LM5020: Why does CS pin discharge very slowly?

Marvin Feng

Part Number: LM5020

Hi Team,

Our customer is using LM5020. However, in the actual application, we found CS pin's voltage discharge very slowly. Please refer to the below waveform.

CH1: OUT; CH2: CS.

However, in our datasheet, we can see it has internal FET to ground for discharging.

1) Can you explain how the internal FET works? Why CS pin doesn't discharge the voltage when FET is off?

(As i understand, if CS triggered 0.5V, internal FET is connected to ground for 50ns and then turn off. After that, CS pin's voltage will be consumed by external cap slowly, correct?)

2) How to understand CS sink impedance?

Thanks!

Marvin

over 4 years ago

0 Youhao Xi over 4 years ago

TI__Guru 68495 points

Hi Marvin,

Please email me the schematic.

Thanks,

Youhao Xi, Applications Engineering

0 Marvin Feng over 4 years ago in reply to Youhao Xi

TI__Intellectual 1340 points

Hi Youhao,

Has sent it to you. Please help this.

Thanks!

Marvin

0 Youhao Xi over 4 years ago in reply to Marvin Feng

TI__Guru 68495 points

Hi Marvin,

The internal pull down FET only turns on briefly by the pulse width of CLK plus the leading edge blanking. So, most of the time it is OFF.

The snubber C16-R11 should be placed across the FET drain and GND. Its current can be picked up by the CS pin. If you remove R15, or C11, the CS voltage will mostly stay at GND after the switch is turned off. However, the CS voltage after the FET is off is ignored by the controller and it should not affect the normal operation.

Thanks,

Youhao

0 Marvin Feng over 4 years ago in reply to Youhao Xi

TI__Intellectual 1340 points

Hi Youhao,

Thanks for your clarification.

It should be a typo error. It should be R16 or C11, correct?

Thanks!

Marvin

0 Youhao Xi over 4 years ago in reply to Marvin Feng

TI__Guru 68495 points

Sorry for the typo. I meant either C16 or R11...

0 Youhao Xi over 4 years ago in reply to Youhao Xi

TI__Guru 68495 points

Can I assume the issue is resolved? Let me close this thread here, and you can always re-open it by adding a new post.

Thanks,

Youhao

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LM5020: Why does CS pin discharge very slowly?