LM5085 problem

Kuo Duke

Expert 2040 points

Other Parts Discussed in Thread: LM5085

LM5085 PFET Buck Switching Controller, compared to the NMOS controller, one less BSTRP bootstrap capacitor, but with a higher Rdson, difficult selection PFET, what are the benefit of LM5085 ??

Also shown in FIG function dargam, which will have a regulator, he is used as a push PMOS used for? But PMOS turn on conditions must be VGS <0, then why regulator?

over 9 years ago

0 Vijay86929 over 9 years ago

TI__Genius 11490 points

Hi Kuo,

PFET advantages:
1. less bootstrap capacitor.
2. 100% duty cycle.

0 Vijay86929 over 9 years ago in reply to Vijay86929

TI__Genius 11490 points

The VCC regulator is to create a negative voltage w.r.t. VIN. PFET source is at VIN and its gate voltage must be pulled lower with respect to source to turn it on.
regards

0 Kuo Duke over 9 years ago in reply to Vijay86929

Expert 2040 points

Dear Vijay,

Thanks for your reply, 100% duty for low output drop voltage is good.
Do you have some attachment to determine 100% duty for only PMOS easy implement?

I measurement gate waveform of PFET result show always voltage low level, why is not PWM waveform?

0 Vijay86929 over 9 years ago in reply to Kuo Duke

TI__Genius 11490 points

Actually it is related to the bootstrap capacitor. NMOS needs bootstrap capacitor which generates VIN + VGATE voltage to control the gate of NMOS. To charge the bootstrap capacitor the NMOS needs to turn off every cycle (so switch node can go to ground and VCC can charge CBOOT). This limits the duty cycle to <100%.
Hope that helps.

0 Vijay86929 over 9 years ago in reply to Kuo Duke

TI__Genius 11490 points

The gate will show "always low" if the PMOS is always on. It depends on VIN, VOUT conditions. It will help to look at the screenshots.

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LM5085 problem