LP8869C-Q1: SWITCHING NOISE IN INPUT WITH RESPECT BRIGHTNESS PWM FREQUENCY & DUTY CYLE

Hi Surya,

What's the PWM frequency you are using now? note that >20KHz is beyond audible noise.

BR, Gracie

There are 3 ways to mitigate the noise:

1. increase the capacitor value to reduce the voltage ripple on the Boost Vout;

2. Make sure the loop gain is stable;

3. Increase PWM frequency >20KHz. 3 is the most direct way.

BR, Gracie

Thanks for the response.

The switching noise is on the INPUT of the LP8869C-Q1, And PWM input is of 210Hz ( and test over the range from 100Hz to 1.5KHz)

Hi Surya,

Vin is quite strange: seems not from the battery but from some DC-DC. Could you pls increase Cin or improve front power quality?

PWM frequency could be improved.

BR, Gracie

Hello Grace,

Yes. But, we have been testing this by isolating other sections from this block input. Still the issue is there.
And we tested by supplying the input with Battery also. 

Tested with increased Cin capacitor also (i.e., 47uF+330uF), and the switching is approximately 220mVpp.

And we observed that, the noise Vpp increasing with the PWM frequency proportionally. (tested over100Hz to 1.5KHz)  


Note: 
with the TPS61194-Q1EVM ( LP8869C-Q1 assembled on it ) also observed the similar switching there on EVM input, Approximately 700mVpp.


Regards,
Surya

Hi Surya,

Could you pls raise the frequency as I suggested? I could probably test this for you tmr.

BR, Gracie

Hi Grace,

Please check the below waveform of the PGND switching with respect to GND.

Will you please check the effect of this on VIN switching.

Surya.