Texas Instruments Incorporated AN-2041 User's Guide

1 Introduction

The LM3434 is an adaptive constant on-time DC/DC buck constant current controller designed to drive a
high brightness LEDs (HB LED) at high forward currents. It is a true current source that provides a
constant current with constant ripple current regardless of the LED forward voltage drop. The board can
accept an input voltage ranging from -9V to -30V with respect to GND. The output configuration allows the
anodes of multiple LEDs to be tied directly to the ground referenced chassis for maximum heat sink
efficacy when a negative input voltage is used.

2 LM3434 Board Description

The evaluation board is designed to provide a constant current in the range of 4A to 20A. The LM3434
requires two input voltages for operation. A positive voltage with respect to GND is required for the bias
and control circuitry and a negative voltage with respect to GND is required for the main power input. This
allows for the capability of using common anode LEDs so that the anodes can be tied to the ground
referenced chassis. The evaluation board only requires one input voltage of -9V to -30V with respect to
GND. The positive voltage is supplied by the LM5002 circuit. The LM5002 circuit also provides a UVLO
function to remove the possibility of the LM3434 from drawing high currents at low input voltages during
startup. Initially the output current is set at the minimum of approximately 4A with the POT P1 fully
counter-clockwise. To set the desired current level a short may be connected between LED+ and LED-,
then use a current probe and turn the POT clockwise until the desired current is reached. The current may
be adjusted with P1 up to 18A. 20A output may be achieved either by bypassing P1 and applying an
analog voltage directly to ADJ or by adjusting the values of R1 and/or R2 to get higher than 1.5V with P1
fully clockwise. PWM dimming FETs are included on-board for testing when the LED can be connected
directly next to the board. A shutdown test post on J2, ENA, is included so that startup and shutdown
functions can be tested using an external voltage.

User's Guide

SNVA431B–March 2010–Revised May 2013

AN-2041 LM3434 20A Evaluation Board

3 Setting the LED Current

The LM3434 evaluation board is designed so that the LED current can be set in multiple ways. There is a
shunt on J2 initially connecting the ADJ pin to the POT allowing the current to be adjusted using the POT
P1. This POT will apply a voltage to the ADJ pin between 0.3V and 1.5V with respect to GND to adjust the
voltage across the sense resistor (R
positive with respect to GND can then be applied to the ADJ test point on the board. A 5MΩ resistor (two
10MΩ resistors in parallel) comes mounted on the board so using the V
the current can be set using the following equation:

I

= V

LED

SENSE/RSENSE

Alternatively the shunt can be removed and the ADJ test point can be connected to the VINX test point to
fix V

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SENSE

at 60mV.

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) R15. The shunt may also be removed and an external voltage

SENSE

SENSE

vs. V

graph in Section 7

ADJ

(1)

1

Life

ACTUAL

= Life

RATED

X 2

T

CORE

- T

ACTUAL

7

( )

I

RMS

=

V

LED

(|VEE|-V

LED

)

|VEE|

I

LED

PWM Dimming

4 PWM Dimming

The LM3434 is capable if high speed PWM dimming in excess of 40kHz. Dimming is accomplished by
shorting across the LED with a FET(s). Dimming FETs are included on the evaluation board for testing
LEDs placed close to the board. The FETs on the evaluation board should be removed if using dimming
FETs remotely placed close to the LED (recommended). If the FETs cannot be placed directly next to the
LED then a snubber across the FETs may be required to protect the FETs and the LM3434 from v=Ldi/dt
voltage transients induced by the fast current changes in the line inductance leading to the LED. This will
slow the edges and limit PWM dimming capabilities at high frequencies.

To use the dimming function apply square wave to the PWM test point on the board that has a positive
voltage with respect to GND. When this pin is pulled high the dimming FET is enabled and the LED turns
off. When it is pulled low the dimming FET is turned off and the LED turns on. A scope plot of PWM
dimming is included in Section 7 showing 30kHz dimming at 50% duty cycle.

5 High Current Operation and Component Lifetime

When driving high current LEDs, particularly when PWM dimming, component lifetime may become a
factor. In these cases the input ripple current that the input capacitors are required to withstand can
become large. At lower currents long life ceramic capacitors may be able to handle this ripple current
without a problem. At higher currents more input capacitance may be required. To remain cost effective
this may require putting one or more aluminum electrolytic capacitors in parallel with the ceramic input
capacitors. Since the operational lifetime of LEDs is very long (up to 50,000 hours) the longevity of an
aluminum electrolytic capacitor can become the main factor in the overall system lifetime. The first
consideration for selecting the input capacitors is the RMS ripple current they will be required to handle.
This current is given by the following equation:

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The parallel combination of the ceramic and aluminum electrolytic input capacitors must be able to handle
this ripple current. The aluminum electrolytic in particular should be able to handle the ripple current
without a significant rise in core temperature. A good rule of thumb is that if the case temperature of the
capacitor is 5°C above the ambient board temperature then the capacitor is not capable of sustaining the
ripple current for its full rated lifetime and a more robust or lower ESR capacitor should be selected.

