LM3500
LM3500 Synchronous Step-up DC/DC Converter for White LED Applications
Literature Number: SNVS231F
February 2005
LM3500
Synchronous Step-up DC/DC Converter for White LED
Applications
General Description
The LM3500 is a fixed-frequency step-up DC/DC converter
that is ideal for driving white LEDs for display backlighting and
other lighting functions. With fully intergrated synchronous
switching (no external schottky diode required) and a low
feedback voltage (500mV), power efficiency of the LM3500
circuit has been optimized for lighting applications in wireless
phones and other portable products (single cell Li-Ion or 3-
cell NiMH battery supplies). The LM3500 operates with a fixed
1MHz switching frequency. When used with ceramic input
and output capacitors, the LM3500 provides a small, low-
noise, low-cost solution.
Two LM3500 options are available with different output volt-
age capabilities. The LM3500-21 has a maximum output
voltage of 21V and is typically suited for driving 4 or 5 white
LEDs in series. The LM3500-16 has a maximum output volt-
age of 16V and is typically suited for driving 3 or 4 white LEDs
in series (maximum number of series LEDs dependent on
LED forward voltage). If the primary white LED network
should be disconnected, the LM3500 uses internal protection
circuitry on the output to prevent a destructive over-voltage
event.
A single external resistor is used to set the maximum LED
current in LED-drive applications. The LED current can easily
be adjusted using a pulse width modulated (PWM) signal on
the shutdown pin. In shutdown, the LM3500 completely dis-
connects the input from output, creating total isolation and
preventing any leakage currents from trickling into the LEDs.
Features
■Synchronous rectification, high efficiency and no external
schottky diode required
■Uses small surface mount components
■Can drive 2-5 white LEDs in series
(may function with more low-VF LEDs)
■2.7V to 7V input range
■Internal output over-voltage protection (OVP) circuitry,
with no external zener diode required
LM3500-16: 15.5V OVP; LM3500-21: 20.5V OVP.
■True shutdown isolation
■Input undervoltage lockout
■Requires only small ceramic capacitors at the input and
output
■Thermal Shutdown
■0.1µA shutdown current
■Small 8-bump thin micro SMD package
Applications
■LCD Bias Supplies
■White LED Backlighting
■Handheld Devices
■Digital Cameras
■Portable Applications
Typical Application Circuit
20065701
© 2007 National Semiconductor Corporation 200657 www.national.com
LM3500 Synchronous Step-up DC/DC Converter for White LED Applications
Connection Diagram
Top View
20065702
8-bump micro SMD
Ordering Information
Maximum
Output Voltage
Order Number Package Type NSC Package
Drawing
Top Mark Supplied As
16V LM3500TL-16 micro SMD TL08SSA S18 250 Units, Tape and Reel
16V LM3500TLX-16 micro SMD TL08SSA S18 3000 Units, Tape and Reel
21V LM3500TL-21 micro SMD TL08SSA S23 250 Units, Tape and Reel
21V LM3500TLX-21 micro SMD TL08SSA S23 3000 Units, Tape and Reel
Pin Description/Functions
Pin Name Function
A1 AGND Analog ground.
B1 VIN Analog and Power supply input.
C1 VOUT PMOS source connection for synchronous rectification.
C2 VSW Switch pin. Drain connections of both NMOS and PMOS power devices.
C3 GND Power Ground.
B3 FB Output voltage feedback connection.
A3 NC No internal connection made to this pin.
A2 SHDN Shutdown control pin.
AGND(pin A1): Analog ground pin. The analog ground pin
should tie directly to the GND pin.
VIN(pin B1): Analog and Power supply pin. Bypass this pin
with a capacitor, as close to the device as possible, connected
between the VIN and GND pins.
VOUT(pin C1): Source connection of internal PMOS power
device. Connect the output capacitor between the VOUT and
GND pins as close as possible to the device.
VSW(pin C2): Drain connection of internal NMOS and PMOS
switch devices. Keep the inductor connection close to this pin
to minimize EMI radiation.
GND(pin C3): Power ground pin. Tie directly to ground plane.
FB(pin B3): Output voltage feedback connection. Set the pri-
mary White LED network current with a resistor from the FB
pin to GND. Keep the current setting resistor close to the de-
vice and connected between the FB and GND pins.
NC(pin A3): No internal connection is made to this pin. The
maximum allowable voltage that can be applied to this pin is
7.5V.
