Selection of Gain Setting Resistor for Dynamic

Headroom Control

By connecting the VDHC pin of LM3432 through a gain setting

resistor to the VDHC pin of the LM3430, the LM3432's DHC

function will regulate the LED rail voltage from the boost con-

verter to a level just enough to keep all LED strings current in

regulation. This gain setting resistor, R19 determines the

maximum depth of the rail voltage can be lowered. The rule

of thumb on the selection of this resistor is about three times

of the resistance of the bottom resistor in the LM3430 output

voltage feedback resistor divider. That approximately allows

the rail voltage lowered by one-quarter of the preset boost

converter output voltage. In this example, the bottom feed-

back resistor, R9 is 3.01 k

Ω, the gain setting resistor, R19

used is 9.09 k

Ω. The small capacitor, C8 connected to the

VDHC output pin is for noise filtering, a 100pF capacitor is

good enough.

The capacitor C35 connected to VDHC pin controls the DHC

function respond time, the value needed depends on the

PWM dimming frequency. In our demonstration board, the

PWM frequency is about 20 kHz, a 100 nF capacitor is used.

The DHC function must be kept slow enough to avoid false

triggering of the protection logics and fast enough to respond

to high PWM dimming frequency with narrow ON pulse width.

Bench side testing is required to determine the optimum value

for specific requirements

REFERENCE DOCUMENTS

Document Number/Type

Title

Datasheet

LM3430 Boost Controller for LED Backlighting

Datasheet

LM3432 6-Channel Current Regulator for LED Backlight

Application

AN-1529

LM3430 Evaluation Board

15

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