BQ24773: What's the minimum bootstrap cap for BQ24773

Shaquille Chen

Part Number: BQ24773

Hi Team,

What's the minimum bootstrap cap for BQ24773? Customer have EMI issue, they can solve this issue by change bootstrap cap from 47nF to 1000p.

They want to know is this work or not.

Shaq.

over 5 years ago

0 Jeff F over 5 years ago

TI__Guru**** 168566 points

Hi Shaquille,

We have not tested with bootstrap < 47nF and therefore can't recommend lower than that value. 1000pF is definitely too small for optimal operation. Can your customer improve the layout, add a series resistor with the bootstrap capacitor, add a 0.1uF across the buck power FETs to improve EMI?

Regards,

Jeff

0 Brandon Chang over 5 years ago in reply to Jeff F

Prodigy 10 points

Hi Jeff,

What's your suggest value of RC on bootstrap and snubber?

Could provide some combination values for reference?

Thanks!

0 Jeff F over 5 years ago in reply to Brandon Chang

TI__Guru**** 168566 points

Hi Brandon,

We have only tested with 0.047uF BTST capacitor. The BQ24773 high side driver on resistances is 4 ohm. If you add an external resistor up 4 ohm, you should reduce the FET turn on time by 1/2, which will reduce EMI and also reduce efficiency. I would not increase above 6ohm series resistor.

Regards,

Jeff

0 Brandon Chang1 over 5 years ago in reply to Jeff F

Prodigy 10 points

Hi Jeff,

We put 20ohm in series on H/L gate can pass the EMI test. Is it acceptable?

or can we try to add RC in series on BTST?(2.2ohm+0.047u? / 2.2ohm+0.1u?) Any recommended value for test?

Thanks!

0 Jeff F over 5 years ago in reply to Brandon Chang1

TI__Guru**** 168566 points

Hi Brandon,

Either 20 ohm is series with the gate drive or 2.2ohm+0.047uF is acceptable.

Regards,

Jeff

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BQ24773: What's the minimum bootstrap cap for BQ24773