SHDN(pin A2): Shutdown control pin. Disable the device with
a voltage less than 0.3V and enable the device with a voltage
greater than 1.1V. The white LED current can be controlled
using a PWM signal at this pin. There is an internal pull down
on the SHDN pin, the device is in a normally off state.
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LM3500
Absolute Maximum Ratings (Note 1)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
VIN −0.3V to 7.5V
VOUT (LM3500-16)(Note 2) −0.3V to 16V
VOUT (LM3500-21)(Note 2) −0.3V to 21V
VSW(Note 2) −0.3V to VOUT+0.3V
FB, SHDN, and NC Voltages −0.3V to 7.5V
Maximum Junction Temperature 150°C
Lead Temperature
(Note 3) 300°C
ESD Ratings (Note 4)
Human Body Model 2kV
Machine Model 200V
Operating Conditions
Ambient Temperature
(Note 5) −40°C to +85°C
Junction Temperature −40°C to +125°C
Supply Voltage 2.7V to 7V
Thermal Properties
Junction to Ambient Thermal
Resistance (θJA)(Note 6)
75°C/W
Electrical Characteristics
Specifications in standard type face are for TA = 25°C and those in boldface type apply over the Operating Temperature Range
of TA = −10°C to +85°C. Unless otherwise specified VIN =2.7V and specification apply to both LM3500-16 and LM3500-21.
Symbol Parameter Conditions Min
(Note 7)
Typ
(Note 8)
Max
(Note 7) Units
IQQuiescent Current, Device Not
Switching FB > 0.54V 0.95 1.2
mA
Quiescent Current, Device
Switching FB = 0V 1.8 2.5
Shutdown SHDN = 0V 0.1 2µA
VFB Feedback Voltage VIN = 2.7V to 7V 0.47 0.5 0.53 V
ΔVFB Feedback Voltage Line
Regulation
VIN = 2.7V to 7V 0.1 0.4 %/V
ICL Switch Current Limit
(LM3500-16)
VIN = 2.7V,
Duty Cycle = 80% 275 400 480
mA
VIN = 3.0V,
Duty Cycle = 70% 255 400 530
Switch Current Limit
(LM3500-21)
VIN = 2.7V,
Duty Cycle = 70% 420 640 770
VIN = 3.0V,
Duty Cycle = 63% 450 670 800
IBFB Pin Bias Current FB = 0.5V (Note 9) 45 200 nA
VIN Input Voltage Range 2.7 7.0 V
RDSON NMOS Switch RDSON VIN = 2.7V, ISW = 300mA 0.43 Ω
PMOS Switch RDSON VOUT = 6V, ISW = 300mA 1.1 2.3
DLimit Duty Cycle Limit (LM3500-16) FB = 0V 80 87 %
Duty Cycle Limit (LM3500-21) FB = 0V 85 94
FSW Switching Frequency 0.85 1.0 1.15 MHz
ISD SHDN Pin Current (Note 10) SHDN = 5.5V 18 30
µASHDN = 2.7V 9 16
SHDN = GND 0.1
ILSwitch Leakage Current
(LM3500-16)
VSW = 15V 0.01 0.5 µA
Switch Leakage Current
(LM3500-21)
VSW = 20V 0.01 2.0
UVP Input Undervoltage Lockout ON Threshold 2.4 2.5 2.6 V
OFF Threshold 2.3 2.4 2.5
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LM3500
Symbol Parameter Conditions Min
(Note 7)
Typ
(Note 8)
Max
(Note 7) Units
OVP Output Overvoltage Protection
(LM3500-16)
ON Threshold 15 15.5 16
V
OFF Threshold 14 14.6 15
Output Overvoltage Protection
(LM3500-21)
ON Threshold 20 20.5 21
OFF Threshold 19 19.5 20
IVout VOUT Bias Current
(LM3500-16)
VOUT = 15V, SHDN = VIN 260 400
µA
VOUT Bias Current
(LM3500-21)
VOUT = 20V, SHDN = VIN 300 460
IVL PMOS Switch Leakage
Current (LM3500-16)
VOUT = 15V, VSW = 0V 0.01 3
µA
PMOS Switch Leakage
Current (LM3500-21)
VOUT = 20V, VSW = 0V 0.01 3
SHDN
Threshold
SHDN Low 0.65 0.3 V
SHDN High 1.1 0.65
Specifications in standard type face are for TJ = 25°C and those in boldface type apply over the full Operating Temperature
Range (TJ = −40°C to +125°C). Unless otherwise specified VIN =2.7V and specification apply to both LM3500-16 and LM3500-21.
