TPS54388-Q1 - the reristor of RT/CLK Pin

Dice-K

Hello,
I have a question about TPS54388-Q1.

I use two TPS54388-Q1 in parallel.
I use them in external clock synchronization.
When I do not use an external clock, I use them at fsw=1.6MHz.
(When we use a single TPS54388-Q1 at fsw=1.6MHz, we use Rt=105kOhm. By equation(8).)

Please teach me the correct method of Rt setting.

Regards,
Daisuke

over 8 years ago

0 JohnTucker over 8 years ago

TI__Guru**** 191445 points

You should probably not tie the two RT/CLK pins directly together. Each should have its own RT clock pin resistor and be driven by an independent buffered clock source.

0 Dice-K over 8 years ago in reply to JohnTucker

Mastermind 6330 points

Hello John,
Thank you for your reply.

Which does your answer mean? (Circuit-A or Circuit-B)
Our customer would like to use in Circuit-A.

Regards,
Daisuke

0 JohnTucker over 8 years ago in reply to Dice-K

TI__Guru**** 191445 points

I think that implementation A is asking for trouble. Circuit B should work. I think you do not even need the 1k ohm resistor.

0 Dice-K over 8 years ago in reply to JohnTucker

Mastermind 6330 points

Hello John,
Thank you for your reply.

Please teach me the reason why parallel connection such as Circuit-A is a problem.

Best regards,
Daisuke

0 JohnTucker over 8 years ago in reply to Dice-K

TI__Guru**** 191445 points

The TPS54388-Q1 sets the clock frequency by applying a regulated voltage of 500 mV at the RT/CLK pin. a resistor to GND sets the RT/CLK pin current which determines the internal switching frequency. Connecting two pins together means they may share current. The frequency set equations will no longer work. Additionally, the RT/CLK pin is extremely sensitive to noise. You should place the RT resistor as close to the IC as possible and minimize any trace length to it.

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TPS54388-Q1 - the reristor of RT/CLK Pin