LM3432BSQ National Semiconductor, LM3432BSQ Datasheet - Page 14

• Manufacturer Part Number: LM3432BSQ
• Description: LM3432/LM3432B 6-Channel Current Regulator for LED Backlight Application; Package: LLP; No of Pins: 24; Qty per Container: 1000; Container: Reel
• Manufacturer: National Semiconductor

www.national.com
Selecting the VIN Capacitor, C
The purpose of this capacitor is to supply transient current to
the device and suppress VIN noise in order to ensure proper
operation. A low ESR ceramic capacitor with good high fre-
quency performance is recommended. The capacitance can
range from 0.1 µF to 1 µF.
Analog Dimming Control
If analog dimming control is required, a capacitor, C
should be connected from the MODE pin to ground instead of
shorting the MODE pin to ground. The relationship between
the PWM dimming frequency and the capacitance of C
is illustrated below.
When V
100%
The LED dimming ratio is calculated from the ratio of minimum
ON duty factor to the maximum ON duty factor. With the
LM3432/LM3432B, the LEDs can be fully turned on up to
100% ON duty factor and the minimum ON duty factor is lim-
ited by the phase delay time, 0.5 µs. The dimming ratio can
be estimated as below.
As an example, if the PWM dimming frequency is set to 500
Hz, the best achievable dimming ratio is:
PWM
≤
1V, D
ON
= 0% and when V
VIN
PWM
≥
3V, D
MODE
MODE
ON
=
14
Driving High Current LEDs
The LM3432/LM3432B can support string currents from 15
mA to 40 mA. If the application needs to drive high current
LEDs that require more than 40 mA per string, the LM3432/
LM3432B provides the alternative of connecting several IOUT
ports together to achieve higher output current per string. With
this approach, there is the obvious trade off between higher
output current and number of strings driven. Two possible
configurations are illustrated in Figure 5.