Symbol Parameter Conditions Min
(Note 7)
Typ
(Note 8)
Max
(Note 7) Units
IQQuiescent Current, Device Not
Switching FB > 0.54V 0.95 1.2
mA
Quiescent Current, Device
Switching FB = 0V 1.8 2.5
Shutdown SHDN = 0V 0.1 2µA
VFB Feedback Voltage VIN = 2.7V to 7V 0.47 0.5 0.53 V
ΔVFB Feedback Voltage Line
Regulation
VIN = 2.7V to 7V 0.1 0.4 %/V
ICL Switch Current Limit
(LM3500-16)
VIN = 3.0V, Duty Cycle = 70% 400
mA
Switch Current Limit
(LM3500-21)
VIN = 3.0V, Duty Cycle = 63% 670
IBFB Pin Bias Current FB = 0.5V (Note 9) 45 200 nA
VIN Input Voltage Range 2.7 7.0 V
RDSON NMOS Switch RDSON VIN = 2.7V, ISW = 300mA 0.43 Ω
PMOS Switch RDSON VOUT = 6V, ISW = 300mA 1.1 2.3
DLimit Duty Cycle Limit (LM3500-16) FB = 0V 87 %
Duty Cycle Limit (LM3500-21) FB = 0V 94
FSW Switching Frequency 0.8 1.0 1.2 MHz
ISD SHDN Pin Current (Note 10) SHDN = 5.5V 18 30
µASHDN = 2.7V 9 16
SHDN = GND 0.1
ILSwitch Leakage Current
(LM3500-16)
VSW = 15V 0.01 0.5 µA
Switch Leakage Current
(LM3500-21)
VSW = 20V 0.01 2.0
UVP Input Undervoltage Lockout ON Threshold 2.4 2.5 2.6 V
OFF Threshold 2.3 2.4 2.5
OVP Output Overvoltage Protection
(LM3500-16)
ON Threshold 15 15.5 16
V
OFF Threshold 14 14.6 15
Output Overvoltage Protection
(LM3500-21)
ON Threshold 20 20.5 21
OFF Threshold 19 19.5 20
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LM3500
Symbol Parameter Conditions Min
(Note 7)
Typ
(Note 8)
Max
(Note 7) Units
IVout VOUT Bias Current
(LM3500-16)
VOUT = 15V, SHDN = VIN 260 400
µA
VOUT Bias Current
(LM3500-21)
VOUT = 20V, SHDN = VIN 300 460
IVL PMOS Switch Leakage
Current (LM3500-16)
VOUT = 15V, VSW = 0V 0.01 3
µA
PMOS Switch Leakage
Current (LM3500-21)
VOUT = 20V, VSW = 0V 0.01 3
SHDN
Threshold
SHDN Low 0.65 0.3 V
SHDN High 1.1 0.65
Note 1: Absolute maximum ratings are limits beyond which damage to the device may occur. Operating Ratings are conditions for which the device is intended
to be functional, but device parameter specifications may not be guaranteed. For guaranteed specifications and test conditions, see the Electrical Characteristics.
Note 2: This condition applies if VIN < VOUT. If VIN > VOUT, a voltage greater than VIN + 0.3V should not be applied to the VOUT or VSW pins.
Note 3: For more detailed soldering information and specifications, please refer to National Semiconductor Application Note 1112: Micro SMD Wafer Level Chip
Scale Package (AN-1112), available at www.national.com.
Note 4: The human body model is a 100 pF capacitor discharged through a 1.5 kΩ resistor into each pin. The machine model is a 200 pF capacitor discharged
directly into each pin.
Note 5: In applications where high power dissipation and/or poor package thermal resistance is present, the maximum ambient temperature may have to be
derated. Maximum ambient temperature (TA-MAX) is dependent on the maximum operating junction temperature (TJ-MAX-OP = 125ºC), the maximum power
dissipation of the device in the application (PD-MAX), and the junction-to ambient thermal resistance of the part/package in the application (θJA), as given by the
following equation: TA-MAX = TJ-MAX-OP – (θJA × PD-MAX).
