TPS552892EVM: power on in FPWM

Hi,

We set Rfsw= 49.9kohm and FPWM mode but frequecy only 195khz, as datasheet caculation it shoud be 400khz, may I know any possible to let frequency going wrong?

Vin =12V Vout = 12V

CH1: SW1

CH2: SW2

CH3: Vout

Thanks!

Hi Jeff,

Thank you for reaching out. I will look into this later today.

Regards,

Mulin

Hi Mulin,

4. Adjust the input voltage to enter Buck Mode or Boost Mode, and the frequency will revert back to the set 400kHz. However, the conversion voltages for different loads will vary.

   4.1 What are the conversion conditions for the three different modes?

   4.2 Why is there a frequency division design in Buck-Boost Mode?

5. After the output voltage is established, it takes 5 cycles to transition from PFM to FPWM.

   5.1 Under no load conditions, since PFM does not lock the minimum frequency, is there a chance that the operating frequency could fall into the audio range during the 5 cycles?

   5.2 Similarly, under no load conditions, referring to the power-on waveform previously provided by Jack, Vout experiences an overshoot issue, where it is set at 12V but spikes up to 13.46V. What methods can be used to improve this?

Thanks!

Jeff

Hi Jeff,

4.1 Do you mean output voltage will change with different input voltage? The voltage shouldn't change unless current limit is triggered. Can you describe Vin Vout Iout spec for different modes?

4.2 Because without this function, frequency will be doubled, switching loss will increase and the efficiency will drop in buck-boost mode.

5.1 It is possible. But the start up process only have 4ms, does this matter to your application?

5.2 You can consider it similar to transient overshoot condition. By adding output caps and optimize compensation parameters(increase cross frequency) can help.

Regards,

Mulin

Hi Mulin,

4.1 Do you mean output voltage will change with different input voltage? The voltage shouldn't change unless current limit is triggered. Can you describe Vin Vout Iout spec for different modes?

=> I mean when input voltage change, due to output voltage is fixed, what is the change threshold between the Boost and buck mode.

5.1 It is possible. But the start up process only have 4ms, does this matter to your application?

=> We concern about the about the maximum 13.46V may cause output abonrmal since it have (13.46-12)/ 12 = 12% error.

Seems in FPWM mode also have this 12% error on your EVM?

5.2 You can consider it similar to transient overshoot condition. By adding output caps and optimize compensation parameters(increase cross frequency) can help.

=> What is the suggest 1) adding output caps value 2) optimize compensation value in Vin11~13V to Vout 12V?

Thanks!

Jeff

Hi Jeff,

4.1 There is some criteria for mode transition. The threshold is not fixed with different output voltage, switching frequency.

5.1 Do you modify the EVM? Normally the error will not be that large. 5% is a reasonable range.

5.2 It depends on your current BOM, including inductor, output cap PN, load current. You can slightly add some ceramic capacitors or change e-cap to a larger one by real test.

Regards,

Mulin

Hi Mulin,

Your waveform is Vout =5V, Would you please try change vout =12V and in FPWM mode? Maybe can see the same voltage diff in start up.

I'm check with customer other BOM they chaned. thanks!

Jeff

Hi Mulin,

Seem becasue customer remove the output capacitor C19, the spkie seems acceptable by customer, I will close this thread. thanks!

Jeff