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TPS63900: early switch to standby power

Actura inc
Prodigy 30 points
Part Number: TPS63900

We are evaluating a preproduction TPS63900. The schematics are effectively as shown in the datasheet page 1, with both, SEL and EN tied to VIN. What we notice is that if VIN drops below 2.4V the circuit switches into secondary power (standby mode). 2.4V on SEL still is a logic high and we were expecting the circuit to still run in main power mode. Is this a known issue that would be addressed in the production release?

  • 0
    TI__Mastermind 21565 points

    Hi Actura:

    Sorry, that I'm confused by secondary power and main power. Is there different power supply in system? May you share a system diagram? What's the Vout you prefer?

  • 0 in reply to Minqiu Xie
    Prodigy 30 points

    The schematics is as per datasheet on page 1 with the SEL pin tied to VIN. Page 3 of the datasheet says:

    SEL: Output voltage select. Selects V O(2) when a high level is applied to this pin. Selects V O(1)
    when a low level is applied to this pin. It must not be left open.


    what we observe is that if VIN drops below 2.4V the chip switches from V O(2) to V O(1), even though the SEL pin still is at a logic high.

  • 0 in reply to Actura inc
    TI__Mastermind 21565 points

    Hi Actura:

    Got it. I'll check it on EVM later today. 

  • 0 in reply to Minqiu Xie
    TI__Mastermind 21565 points

    HI Actura:

    I checked on EVM. There is no issue as you described. 

    What's your Vout and load condition? And why do you think it's Vout toggled?  

    I afraid it may caused by power supply has limited capability. You can set unlimited input current and do the test without the load. 

  • 0 in reply to Minqiu Xie
    Prodigy 30 points

    We tested again. The load current is 1.4 mA @3.3V via a LED/resistor. VIN is fed from a discharging supercapacitor. The configuration resistors are:

        CFG1    36.5k
        CFG2    510 Ohm
        CFG3    0

    As before, at 2.32 V on VIN we are observing a switch of the output voltage from 3.3V down to 1.91 V. The 1.91 V remains constant until the shutoff at VIN.

    This may just be a single part, if it is a production issue we would need to redesign the power supply however.

  • 0 in reply to Actura inc
    TI__Mastermind 21565 points

    HI Actura:

    That's an interesting phenomenon if you make sure the Vin is stable. 

    May you catch the waveform at Vin, Vout, L1, L2 when Vout drops? And what will happen if you set Vo(1) larger than Vo(2)?

    For easier to debug, you can replace the supercap with a DC power supply, and regulate the Vin smooth.