Note 6: Junction-to-ambient thermal resistance (θJA) is highly application and board-layout dependent. The 75ºC/W figure provided was measured on a 4-layer
test board conforming to JEDEC standards. In applications where high maximum power dissipation exists, special care must be paid to thermal dissipation issues
when designing the board layout.
Note 7: All limits guaranteed at room temperature (standard typeface) and at temperature extremes (bold typeface). All room temperature limits are production
tested, guaranteed through statistical analysis or guaranteed by design. All limits at temperature extremes are guaranteed via correlation using standard Statistical
Quality Control (SQC) methods. All limits are used to calculate Average Outgoing Quality Level (AOQL).
Note 8: Typical numbers are at 25°C and represent the most likely norm.
Note 9: Feedback current flows out of the pin.
Note 10: Current flows into the pin.
Typical Performance Characteristics
Switching Quiescent Current vs VIN
20065755
Non-Switching Quiescent Current vs VIN
20065756
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LM3500
2 LED Efficiency vs LED Current
L = Coilcraft DT1608C-223,
Efficiency = 100*(PIN/(2VLED*ILED))
20065757
2 LED Efficiency vs LED Current
L = TDK VLP4612T-220MR34,
Efficiency = 100*(PIN/(2VLED*ILED))
20065779
3 LED Efficiency vs LED Current
L = Coilcraft DT1608C-223,
Efficiency = 100*(PIN/(3VLED*ILED))
20065758
3 LED Efficiency vs LED Current
L = TDK VLP4612T-220MR34,
Efficiency = 100*(PIN/(3VLED*ILED))
20065780
4 LED Efficiency vs LED Current
L = Coilcraft DT1608C-223,
Efficiency = 100*(PIN/(4VLED*ILED))
20065759
4 LED Efficiency vs LED Current
L = TDK VLP4612T-220MR34,
Efficiency = 100*(PIN/(4VLED*ILED))
20065781
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LM3500
2 LED Efficiency vs VIN
L = Coilcraft DT1608C-223,
Efficiency = 100*(PIN/(2VLED*ILED))
20065769
3 LED Efficiency vs VIN
L = Coilcraft DT1608C-223,
Efficiency = 100*(PIN/(3VLED*ILED))
20065770
4 LED Efficiency vs VIN
L = Coilcraft DT1608C-223,
Efficiency = 100*(PIN/(4VLED*ILED))
20065773
SHDN Pin Current vs SHDN Pin Voltage
20065761
Output Power vs VIN: LM3500-16
(L = Coilcraft DT1608C-223)
20065784
Output Power vs Temperature: LM3500-16
(L = Coilcraft DT1608C-223)
20065785
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LM3500
Switch Current Limit vs VIN: LM3500-16
20065762
Switch Current Limit vs Temperature
LM3500-16, VOUT=8V
20065763
Switch Current Limit vs Temperature
LM3500-16, VOUT=12V
20065776
Switch Current Limit vs VIN: LM3500-21
20065791
Switch Current Limit vs Temperature
LM3500-21, VOUT=8V
20065792
Switch Current Limit vs Temperature
LM3500-21, VOUT=12V
20065793
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LM3500
Switch Current Limit vs Temperature
LM3500-21, VOUT=18V
20065794
Oscillator Frequency vs VIN
20065764
VOUT DC Bias vs VOUT Voltage: LM3500-16
20065765
VOUT DC Bias vs VOUT Voltage: LM3500-21
20065795
FB Voltage vs Temperature
20065766
FB Voltage vs VIN
20065767
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LM3500
NMOS RDSON vs VIN
(ISW = 300mA)
20065774
PMOS RDSON vs Temperature
20065775
Typical VIN Ripple
20065768
LM3500-16, 3 LEDs, RLED = 22Ω, VIN = 3.0V
1) SW, 10V/div, DC
3) IL, 100mA/div, DC
4) VIN, 100mV/div, AC
T = 250ns/div
Start-Up: LM3500-16
20065771
3 LEDs, RLED = 22Ω, VIN = 3.0V
1) SHDN, 1V/div, DC
2) IL, 100mA/div, DC
3) ILED, 20mA/div, DC
T = 100µs/div
Start-Up: LM3500-21
20065796
3 LEDs, RLED = 22Ω, VIN = 3.0V
1) SHDN, 1V/div, DC
4) IL, 100mA/div, DC
2) VOUT, 10/div, DC
T = 200µs/div
VCONT = 2.7V
SHDN Pin Duty Cycle Control Waveforms
20065772
LM3500-16, 3 LEDs, RLED = 22Ω, VIN = 3.0V, SHDN frequency = 200Hz
1) SHDN, 1V/div, DC
2) IL, 100mA/div, DC
3) ILED, 20mA/div, DC
4) VOUT, 10V/div, DC
T = 1ms/div
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LM3500
