LM3434_1 Datasheet, PDF (1/8 Pages) National Semiconductor (TI) – Evaluation Board adaptive constant on-time DC/DC buck

LM3434 20A Evaluation
Board
National Semiconductor
Application Note 2041
Clinton Jensen
May 3, 2010
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
w.r.t. 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.
LM3434 Board Description
The evaluation board is designed to provide a constant cur-
rent 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 pro-
vides a UVLO function to remove the possibility of the LM3434
from drawing high currents at low input voltages during start-
up. Initially the output current is set at the minimum of ap-
proximately 4A with the POT P1 fully counter-clockwise. To
set the desired current level a short may be connected be-
tween LED+ and LED-, then use a current probe and turn the
POT clockwise until the desired current is reached. The cur-
rent may be adjusted with P1 up to 18A. 20A output may be
acheived either by bypassing P1 and applying an analog volt-
age 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.
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 volt-
age to the ADJ pin between 0.3V and 1.5V w.r.t. GND to
adjust the voltage across the sense resistor (R_SENSE) R15.
The shunt may also be removed and an external voltage pos-
itive w.r.t. 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_SENSE vs. V_ADJ
graph in the Typical Performance Characteristics section the
current can be set using the following equation:
I_LED = V_SENSE/R_SENSE
Alternatively the shunt can be removed and the ADJ test point
can be connected to the VINX test point to fix V_SENSE at 60mV.
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 eval-
uation 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 (recom-
mended). If the FETs cannot be placed directly next to the
LED then a snubber across the FETs may be required to pro-
tect the FETs and the LM3434 from v=Ldi/dt voltage tran-
sients 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 w.r.t. 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 dim-
ming is included in the Typical Performance Characteristics
section showing 30kHz dimming at 50% duty cycle.
High Current Operation and
Component Lifetime
When driving high current LEDs, particularly when PWM dim-
ming, component lifetime may become a factor. In these
cases the input ripple current that the input capacitors are re-
quired to withstand can become large. At lower currents long
life ceramic capacitors may be able to handle this ripple cur-
rent without a problem. At higher currents more input capac-
itance 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 oper-
ational 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 con-
sideration for selecting the input capacitors is the RMS ripple
current they will be required to handle. This current is given
by the following equation:
The parallel combination of the ceramic and aluminum elec-
trolytic 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 temper-
ature of the capacitor is 5°C above the ambient board tem-
perature 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 ca-
pacitor lifetime are the rated lifetime and the ambient operat-
ing 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
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