LM74700-Q1: start up time

Dongbao Zhou

Part Number: LM74700-Q1

Hi team,

Is it possible to speed up the power up time by adding a 12V voltage source to VCAP? Main purpose is to decrease the charging pump start up time.

Thanks

Dongbao

over 5 years ago

0 Karikalan Selvaraj over 5 years ago

TI__Genius 16305 points

Hi Dongbao,

VCAP is already referenced to ANODE and it begins with the voltage same as ANODE.

Best method is to use VCAP capacitor that is 10x the input capacitance of MOSFETs used.

Regards,

Kari.

0 Dongbao Zhou over 5 years ago in reply to Karikalan Selvaraj

TI__Expert 4170 points

Hi Kari,

The main problem is Vin ramp up slowly, diode conduction time is too long. Customer want to charge the VCAP before charge pump start to speed up MOSFET conduction.

Dongbao

0 Dongbao Zhou over 5 years ago in reply to Dongbao Zhou

TI__Expert 4170 points

12V is separate power supply from Vbat, it didn't affect by the Vbat.

0 Karikalan Selvaraj over 5 years ago in reply to Dongbao Zhou

TI__Genius 16305 points

Hello Dongbao,

Yes, if the VIN is slowly ramping up, but in the meanwhile VCAP is already charged to 12V, then it will certainly enable the MOSFET to quickly turn on as soon as VIN reaches VIN POR threshold (max 3.9V).

And during the normal operation, diode will prevent VCAP discharge back to 12V, so it is not a concern.

Regards,

Kari.

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LM74700-Q1: start up time