Typical VOUT Ripple, OVP Functioning: LM3500-16
20065782
VOUT open circuit and equals approximately 15V DC, VIN = 3.0V
3) VOUT, 200mV/div, AC
T = 1ms/div
Typical VOUT Ripple, OVP Functioning: LM3500-21
20065797
VOUT open circuit and equals approximately 20V DC, VIN = 3.0V
1) VOUT, 200mV/div, AC
T = 400µs/div
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LM3500
Operation
20065704
FIGURE 1. LM3500 Block Diagram
The LM3500 utilizes a synchronous Current Mode PWM con-
trol scheme to regulate the feedback voltage over almost all
load conditions. The DC/DC controller acts as a controlled
current source ideal for white LED applications. The LM3500
is internally compensated preventing the use of any external
compensation components providing a compact overall solu-
tion. The operation can best be understood referring to the
block diagram in Figure 1. At the start of each cycle, the os-
cillator sets the driver logic and turns on the NMOS power
device conducting current through the inductor and turns off
the PMOS power device isolating the output from the VSW pin.
The LED current is supplied by the output capacitor when the
NMOS power device is active. During this cycle, the output
voltage of the EAMP controls the current through the inductor.
This voltage will increase for larger loads and decrease for
smaller loads limiting the peak current in the inductor mini-
mizing EMI radiation. The EAMP voltage is compared with a
voltage ramp and the sensed switch voltage. Once this volt-
age reaches the EAMP output voltage, the PWM COMP will
then reset the logic turning off the NMOS power device and
turning on the PMOS power device. The inductor current then
flows through the PMOS power device to the white LED load
and output capacitor. The inductor current recharges the out-
put capacitor and supplies the current for the white LED
branches. The oscillator then sets the driver logic again re-
peating the process. The Duty Limit Comp is always opera-
tional preventing the NMOS power switch from being on more
than one cycle and conducting large amounts of current.
The LM3500 has dedicated protection circuitry active during
normal operation to protect the IC and the external compo-
nents. The Thermal Shutdown circuitry turns off both the
NMOS and PMOS power devices when the die temperature
reaches excessive levels. The LM3500 has a UVP Comp that
disables both the NMOS and PMOS power devices when
battery voltages are too low preventing an on state of the
power devices which could conduct large amounts of current.
The OVP Comp prevents the output voltage from increasing
beyond 15.5V(LM3500-16) and 20.5V(LM3500-21) when the
primary white LED network is removed or if there is an LED
failure, allowing the use of small (16V for LM3500-16 and 25V
for LM3500-21) ceramic capacitors at the output. This com-
parator has hysteresis that will regulate the output voltage
between 15.5V and 14.6V typically for the LM3500-16, and
between 20.5V and 19.5V for the LM3500-21. The LM3500
features a shutdown mode that reduces the supply current to
0.1uA and isolates the input and output of the converter.
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LM3500
Application Information
ADJUSTING LED CURRENT
The White LED current is set using the following equation:
The LED current can be controlled using a PWM signal on the
SHDN pin with frequencies in the range of 100Hz (greater
than visible frequency spectrum) to 1kHz. For controlling LED
currents down to the µA levels, it is best to use a PWM signal
frequency between 200-500Hz. The LM3500 LED current can
be controlled with PWM signal frequencies above 1kHz but
the controllable current decreases with higher frequency. The
maximum LED current would be achieved using the equation
above with 100% duty cycle, ie. the SHDN pin always high.