The other main considerations for aluminum electrolytic capacitor lifetime are the rated lifetime and the
ambient operating temperature. An aluminum electrolytic capacitor comes with a lifetime rating at a given
core temperature, such as 5000 hours at 105°C. As dictated by physics the capacitor lifetime should
double for each 7°C below this temperature the capacitor operates at and should halve for each 7°C
above this temperature the capacitor operates at. A good quality aluminum electrolytic capacitor will also
have a core temperature of approximately 3°C to 5°C above the ambient temperature at rated RMS
operating current. So as an example, a capacitor rated for 5,000 hours at 105°C that is operating in an
ambient environment of 85°C will have a core temperature of approximately 90°C at full rated RMS
operating current. In this case the expected operating lifetime of the capacitor will be approximately just
over 20,000 hours. The actual lifetime (Life

Where Life

is the rated lifetime at the rated core temperature T

RATED

) can be found using the equation:

ACTUAL

CORE

.

For example: If the ambient temperature is 85°C the core temperature is 85°C + 5°C = 90°C. (105°C ­90°C)/7°C = 2.143. 2^2.413 = 4.417. So the expected lifetime is 5,000 × 4.417 = 22,085 hours. Long life
capacitors are recommended for LED applications and are available with ratings of up to 20,000 hours or
more at 105°C.

(2)

(3)

2

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6 Bill of Materials

ID Part Number Type Size Parameters Qty Vendor

U1 LM3434 LED Driver WQFN-24 1 Texas

U2 LM5002 Boost Regulator SOIC-8 1 Texas

C1 C0805C331J5GACTU Capacitor 0805 330pF, 50V 1 Kemet
C2 GRM31CR60J476KE19L Capacitor 1206 47µF, 6.3V 1 Murata
C3 EKY-350ELL151MHB5D Capacitor MULTICAP 150µF, 35V 1 United Chemi-

C4, C5, C6 GRM32ER6YA106KA12 Capacitor 1210 10µF, 35V 2 Murata

C7 C0805C104J5RACTU Capacitor 0805 0.1µF, 50V 1 Kemet

C8, C13 HMK212BJ103KG-T Capacitor 0805 10nF, 100V 2 Taiyo Yuden

C9 OPEN 0805

C10, C11 GRM21BC81E475MA12 Capacitor 0805 4.7µF, 25V 2 Murata

C12 0805YD105KAT2A Capacitor 0805 1µF, 16V 1 AVX
C14 B37941K9474K60 Capacitor 0805 0.47µF, 16V 1 EPCOS Inc .
C15 GRM21BF51E225ZA01L Capacitor 0805 2.2µF, 25V 1 Murata
C17 OPEN 0805
C18 08055C104JAT2A Capacitor 0805 0.1µF, 50V 1 AVX

D1, D2 MBR0540 Diode SOD-123 40V, 500mA 2 Fairchild

D3 MBRS240LT3 Diode SMB 40V, 2A 1 ON

D4 OPEN SMB

J2 B8B-EH-A(LF)(SN) Connector 1 JST Sales

J1 1761582001 Connector 1 Weidmuller

Jled 87438-0843 Connector 1 Molex

L1 LPS3008-104ML Inductor 3008 100µH, 150mA 1 Coilcraft
L2 SER2915H-103KL Inductor SER2900 10µH, 21.5A 1 Coilcraft

L3, L4, L5, L6 MPZ2012S300A Ferrite Bead 0805 30Ω @ 100MHz 4 TDK

L7 MPZ2012S101A Ferrite Bead 0805 100Ω @ 100MHz 1 TDK
P1 3352T-1-103LF Potentiometer BOURNS2 10kΩ 1 Bourns

Q1, Q2, Q3, Q4, Si7790DP FET PowerPAK 40V, 6mΩ 2 Vishay-Siliconix

Q5, Q6

Q7 MMDT3906-7-F Dual PNP SOT363_N 1 Diodes Inc.
Q8 ZXTN25040DFHTA NPN SOT-23B 1 Zetex
Q9 ZXTP25040DFHTA PNP SOT-23B 1 Zetex
R1 ERJ-6ENF2942V Resistor 0805 29.4kΩ 1 Panasonic
R2 ERJ-6ENF2491V Resistor 0805 2.49kΩ 1 Panasonic

R3, R30, R31 ERJ-6ENF1002V Resistor 0805 10kΩ 3 Panasonic

R4 ERJ-6GEYJ393V Resistor 0805 39kΩ 1 Panasonic
R5 ERJ-6GEYJ101V Resistor 0805 100Ω 1 Panasonic
R7 OPEN

R14 ERJ-6ENF49R9V Resistor 0805 49.9Ω 1 Panasonic

R8 ERJ-6ENF2002V Resistor 0805 20kΩ 1 Panasonic

R10 ERJ-6ENF4991V Resistor 0805 4.99kΩ 1 Panasonic

R11, R12 ERJ-6ENF6192V Resistor 0805 61.9kΩ 2 Panasonic

R13 ERJ-6GEYJ103V Resistor 0805 10kΩ 1 Panasonic

R15a, R15b WSL25125R0100FEA Resistor CR6332-2512 0.01Ω 2 Vishay

Bill of Materials

Table 1. Bill of Materials

Instruments

Instruments

con

Semiconductor

America, Inc.

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