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Original question:

BQ24070EVM: Overriding the short circuit protection by pulsing the DPPM pin?

BQ24070: Introduced a switch between the OUT pins and the load which is time delayed from the startup of VIN; How long is enough?

Jerry Molnar
Prodigy 210 points
Part Number: BQ24070

Hi Garrett,


One more question!
For the case of:
- no battery present.
- there is a load connected to the OUT pins.
- the device is powered off.
- 10nF cap on DPPM

As we've discussed, when the 5V is turned on to VIN, the BQ24070 will think there is a short circuit at the load and enter the "10ma" mode and not provide any load current.
To smooth the operation of our implemenation, I've introduced a series pass transistor between the BQ24070 and the load.
In the situation as per above, the delay circuit is "triggered" by the assertion the PG pin.

Two questions:
[1] I see that in this configuration (No battery), PG is asserted when VIN reaches ~3V. Is that what you'd expect. Do you think that threshold will have a large tolerance?

[2] The main question is that the delay from PG being asserted to the load switch being turned on is approx 50ms.
Is that sufficient time for the chip to settle and not be bothered by a load being present? (i.e not enter the 10ma short circuit mode)
(I can easily make it longer if needed, just trying to get a feel for how long the chip needs).

In my worst case for VIN rise time, it takes about 5ms for VIN to get to 3V and another 20ms to get to 5V.
My typical case is that it takes about 1ms for VIN to get to 5V.

Thanks once again and Best Regards
Jerry

  • 0
    TI__Mastermind 31510 points

    Hi Jerry, 

    Glad we were able to resolve your last question. Please see my comments below in regards to your 2 new questions. 

    1) PG LED turns on (i.e. PG pin drives low) at approx. 2.55V in my lab testing with no battery attached. The BQ24070 datasheet defines PG turn on as when input voltage is above battery voltage. Since no battery is present in this case PG led is turning on when Vin increases above the V(UVLO) threshold defined on page 8 of the datasheet. The V(UVLO) specification also lists the tolerance as 2.45V to 2.65V. 

    2) A 50ms delay from PG assertion to the load being connected to the BQ24070 output should be more than sufficient. Below waveform is taken on the BQ24070EVM. Out pin voltage settles is less time than 50ms. Even with your worst case VIN rise time it appears to me that 50ms delay to load connect should still be sufficient.

    Best Regards,

    Garrett 

  • 0 in reply to Garrett Kreger
    Prodigy 210 points

    Hi Garrett,

    Thanks for your response!
    Just to confirm, perhaps a different way to ask my question is whether all the necessary signals have settled internally to the chip so it won't trigger the short circuit detection when the load is connected.
    Is the output voltage settling a proxy for saying that yes, any (in spec) load can be connected after the output voltage has settled?

    Thanks in advance!
    Jerry Molnar

  • 0 in reply to Jerry Molnar
    TI__Mastermind 31510 points

    Hi Jerry, 

    The output voltage settling is a proxy for saying yes the load can be connected. With no battery connected Out pin voltage just needs to rise above 1V before the load is connected to avoid the short circuit condition. 

    For my tests with the BQ24070EVM I was turning on my load immediately after turning on my power supply without issue. 

    Best Regards,

    Garrett 

  • 0 in reply to Garrett Kreger
    Prodigy 210 points

    Hi Garrett,

    Great! Thank you very much once again!

    All the best
    Jerry