LED-DRIVE CAPABILITY
The maximum number of LEDs that can be driven by the
LM3500 is limited by the output voltage capability of the
LM3500. When using the LM3500 in the typical application
configuration, with LEDs stacked in series between the
VOUT and FB pins, the maximum number of LEDs that can be
placed in series (NMAX) is dependent on the maximum LED
forward voltage (VF-MAX), the voltage of the LM3500 feedback
pin (VFB-MAX = 0.53V), and the minimum output over-voltage
protection level of the chosen LM3500 option (LM3500-16:
OVPMIN = 15V; LM3500-21: OVPMIN = 20V). For the circuit to
function properly, the following inequality must be met:
(NMAX × VF-MAX) + 0.53V ≤ OVPMIN
When inserting a value for maximim LED VF, LED forward
voltage variation over the operating temperature range
should be considered. The table below provides maximum
LED voltage numbers for the LM3500-16 and LM3500-21 in
the typical application circuit configuration (with 3, 4, 5, 6, or
7 LEDs placed in series between the VOUT and FB pins).
# of LEDs
(in series)
Maximum LED VF
LM3500-16 LM3500-21
3 4.82V 6.49V
4 3.61V 4.86V
5 2.89V 3.89V
6 X 3.24V
7 X 2.78V
For the LM3500 to operate properly, the output voltage must
be kept above the input voltage during operation. For most
applications, this requires a minimum of 2 LEDs (total of 6V
or more) between the FB and VOUT pins.
OUTPUT OVERVOLTAGE PROTECTION
The LM3500 contains dedicated circuitry for monitoring the
output voltage. In the event that the primary LED network is
disconnected from the LM3500-16, the output voltage will in-
crease and be limited to 15.5V (typ.). There is a 900mV
hysteresis associated with this circuitry which will cause the
output to fluctuate between 15.5V and 14.6V (typ.) if the pri-
mary network is disconnected. In the event that the network
is reconnected regulation will begin at the appropriate output
voltage. The 15.5V limit allows the use of 16V 1µF ceramic
output capacitors creating an overall small solution for white
LED applications.
In the event that the primary LED network is disconnected
from the LM3500-21, the output voltage will increase and be
limited to 20.5V (typ.). There is a 1V hysteresis associated
with this circuitry which will cause the output to fluctuate be-
tween 20.5V and 19.5V (typ.) if the primary network is dis-
connected. In the event that the network is reconnected
regulation will begin at the appropriate output voltage. The
20.5V limit allows the use of 25V 1µF ceramic output capac-
itors.
RELIABILITY AND THERMAL SHUTDOWN
The maximum continuous pin current for the 8 pin thin micro
SMD package is 535mA. When driving the device near its
power output limits the VSW pin can see a higher DC current
than 535mA (see INDUCTOR SELECTION section for aver-
age switch current). To preserve the long term reliability of the
device the average switch current should not exceed 535mA.
The LM3500 has an internal thermal shutdown function to
protect the die from excessive temperatures. The thermal
shutdown trip point is typically 150°C. There is a hysteresis of
typically 35°C so the die temperature must decrease to ap-
proximately 115°C before the LM3500 will return to normal
operation.
INDUCTOR SELECTION
The inductor used with the LM3500 must have a saturation
current greater than the cycle by cycle peak inductor current
(see Typical Peak Inductor Currents table below). Choosing
inductors with low DCR decreases power losses and increas-
es efficiency.
The minimum inductor value required for the LM3500-16 can
be calculated using the following equation:
The minimum inductor value required for the LM3500-21 can
be calculated using the following equation:
For both equations above, L is in µH, VIN is the input supply
of the chip in Volts, RDSON is the ON resistance of the NMOS
power switch found in the Typical Performance Characteris-
tics section in ohms and D is the duty cycle of the switching
regulator. The above equation is only valid for D greater than
or equal to 0.5. For applications where the minimum duty cy-
cle is less than 0.5, a 22µH inductor is the typical recommen-
dation for use with most applications. Bench-level verification
of circuit performance is required in these special cases, how-
ever. The duty cycle, D, is given by the following equation:
where VOUT is the voltage at pin C1.
