This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

TPS7A20: TPS7A2055 low output voltage

Part Number: TPS7A20

Hi,

i have trouble working with the TPS7A2055PDBVR LDR regulator. I want to use this chip to generate a 5.5V output voltage from a 6 V source.

In my design i have an output cap with 22uF and an input cap >1 uF.

The load is connected with a PMOS. If to mosfet is open, the output voltage is around 5.3V with an input voltage ov 6.00V. This is not as expected but could work.

As soon as i connect the load (max 50mA), the voltage drops to below 4 V.

The EN-Pin is connected to the input voltage.

Can you explain this behavior?

Moritz Speidel

  • Hi Moritz, 

    Is it possible to capture a scope shot for the device Vin, Vout, when it starts up without any load, and right after connecting the 50mA load? Also, is it possible to remove the PMOS and test it by applying a resistive load, or simply applying the load directly at Vout? 

    Best, 

    Edgar Acosta

  • Hi Edgar,

    I took some screenshots from my scope, sadly it has no internal USB :(.

    Startup

    Startup with no Load                                                 Connecting Load (removing PMOS-Voltage)

    VIn shows no issues with voltage drops either when initially connecting the load or while load transients.

    I tried to connect a 34 Ohm resistor in parallel to the pmos-path (with pmos non conducting). Vout then dropped to 2.4V.

    Is the device perhaps broken?

    Best,

    Moritz

  • Hi Moritz, 

    Thank you for sharing the scope shots. 

    For the following scope shot: 

     This is as soon as you connect the load, correct? Is Vbus still Vin or is this only for the PMOS? Also, where is the Vout being measured? is it right across the LDO or is it down the line? 

    If Vbus is Vin, it seems that once it gets loaded, Vin collapses. 

    What are you using for the 15V rail and what is the current limit on this 15V? 

    I would try doing the following: 

    Isolate the L78M device from TPS7A20 by bypassing the L78M. This can be achieved by not turning on the L78M device and apply an external 6V to the TPS7A20 and run the tests again. 

    It would be odd that the device is damaged as it does turn on normally. If for any reason this is still a possibility, I would recommend changing the TPS7A20 with a new one and rerun the tests. 

    Lastly, can you also share the schematic with the load? 

    Best, 

    Edgar Acosta

  • Hi Edgar,

    VBus is only the control voltage for the PMOS. I also measured Vin and the 15V-Rail. They are not affected by applying the load.

    The 15V are supplied by a Keitley 230 with 100 mA current limit.

    Vout is measured on the cap directly after the TPS (<1mm distance), so resitive drops should not play a big role.

    If i supply the 6V rail directly, the same thing happens, if i connect the load.

    There is nothing connected to any connector except the +15V and GND.

    Best, Moritz

  • Hi Moritz, 

    I've been going through the Raspberry Pi Pico Data Sheet, and they show the same configuration when the USB is not being used. With this said, then Vsys could be powered with 1.8V to 5.5V, which is the rail generated by the TPS7A20: 

    It does seem odd that the upper power stream does not collapse. 

    Have you tried replacing the LDO? 

    Lastly, is there a way to do something like this: 

    Remove the PMOS and/or isolate the LDO from the MCU to prevent any damage, and load the LDO with a 110 Ohm resistor to get 50mA? 

    Lastly, is there a way to increase the current limit of the 15V supply. I want to rule out inrush current, although I do not believe this is the case since no other rail collapses.

    Best, 

    Edgar Acosta

  • Hi Edgar,

    i only had the time to try supplying 3-5V directly via the PMOS. Everything worked fine. I will try to use a psu with higer current rating as 100 mA is the most mine can do. Also i am going to replace the chip with a new one.

    As soon as i have new info, i will get back.

    Thanks,

    Moritz

  • Hi Moritz, 

    Thank you for the patience and sharing the information. 

    From the Scope shots you shared it almost seems that the TPS7A20 enters current limit and or that it got damaged via reverse current. I also read from the Raspberry Pi DS that it is recommended to use a Schottky Diode in the same direction as the body diode of the PMOS. 

    The same methodology can be applied to the LDO to protect it from any possible reverse current scenario. 

    Let me know once a new LDO has been installed. I highly recommend testing it separately from the Raspberry pi and capturing Startup and Shutdown waveforms and try to load it as previously described to discard any possible damage before interacting with the MCU. 

    Best, 

    Edgar Acosta

  • Hi Edgar,

    i replaced the LDO and added a Schottky Diode and now everything works!

    It seems as the old one was broken. Thanks a lot for your support.

    Have a nice Christmas-time!

    Best,

    Moritz