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TPS28225-Q1: Switching doesn't start again if the PWM stops and than starts again

Ovidiu Fechete
Prodigy 70 points
Part Number: TPS28225-Q1


Hi,

I have a possible problem (the behavior is not the desired one for my application) with the restart of the IC. So the IC drives two Mosfets, the drive is as intended, it works perfectly. The issue appears once the PWM goes low. So the IC stops the drive, but when the PWM reappears, the drive doesn't start again. So my intended behavior would be for the IC to restart automatically when the PWM appears again. Can anybody give me any advice? Thank you. 

  • 0
    Prodigy 70 points

    As an add on, I would like to say that if I toggle the EN pin after the process described above, the IC starts driving the fets. 

  • 0
    Prodigy 70 points

    ok, it seems that my bootstrap cap is too large. That solves my problem. 

  • 0 in reply to Ovidiu Fechete
    TI__Prodigy 430 points

    Hi Ovidiu,

    It looks like you were able to debug your system. If you have any more questions regarding this topic, please let me know, I will be happy to help.

    Regards,
    Zihao

  • 0
    Prodigy 70 points

    Actually, I didn't solve it. As a charge pump is connected to the bootstrap pin, I can't change that bootstrap cap too much, so I have to increase the value of the cap back to the original value. So the IC still doesn't start as described earlier.

    1. How can I see if the bootstrap capacitor is correctly calculated? 

    2. Do you have any other hints? 

  • 0 in reply to Ovidiu Fechete
    TI__Prodigy 430 points

    Hi Ovidiu,

    Please see below for my answers. 

    Ovidiu Fechete said:
    1. How can I see if the bootstrap capacitor is correctly calculated? 

    We have a FAQ that walks through correctly calculating the needed bootstrap capacitance. We also have an FAQ that covers biasing methods that includes your charge pump here: FAQ. This application note covers calculations and component selection for a charge pump configuration. 

    Ovidiu Fechete said:
    2. Do you have any other hints? 

    I see that you mentioned the device does not drive until you toggle the Enable pin. Can you confirm that the pin is HIGH while you switch ON the PWM and the IC does not drive? You have a switch and pull-up resistor on the EN pin so I want to make sure this switch is open when the PWM is HIGH so that the Enable is HIGH as well. This is more of a "sanity check" so we can start eliminating any reasons as to why you are seeing issues. 

    You also mentioned a charge pump for the high side for this part. Are you needing the IC's high side output to have a long ON time or low frequency (including DC)? If this is the case, then you can use the low-side's output to generate the high side bias for the charge pump capacitor, and not have a bootstrap circuitry since the bootstrap circuitry will not be adequate. If you do not plan on having long ON time or low frequency (including DC), then you typically don't need the charge pump, you would just need the bootstrap capacitor. This is a much simpler method and also a cost-effective method for biasing the high side. The previously mentioned FAQs have method to calculate the needed bootstrap capacitances, and the application note covers charge pump applications. 

    Please let me know if you have any other questions, I will be happy to help. 

    Regards,

    Zihao