIO SW Package 20-Lead Plastic Small Outline (Wide .300 Inch) (Reference LTC DWG # 05-08-1620) .050 BSC .045 .005 .030 .005 TYP .496 - .512 (12.598 - 13.005) NOTE 4 N 20 18 17 16 15 14 13 12 11 N .325 .005 .420 MIN 19 .394 - .419 (10.007 - 10.643) NOTE 3 1 2 3 N/2 N/2 RECOMMENDED SOLDER PAD LAYOUT .005 (0.127) RAD MIN .009 - .013 (0.229 - 0.330) NOTE: 1. DIMENSIONS IN .291 - .299 (7.391 - 7.595) NOTE 4 .010 - .029 x 45 (0.254 - 0.737) 1 2 3 4 5 6 7 8 .093 - .104 (2.362 - 2.642) 9 10 .037 - .045 (0.940 - 1.143) 0 - 8 TYP NOTE 3 .016 - .050 (0.406 - 1.270) .050 (1.270) BSC .014 - .019 (0.356 - 0.482) TYP .004 - .012 (0.102 - 0.305) S20 (WIDE) 0502 INCHES (MILLIMETERS) 2. DRAWING NOT TO SCALE 3. PIN 1 IDENT, NOTCH ON TOP AND CAVITIES ON THE BOTTOM OF PACKAGES ARE THE MANUFACTURING OPTIONS. THE PART MAY BE SUPPLIED WITH OR WITHOUT ANY OF THE OPTIONS 4. THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS. MOLD FLASH OR PROTRUSIONS SHALL NOT EXCEED .006" (0.15mm) 1795fa 10 LT1795 U PACKAGE DESCRIPTIO FE Package 20-Lead Plastic TSSOP (4.4mm) (Reference LTC DWG # 05-08-1663) Exposed Pad Variation CA 6.40 - 6.60* (.252 - .260) 4.95 (.195) 4.95 (.195) 20 1918 17 16 15 14 13 12 11 6.60 0.10 2.74 (.108) 4.50 0.10 2.74 6.40 (.108) BSC SEE NOTE 4 0.45 0.05 1.05 0.10 0.65 BSC 1 2 3 4 5 6 7 8 9 10 RECOMMENDED SOLDER PAD LAYOUT 1.20 (.047) MAX 4.30 - 4.50* (.169 - .177) 0 - 8 0.09 - 0.20 (.0036 - .0079) 0.45 - 0.75 (.018 - .030) NOTE: 1. CONTROLLING DIMENSION: MILLIMETERS MILLIMETERS 2. DIMENSIONS ARE IN (INCHES) 3. DRAWING NOT TO SCALE 0.65 (.0256) BSC 0.195 - 0.30 (.0077 - .0118) 0.05 - 0.15 (.002 - .006) FE20 (CA) TSSOP 0203 4. RECOMMENDED MINIMUM PCB METAL SIZE FOR EXPOSED PAD ATTACHMENT *DIMENSIONS DO NOT INCLUDE MOLD FLASH. MOLD FLASH SHALL NOT EXCEED 0.150mm (.006") PER SIDE 1795fa Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights. 11 LT1795 W W SI PLIFIED SCHEMATIC SHDN TO ALL CURRENT SOURCES SHDNREF V+ Q5 Q10 Q2 D1 Q6 Q1 Q11 Q15 Q9 V- +IN CC -IN V- 50 COMP RC OUTPUT V+ V+ Q12 Q3 Q8 Q16 Q14 D2 Q4 Q7 Q13 V- 1795 SS RELATED PARTS PART NUMBER DESCRIPTION COMMENTS LT1497 Dual 125mA, 50MHz Current Feedback Amplifier 900V/s Slew Rate LT1207 Dual 250mA, 60MHz Current Feedback Amplifier Shutdown/Current Set Function LT1886 Dual 200mA, 700MHz Voltage Feedback Amplifier Low Distortion: -72dBc at 200kHz 1795fa 12 Linear Technology Corporation 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 FAX: (408) 434-0507 www.linear.com LT/TP 0603 1K REVA * PRINTED IN USA LINEAR TECHNOLOGY CORPORATION 1999