Re: PWMing of White LED LCD back light
- From: Mochuelo <b@xxx>
- Date: Sat, 23 Jul 2005 17:43:39 +0200
On Sat, 23 Jul 2005 15:42:36 +0200, Sylvain Munaut <com.246tNt@tnt>
wrote:
>> First, you don't need the NPN. If your MCU is fed also with 5 V, the
>> PWM output can directly command the PMOS. You don't even need a
>> resistor. You might want to include it, in series with the gate of the
>> PMOS, if you were concerned about radiation (EMI). The PMOS must have
>> a gate threshold voltage lower than 5 V (as it was also in your
>> circuit). Just take into account that the LEDs will be on while your
>> MCU output is 0. No problem at all. However, if you still want to
>> complement it, most MCUs with PWM allow you to invert the output
>> inside the MCU itself.
>
>No, the MCU is a Au1100 (32bit MIPS) and has 3.3V IO and when in sleep
>or early boot phase it's output might be Tristated or low and the
>backlight must be off.
Ok, if the MCU IO is 3.3 V and the backlight input is 0 to 5 V, it
makes sense using an extra transistor. However, if you have access to
the positive and the negative node of the backlight subcircuit (i.e.,
if it is not internally grounded), I would use a single NMOS (with a
gate threshold voltage lower than 3.3 V) and a high-ohmic resistor
from gate to GND. Source to GND; drain to backlight negative;
backlight positive to 5 V. The resistor would guarantee that the gate
is low when your MCU is in sleep mode (regardless on whether the PWM
output is high-z or low).
.
- References:
- PWMing of White LED LCD back light
- From: Sylvain Munaut
- Re: PWMing of White LED LCD back light
- From: Mochuelo
- Re: PWMing of White LED LCD back light
- From: Sylvain Munaut
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