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Powering up TPS65070 with huge capacitors on SYS

Vsevolod Anisimov
Prodigy 240 points
Other Parts Discussed in Thread: TPS65070

Greetings,

I have a device with big external load (valve and compressor), when they start working the consumption increases in 10(!!) times. So i need a huge capacitors (2 x 470 uF) on SYS output of TPS65070 to prevent in from unstability. But i have problems with startup.

I am not new with TPS65070, it is my 3rd device based on this chip. But two previous devices didn`t need so big capacitors, i used only 2 x 22 uF on SYS pin.

The problem is when i start the device (from VBAT pins, PB_IN always tied low) it does not start every time. I looked deeper and the only difference with previous devices is more slowly increase of SYS voltage (nearby 50 ms from 0 to 1.8V, then vefy fast to 4 V). I bet it is because of charging big capacitors.

So question finally is does power on sequence depend of SYS increase speed? When i power on device with not fully discharged big capacitors (so SYS increase speed becomes smaller), device turn on without problems.

Best wishes,

Vsevolod Anisimov 

  • 0
    TI__Expert 6060 points

    Hello Anisimov,

    The slow increase of the SYS voltage in the range 0V < VSYS < 1.8V is the expected behavior of the device.

    There is a soft start function inplemented on the SYS output.

    In case the device is only powered from VBAT :

    The device is turned on by pulling /PB_IN low. SYS voltage will ramp up with soft start (to avoid high inrush current from the battery) untill the VSYS reached 1.8V. In this phase SYS voltage ramps up with a constant current of 30mA. SYS voltage should not be loaded with any external load during softstart. If there is an external load >= 30mA SYS voltage can not ramp up properly ==> device gets stuck in soft start phase.

    See page 25 (SYS output) of the datasheet for further information.

    Make sure any externnal load is disconneced / disabled until VSYS reached 1.8V.

     

    In case the device is powered from AC/ USB the soft start current is limited by an internal resistor of typ.500Ohms.

    The soft start current in the range 0V < VSYS < 1.8V can be calculated V(AC, USB) / 500Ohms (typ.).

    The tolerance of the internal softstart resistor is in the range +/- 20%