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TPS61322: Would not start at 1.1V input voltage when loaded with 50mA

Part Number: TPS61322

Hi,

We intend to use TPS61322 in an single cell NiMH application that is Vin 1 to 1.5V and Vout 2.2V. I did a test board with the device. The load is 50ohm resistor (100//100) i..e 44mA output current. Input and output capacitors are X5R 22uF ceramics  and the inductor is "DFE252012P-4R7M=P2" i.e. the first one suggested in data sheet example. Without the Schottky diode the starting voltage is 1.2V, with the diode (no snubber), it's 1.1V. I was expecting (and need) to start at 0.9V. Any suggestion on how to achieve this? 

Regards

Ramin Fardi

  • Hi Ramin,

    Thanks for your questions and using our device.

    Could you please measure voltage on VOUT pin when device is fails in start-up? Refer to datasheet TPS61322 needs 0.83V at VOUT pin to start-up. If you have 1.2Vin and there is load on VOUT, after the body diode forward drop the voltage on VOUT pin may be below 0.83V.

    -Wenhao

  • Hi Wenhao,

    Thanks for answering quickly,

    Here are som measurement (without the Schottky diode)

    Vin(start)     Vout

    0.9              0.78

    1.0              1.34

    1.1              1.51

    1.2              2.21

    I look at SW pin and can see that the device start switching 1.0V but the switch waveform isn't correct, see the plots below for Vin 1.1V and 1.2V

    Below Switch wave form Vin=1.1V 

    Below Switch wave form Vin=1.2V 

    Regards/Ramin

  • Hi Ramin,

    So did output start up after VOUT reaches to spec on your side? Do you still attach load when start-up?

    Vin(start)     Vout

    0.9              0.78

    1.0              1.34

    1.1              1.51

    1.2              2.21

    -Wenhao

  • No. The output is clearly larger than Vvout_start of 0.87V but the device would not start switching correctly and thus would not reach 2.2V that it is supposed to reach. And yes the load of 44mA is always connected.  For correct operation the device seems to need an start up input of at least 1.2V. But the data sheet states 0.9V as minimum input voltage. So either I am doing something wrong or the data sheet (or my interpretation of it) is wrong. 

    Regards/Ramin   

  • Hi Ramin,

    Thank you for your clarification. I saw where is your problem. I attached the start-up spec below again. As you can see the 0.87V is tested under Rload>250Ohm. Typically for a low input start-up converter the load-into-start up strength is very limited. So please try to remove load and re-test again, to see if TPS61322 starts up correctly on your board. If no, I can try to test on bench on my side. Thanks!

    -Wenhao