NCS29001

http://onsemi.com

10

SOFT START WITH PWM INPUT

Figure 7 below shows an example of a soft start when the device is powered up from standby with a PWM input. The PWM

signal here is at 100 Hz with a duty cycle of 30%. In this case the LED reaches 100% of its programmed value in 100 ms. This

time can be decreased if the PWM signal runs at a higher duty cycle.

100ms with 30% dimming duty cycle,

100Hz

Vfbp

Vfbn & LED

current

LED current reaches 100%

when Vfbn crosses Vfbp

Soft start with PWM dimming active

Vcomp is kept during

dimming off

PWMin and PWMout

Back light LED brightness gradually

rise to the set value

Figure 7. Soft Start with PWM Input

GATE AND PWMOUT PIN DRIVER CIRCUIT

Since external transistors are required for the boost converter and PWM dimming functions, the device contains an internal

10 V regulator to drive the gate of these transistors. In the case of the PWM transistor this also functions as a level translator

for the PWMin input pin. When selecting external components it is important that the transistor has enough gate drive to ensure

voltage is 8.5 V the gate drivers will be limited to around 7.7 V.