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LM3500
Typical Peak Inductor Currents (mA)
VIN
(V)
# LEDs
(in
series)
LED Current
15
mA
20
mA
30
mA
40
mA
50
mA
60
mA
2.7 2 82 100 134 160 204 234
3 118 138 190 244 294 352
4 142 174 244 322 X X
5 191 232 319 413 X X
3.3 2 76 90 116 136 172 198
3 110 126 168 210 250 290
4 132 158 212 270 320 X
5 183 216 288 365 446 X
4.2 2 64 76 96 116 142 162
3 102 116 148 180 210 246
4 122 146 186 232 272 318
5 179 206 263 324 388 456
CIN = COUT = 1 µF
L = 22 µH, 160 mΩ DCR max. Coilcraft DT1608C-223
2 and 3 LED applications: LM3500-16 or LM3500-21; LED VF = 3.77V at
20mA; TA = 25°C
4 LED applications: LM3500-16 or LM3500-21; LED VF = 3.41V at 20mA;
TA = 25°C
5 LED applications: LM3500-21 only; LED VF = 3.28V at 20mA; TA = 25°C
The typical cycle-by-cycle peak inductor current can be cal-
culated from the following equation:
where IOUT is the total load current, FSW is the switching fre-
quency, L is the inductance and η is the converter efficiency
of the total driven load. A good typical number to use for η is
0.8. The value of η can vary with load and duty cycle. The
average inductor current, which is also the average VSW pin
current, is given by the following equation:
The maximum output current capability of the LM3500 can be
estimated with the following equation:
where ICL is the current limit. Some recommended inductors
include but are not limited to:
Coilcraft DT1608C series
Coilcraft DO1608C series
TDK VLP4612 series
TDK VLP5610 series
TDK VLF4012A series
CAPACITOR SELECTION
Choose low ESR ceramic capacitors for the output to mini-
mize output voltage ripple. Multilayer X7R or X5R type ce-
ramic capacitors are the best choice. For most applications,
a 1µF ceramic output capacitor is sufficient.
Local bypassing for the input is needed on the LM3500. Mul-
tilayer X7R or X5R ceramic capacitors with low ESR are a
good choice for this as well. A 1µF ceramic capacitor is suf-
ficient for most applications. However, for some applications
at least a 4.7µF ceramic capacitor may be required for proper
startup of the LM3500. Using capacitors with low ESR de-
creases input voltage ripple. For additional bypassing, a
100nF ceramic capacitor can be used to shunt high frequency
ripple on the input. Some recommended capacitors include
but are not limited to:
TDK C2012X7R1C105K
Taiyo-Yuden EMK212BJ105 G
LAYOUT CONSIDERATIONS
The input bypass capacitor CIN, as shown in Figure 1, must
be placed close to the device and connect between the VIN
and GND pins. This will reduce copper trace resistance which
effects the input voltage ripple of the IC. For additional input
voltage filtering, a 100nF bypass capacitor can be placed in
parallel with CIN to shunt any high frequency noise to ground.
The output capacitor, COUT, should also be placed close to
the LM3500 and connected directly between the VOUT and
GND pins. Any copper trace connections for the COUT capac-
itor can increase the series resistance, which directly effects
output voltage ripple and efficiency. The current setting re-
sistor, RLED, should be kept close to the FB pin to minimize
copper trace connections that can inject noise into the sys-
tem. The ground connection for the current setting resistor
should connect directly to the GND pin. The AGND pin should
connect directly to the GND pin. Not connecting the AGND
pin directly, as close to the chip as possible, may affect the
performance of the LM3500 and limit its current driving capa-
bility. Trace connections made to the inductor should be
minimized to reduce power dissipation, EMI radiation and in-
crease overall efficiency. It is good practice to keep the VSW
routing away from sensitive pins such as the FB pin. Failure
to do so may inject noise into the FB pin and affect the regu-
lation of the device. See Figure 2 and Figure 3 for an example
of a good layout as used for the LM3500 evaluation board.
www.national.com 14
LM3500
20065777
FIGURE 2. Evaluation Board Layout (2X Magnification)
Top Layer
20065778
FIGURE 3. Evaluation Board Layout (2X Magnification)
Bottom Layer (as viewed from the top)
15 www.national.com
LM3500
20065709
FIGURE 4. 2 White LED Application
20065754
FIGURE 5. Multiple 2 LED String Application
www.national.com 16
LM3500
20065783
FIGURE 6. Multiple 3 LED String Application
20065790
FIGURE 7. LM3500-21 5 LED Application
17 www.national.com
LM3500
www.national.com 18
LM3500
Physical Dimensions inches (millimeters) unless otherwise noted
8-Bump Micro SMD Package (TL)
For Ordering, Refer to Ordering Information Table
NS Package Number TLA08SSA
X1 = 1.92mm (±0.03mm), X2 = 1.92mm (±0.03mm), X3 = 0.6mm (±0.075mm)
19 www.national.com
LM3500
Notes
LM3500 Synchronous Step-up DC/DC Converter for White LED Applications
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