TPS55288: Clock synchronisation range

Michael Mätzler

Part Number: TPS55288

Hi all,

I would like to use the clock synchronisation on the TPS55288 however it only works in a very small range.

FSW is connected to ground via 68k resistor. Without synchronisation the measured frequency is ~289kHz. The clock is generated with an waveform generator (4Vpp, 2V Offset, square wave) and connected to the DITH/SYNC pin. The capacitor for frequency dithering is removed. The switching frequency gets synchronised between 281kHz and 291kHz, above or below the synchronisation gets lost and the TPS switches with 289kHz again. However the datasheet specifies the pulling range with +-30%.

Have tried it with 3 different boards and got the same result.

Best regards,

Michael

over 4 years ago

0 Zack Liu over 4 years ago

TI__Mastermind 34231 points

Hi Michael,

Thank you for reaching out. I'll take a bench test and let you know the results by Tuesday.

0 Michael Mätzler over 4 years ago

Intellectual 320 points

Hi Zack,

many thanks in advance. Also tried it with the TPS55288EVM-045 and got same results

+1 Zack Liu over 4 years ago in reply to Michael Mätzler

TI__Mastermind 34231 points

Hi Michael,

What's the working mode, auto PFM mode or forced PWM mode, do you set now?

Please note that TPS55288 external synchronization function only works in forced PWM mode.

0 Michael Mätzler over 4 years ago in reply to Zack Liu

Intellectual 320 points

Hi Zack.

The working mode on the eval and on my own board was PFM at light loads cause I was not aware this affects the clock synchronisation at high loads. Nevertheless it does neither work in forced PWM mode.

0 Zack Liu over 4 years ago in reply to Michael Mätzler

TI__Mastermind 34231 points

Hi Michael,

I just run a bench test with TPS55288EVM. The external clock synchronization function works well. The Fsw pin resistor is 68kohm, the device switching frequency could follow the external clock well in the +-30% range.

Could you share some waveform? Maybe DITH/SYNC pin, SW1 and SW2 pin waveform.

0 Michael Mätzler over 4 years ago in reply to Zack Liu

Intellectual 320 points

There was an wrong i2c command and the controller was not set correcty to forced pwm. Now it is working just fine.

Thank you

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TPS55288: Clock synchronisation range