CSD97396Q4M: Power loss at higher output voltages & the bootstrap supply.

Mark Barnes

Part Number: CSD97396Q4M

Question 1:
Looking at the Normalized Power Loss vs Output Voltage chart in the datasheet, it's clear that the part is optimised for operation with an output voltage of 1.8V.
Will it still be functional at an output voltage of 20V? Can I reliably extrapolate the normalised power loss line linearly to a higher voltage?
For example, for 20V output, the normalised power loss would be about 4.75, therefore I wouldn't want to go above about 10A of continuous current in order to not exceed the package maximum power of 8W.(assuming I can heatsink appropriately)
Perhaps I should be looking at different devices? It's simply that the integrated MOSFET driver & matched MOSFETs is really appealing to me as a designer wanting to keep things simple.

Question 2:
Can I safely backfeed an isolated 5V supply into the BOOT pin for the situation where the output very rarely changes? (therefore the bootstrap cap doesn't get charged often enough)

Question 3:
What's the minimum switching frequency and minimum LO time in order to adequately keep the bootstrap capacitor charged?

over 5 years ago

0 Brett Barr1 over 5 years ago

TI__Mastermind 18200 points

Mark,
Thanks for posting. I've reached out to some experts and should have an answer for you shortly.

One question I have is what do you expect your input voltage to be? I think in theory the device can do 20Vout and you can extrapolate the power loss curve linearly, but there may be other complications, because at the max recommended Vin (24V) , you are definitely approaching the max duty cycle of 85%.

Hopefully I can get back to you about all of these questions shortly.

0 Mark Barnes over 5 years ago in reply to Brett Barr1

Prodigy 70 points

Hi Brett,

It's 24V as you say. I'm rapidly coming to the conclusion that I may just have to go for a separate gate driver & mosfet combo, this IC is just a little too niche for a very specific purpose.

Mark

0 Brett Barr1 over 5 years ago in reply to Brett Barr1

TI__Mastermind 18200 points

Mark,
Something else out apps team asked - is this being used for buck?

It should be noted that this power stage was designed for low duty cycle applications, so the power loss efficiency won't be great >50% DC.

0 Mark Barnes over 5 years ago in reply to Brett Barr1

Prodigy 70 points

I was going to use it in a couple of ways, one of them being as a Buck.

From the sounds of it, it's wildly inappropriate for my application so I'll move on to a driver + mosfets design. :)

Thanks for your help.

0 Brett Barr1 over 5 years ago in reply to Mark Barnes

TI__Mastermind 18200 points

That would probably be a better optimized (and more flexible) option.

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CSD97396Q4M: Power loss at higher output voltages & the bootstrap supply.