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TPS54331: Tps54331 output 14.3v problem

user6555887
Prodigy 230 points
Part Number: TPS54331
Other Parts Discussed in Thread: TPS62933

Dear sir:

  We use tps54331 to design a circuit with an output of 14.3V

Our input power supply is a 4s2p battery pack with a voltage range of 14.4V ~ 16.8V.

The circuit diagram is as follows:

The problem points are as follows:

1.When the voltage of the battery pack is above 16.0V, the no-load output is stable and can carry load; When the battery voltage is less than 16.0V, the output is sawtooth wave, which can be stable when the load is about 100mA, and the output drops to 0V when the load is more than 100mA

Does this phenomenon mean that VIN Vout is too small, resulting in unstable output.I tried to add a diode between the input voltage and boot. In this way, the input voltage is less than 16.0v, and the output voltage is also stable, but the load cannot be added. Once the load is added, the output is basically reduced to 0V.

I have measured the waveforms of boot and pH as follows:

Vin=15.9V  ch1:PH CH2:BOOT CH3:VOUT  CH4:IOUT

Vin=16.3V  ch1:PH CH2:BOOT CH3:VOUT  CH4:IOUT

Can tps54331 output stably only when the capacitance voltage between boot and pH is greater than 2.1V?

Can tps54331 output stably only when the capacitance voltage between boot and pH is greater than 2.1V? 14.4V ~ 16.8v input cannot output 14.3v? If you want to achieve 14.3v output, are there any improvement measures?

2.How to implement 100% duty cycle?

Thank you very much!

  • 0
    TI__Expert 7520 points

    Hi 

    Here are my comments:

    1. Yes, the voltage of the BOOT cap should be higher than 2.1V (typ). BOOT is used to supply for the driver of the highside MOS. if the voltage of the boot cap is less than the threshold voltage of the highside MOS, the MOS cannot be tuned on. TPS54331 can only support 91% duty cycle.

    2. If you want to implement 100% duty cycle, you can refer to the app below which using an external voltage source and diode to charge the boot cap.

    100%dutycycle.pdf 

    BR

    Ruby

  • 0 in reply to Ruby Li
    Prodigy 230 points

    Hi Ruby:

      The situation of light load is not considered now, because this part of the circuit will always be connected to the load in practical application. Now I have several questions to ask you when testing the connected load.

      The IC input voltage is the battery pack of 4s2p, and the voltage range is 14.4V ~ 16.8v.When the battery voltage drops below 15.5v, the measured waveform is as follows:

      CH1:VIN  CH2:VOUT  CH3:PH  CH4:IOUT(IOUT is the inductance back-end current)

      

       In my opinion, the above waveforms are abnormal, whether pH or IOUT. PH should be a square wave of a specific duty, and IOUT is a continuous and regular sawtooth wave. These two waveform abnormalities represent that the IC is working in an abnormal state at this time, and the IC can not bear such a large load at this time. If it works in this state for a long time, it will damage the IC. Am I right?

       There's another question that's confusing.When the input voltage is 16.4V, the pH waveform is different when the load is 180mA and 465ma. When the load is 180mA, the pH waveform will shake in the blue circle in the picture。

      Is this normal? Why does this happen?

        Load=465mA                                                                                 Load=180mA

      

        Just joined the power supply design, many questions are not very clear, and we look forward to your answers.

       Thank you very much!

       

  • +1 in reply to user6555887
    TI__Expert 7520 points

    Hi 

    For the first question, when Vin is 15.5V, the expected duty cycle should be 92%, but TPS54331 can only support 91% (typ). This means that the boot cap voltage can be dropped under the threshold of the highside MOS, so the IC won't work correctly. If it work at the situation for a long time, it may be damaged. I would recommend you to use TPS62933 instead.

    For the second question, It is normal to have the waveform. When the load is 180mA, you may find from the inductor current waveform that it works into DCM, so there is a ring caused by the parasitic cap and parasitic inductor when the highside and lowside MOSFETS are off.

    Hope it helps.

    BR

    Ruby