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bq24075: how to prevent short circuit protection action in transient state

Tito Massimo Genitti
Prodigy 20 points
Other Parts Discussed in Thread: BQ24075

Hello,

in my project I use a regulated +5V power source for powering a GPRS module and some others sensor IC.

I use a bq24075 for battery management with a single cell 1200 mAh Li-Ion battery. It works perfectly when there is the main +5V generated from a Buck (12V -> 5V). The battery is charged without any issue.

When I disconnect the main power source the battery start to provide current via power path and a Boost IC provide again the +5V power. All works again.

I get some troubles only when I connect the battery directly to the device with main power disconnected and SYSOFF signal pulled to GND. In this case the battery alone has to supply via power path the Boost DC-DC converter to generate +5V. When I connect the battery I see Boost conv. not working properly.

I found the problem: the short circuit circuitry. The short circuit protection start to work as soon as I plug the battery. I think that high current (large capacitor) in the transient state make to occur the protection system. To verify this behaviour I put a 0.2 Ohm resistor before the load and all start to work fine again. 

My question is: can I prevent the short circuit protection action without adding a resistor? Is there any lack after bq24075 OUT pin ? Do I need some soft start IC  ?

Thank you for your help,

Best Regards,

Tito

 

From datasheet, this is the case in which I fall:

INPUT SOURCE NOT CONNECTED

When no source is connected to the IN input, OUT is powered strictly from the battery. During this mode the

current into OUT is not regulated, similar to Battery Supplement Mode, however the short circuit circuitry is

active. If the OUT voltage falls below the BAT voltage by 250mV for longer than tDGL(SC2), OUT is turned off. The

short circuit recovery timer then starts counting. After tREC(SC2), OUT turns on and attempts to restart. If the short

circuit remains, OUT is turned off and the counter restarts. This ON/OFF cycle continues until the overload

condition is removed.

  • 0
    TI__Mastermind 31120 points

    Not sure how to get around the short circuit without doing something like you tried....adding the resistor in series.

    There is a short deglitch before the short circuit kicks in so maybe a schottky dioded betweent the battery and the OUT pin would shunt some of the intial current into the output and the pulse duration above 4A would be shorter than 250us.  This is only suggested if you never want to use the SYSOFF pin, since this would bypass the BAT FET.

    The IC after opening the FET due to a short, will close it again after a delay so if the transmitting can be help off until after the OUT capacitance has been charged may solve the issue.

     

    May want to try the bq2403x family.  It has a delay on the Short Circuit Protection by putting a capacitor on the DPPM pin, thus allowing inrush current.

  • 0 in reply to Charles Mauney
    Prodigy 20 points

    Dear Charles,

    thank you for your help. I can't use your suggested solution because I selected bq24075 really for SYSOFF function that is very important in my application.

    I'm going to try to insert a resistor after OUT pin in parallel with a open FET. When the system starts correctly I will close the FET by micro. In this way the resistor is bypassed during regular working operations and the power consumption is not affected by resistor.

    What do you think about this solution ? 

    Thank you in advance,

    Best regards

    Tito