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TPS7A90: Output voltage drops after start up

Kazuki Itoh
Expert 7835 points
Part Number: TPS7A90

Hello team,

My customer uses TPS7A90 for 3.3Vin / 1.8Vout.

My customer sees the output voltage drops after the start-up, and then gradually goes up to the target voltage very slowly in 200ms.

Please find the internal link here for the waveform and schematic. (Please connect to VPN for download).

Could you please help to resolve this issue?

Regards,

Itoh

  • 0
    TI__Expert 7140 points

    Hello Itoh, 

    Would you happen to have a scope shot of Vin? There could be a chance that there is a voltage drop across the inductor during inrush current. 

    Alternatively, Do you have a plot of Iout? The device can be going into current limit. 

    Best,

    Juliette

  • 0 in reply to Juliette
    TI__Expert 7835 points

    Hello Juliette-san,

    Please find the internal link here for VIN/VOUT waveforms. (Please connect to VPN for download)

    Due to the PCB layout, it is difficult to observe the inrush current.

    Will reduce Cout then check if  there is issues comes from inrush current.

    Regards,

    Itoh

  • 0 in reply to Kazuki Itoh
    TI__Expert 7140 points

    Hi Itoh, 

    I am not sure if it would be an inrush current issue.

    When the Output voltage goes down, does the Vin also go down?

    Also what load are you using for the device?

    Best,

    Juliette

  • 0 in reply to Juliette
    TI__Expert 7835 points

    Hello Juliette-san,

    No load in the test condition. It has just 10uF cap, feedback resistors, and PU resistor for PG output at the output.

    Please find the internal link here for schematic and waveforms. (Please connect to VPN for download)

    VIN doesn't drop when Vout drops. Now, the issue is that the VOUT selling takes several 100ms.

    My customer expects to settle quickly like the waveform in the datasheet.

    Regards,

    Itoh

  • 0 in reply to Kazuki Itoh
    TI__Expert 7140 points

    Hello Itoh, 

    Given the screenshots that you have provided, I am led to believe that this issue may be caused by the internal current source. 

    When the device starts up, the NR cap is being charged by the current source. Once the Output reaches 95% of its nominal value, the internal current source turns off, and the NR cap charges through the resistor.

    A smaller NR cap should decrease the time at which the final 5% of the Cap charges up with. 

    Best,

    Juliette

  • 0 in reply to Juliette
    TI__Expert 7835 points

    Hello Juliette-san,

    My customer tried to change the NR cap from 1uF to 0.1 uF, but it still takes 50ms to settle from 95%. Please check the document I shared with you again.

     Also, the datasheet Fig. 35 shows very quick start up and the scopeshot doesn't show the slowing down from 95% output.

    My customer would like to obtain the similar quick start up as Fig. 35.

    Could you please comment?

    Regards,

     Itoh

  • +1 in reply to Kazuki Itoh
    TI__Expert 7140 points

    Hi Itoh, 

    In the screenshots you showed, it seems like lowering the capacitor value to 0.1uF reduced the settling time to 50ms; whereas, it was greater than 200ms when the CNR capacitor was at 1uF. 

    This delay, as stated before, the Fast Charge circuitry disconnects when VFB is 97% of VREF (mistakenly wrote 95%). The charging current will drop from 100uA to 6.2uA. The 6.2uA current is not necessarily the charging current of the circuit, instead the internal resistor becomes and RC filter with CNR. [RC Charging Filter]

    1Tau is the time necessary to get us to the final 3% of VREF.

    tau = R*C

    280k * 1uF = 280ms in addition to the initial 7.76ms rise to 97% = 287.76ms

    280k * 0.1uF = 28ms in addition to the initial 776us rise to 97%

    As for Figure 35:

    This device is only going up to 0.8V [compared to your 1.8V] so the 3% value needed to be achieved by the RC charging circuit is lower than the 3% value for 1.8V. Furthermore, this graph does not seem to be centered at 0V, meaning that 0.8V is also not being achieved. 

    please try lowering the capacitor further. 

    Best,

    Juliette