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TPS62743: Vout regulation at light load

Vasily T
Prodigy 60 points
Part Number: TPS62743
Other Parts Discussed in Thread: TPS62745

Hi!
I use TPS62743 in li-ion battery-powered application. Supply current 12uA in sleep mode and 8mA in active mode. Vout programmed to 3.0V.

Cin 4.7uF Murata GRM188R61E475KE11

Cout 10uF Murata GRM188R61A106KE69

L 2.2uH Murata LQM2MPN2R2MG0

I see that at Iout less than 5 mA device doesn't regulate the output voltage.

Vin=3.8V Vout=3.2V,

Vin=4.0V Vout=3.4V and so on.

I see no pulses on SW pin. I see pulses on SW pin and regulated Vout if input voltage between 3.3 and 3.6V.

If Iout greater than 5mA device works properly.

On screenshots Yellow - Vin, Green - SW

Vin = 3.6V

Vin = 4.0V

  • 0
    TI__Mastermind 25430 points

    Hi Vasily,

    Could you please share the schematic?

    Regards,

    Febin

  • 0 in reply to Febin
    Prodigy 60 points

    Hi Febin

    Simplified schematic

  • 0 in reply to Vasily T
    TI__Mastermind 25430 points

    Hi Vasily,

    The schematic looks fine. It could be a probable mistake in the measurement.

    I just noticed that both the plots use the same time scale. Could you please change the time scale and also adjust trigger accordingly for no load and try again. Even at no load, there will be switching and you should be able to see the waveform.

    Regards,

    Febin

  • 0 in reply to Febin
    Prodigy 60 points

    Febin, thanks for reply.

    There's no mistake in measurements. I measure Vout by DC voltmeter and i have the same result.

    Vin=3.8V Vout=3.2V,

    Vin=4.0V Vout=3.4V

  • 0 in reply to Vasily T
    TI__Mastermind 25430 points

    Hi Vasily,

    You mentioned that you can't see the SW waveform. For this I suggested the above changes.

    Secondly, please provide on the same plot: Vin, Vout, Iout waveforms for 5mA and for a higher load separately. (Please measure across C2 and C3)

    Please ensure you use a stable input supply.

    Regards,

    Febin

  • 0 in reply to Febin
    Prodigy 60 points

    Hi, Febin

    Iout measured on 1 ohm shunt resistor by subtraction scope channels 3 and 4. Shunt resistor connected between L1 and C3.

    Channel 3 is L1 output.

    1. no load

    Vin=3.5V Vout=3V

    Vin=3.8V Vout=3.2V no pulses on SW pin

    2.  load resistor 560 ohms

    Vin=3.5V Vout=3V

    Vin=4.1V Vout=3.15V no pulses on SW pin

    3. load resistor 56 ohms

    Vin=3.6V Vout=3V

    Vin=4.4V Vout=3V device works fine

    4. Increasing Vin manually

    load resistor 560 ohms

    At Vin=3.7V device stops switching and Vout tracking Vin.

  • 0 in reply to Vasily T
    TI__Mastermind 25430 points

    Hi Vasily,

    Thank you for all the information. 

    I need some time to look into this in detail and hopefully will get back to you by Friday.

    Thanks for your understanding.

    Regards,

    Febin

  • 0 in reply to Febin
    TI__Mastermind 25430 points

    Hi Vasily,

    I am sorry for the delayed response. Thanks for your patience!

    So, I tried to repeat your measurements with TPS62745 as we did not have the TPS62743 EVM in our local inventory. Both TPS62743 and TPS62745 have the same die and the internal working remains the same. 

    I could measure SW pulses at no load, 56 Ohm & 560 Ohm Loads. I changed Vin from 3.5V to 4.1V for an output of 3V and the output voltage is regulating at all times. Please find below the measurement plots.

    I would like to give you a few suggestions:

    1. Please try to change the time scale if you do not see the SW pulses.

    2. The device could have been damaged. Please try with a new device.

    3. Please use our EVM instead of the IC on the board.

    4. If you may wish, I can take a look at the detailed layout and schematic to find the root cause.

    Please let me know your feedback. 

    Regards,

    Febin 

    TPS62745.zip

  • 0 in reply to Febin
    TI__Mastermind 25430 points

    Hi Vasily,

    Do you have some updates to share?

    Regards,

    Febin

  • 0 in reply to Febin
    Prodigy 60 points

    Hi, Febin,

    I have 10 boards and all of them have the same issue. I'm planning to do further research of them.

  • 0 in reply to Vasily T
    TI__Mastermind 25430 points

    Hi Vasily,

    Will you be fine to take this thread internal? Then you can take your time to do the research and please send me an email on the progress.

    If it would be fine with you, could you please share the complete schematic and layout through email : febin.hameed@ti.com

    Regards,

    Febin

  • 0 in reply to Febin
    TI__Mastermind 25430 points

    Hi Vasily,

    I have not heard back from you. Do you have some updates to share?

    Regards,

    Febin

  • 0 in reply to Febin
    Prodigy 60 points

    Hi, Febin.

    I'm sorry for the delay.

    I use MAX16020  powered directly from li-ion cell.  MAX16020L have push-pull outputs. I found leakage from LOW_LINE and BATT_OK outputs to power rail through MCUs input pins.

    I should use the open-drain outputs version of MAX16020.

    Thank you for your help.

  • 0 in reply to Vasily T
    TI__Mastermind 25430 points

    Hi Vasily,

    Thanks for confirming!

    Regards,

    Febin