LM3430_07 NSC [National Semiconductor], LM3430_07 Datasheet - Page 15

• Manufacturer Part Number: LM3430_07
• Description: LED Backlighting Solution with LM3430 and LM3432
• Manufacturer: NSC [National Semiconductor]

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
REFERENCE DOCUMENTS
Document Number/Type
Datasheet
Datasheet
AN-1529
15
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 kΩ, the gain setting resistor, R19
used is 9.09 kΩ. 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
LM3432 6-Channel Current Regulator for LED Backlight
LM3430 Boost Controller for LED Backlighting
LM3430 Evaluation Board
Application
Title
www.national.com