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bq25504EVM operation under no load to charge battery

Ashim Gurung
Intellectual 330 points

In reference to the user guide of the bq25504EVM, I am quite confused about the EVM operation.

For cold start operation, at least 0.4 V, 170 microAmp, or 50 microWatts input power is required under no load condition. If I want to connect a 3 V battery at the BAT pin and charge with using a dc supply with the above input power, how will the battery get charged under no load condition?

I am confused about about the working relationship between the Vstor and Vbatt junctions.

How does the Vstor come into play? Does the battery charging operation depend on what voltage the battery is currently at? 

Any help will be greatly appreciated.

  • 0
    Intellectual 330 points

    is whatever voltage appears at STOR, the battery gets?

  • 0 in reply to Ashim Gurung
    TI__Guru**** 166106 points

    There is PFET between VSTOR and VBAT with a diode that allows current flow from VBAT to VSTOR all the time.  So VBAT = VSTOR - Vdiode until the PFET closes.  The PFET only closes (allowing current flow in both directions) when VSTOR > VBAT_UV.  The PFET is used to prevent battery overdischarge.

  • 0 in reply to Jeff F
    TI__Guru**** 166106 points

    If you attach a battery to VBAT when both VBAT and VSTOR < 200mV, the PFET will turn on for 50ms, check to see if VSTOR cap charges above VBAT_UV and if so, start charging immediately from the input supply.  If the attached battery is not above VBAT_UV then VSTOR will collapse after the PFET turns on and you will have to wait until the cold start circuit charges VSTOR above VSTOR_CHGEN ~=1.8V.

  • 0 in reply to Jeff F
    Intellectual 330 points

    Thanks Jeff for the reply.

    Q1. You said: If you attach a battery to VBAT when both VBAT and VSTOR < 200mV, the PFET will turn on for 50ms. 

    But didn't you say PFET will only turn on when Vstor>Vbatt_uv ?

    Q2. To charge a battery (initial voltage is > 2.7 V), do I need to first charge the Vstor to higher than Vbatt_uv before connecting the battery? (no load condition, objective: charging the battery)

    Q3. If I connect the battery with Vstor>Vbatt_uv  but less than 2.7 V, the PFET turns on and the battery will be discharged instead of charging to increase the Vstor. Is this right? (no load condition, objective: charging the battery)

    Thanks! 

  • 0 in reply to Ashim Gurung
    TI__Guru**** 166106 points

    Regarding Q1, only at battery attach when VBAT and VSTOR are near 0V will the PFET turn on briefly to provide an initial charge charge onto the VSTOR cap.  If the VSTOR voltage > VBAT_UV then the PFET will stay on.  If not or is the load on VSTOR pulls its voltage below VBAT_UV the PFET will close until the charger raises VSTOR to VBAT_UV.

    Regarding Q2, No.  The battery should be attached the entire time.  VBAT is actually connected to VSTOR through the body diode of the internal PFET.  The purpose of the internal PFET is to prevent battery undervoltage.

    Regarding Q3, when VSTOR >VBAT_UV, the PFET closes so that VBAT=VSTOR.   When there is more than enough input power to provide the system load on VSTOR, the extra energy goes to the battery through the body diode, if the VSTOR<VBAT_UV or through the PFET if VSTOR>VBAT_UV.  If there is not enough input power to power the system load on VSTOR, then the battery will provide the system load until VSTOR <VBAT_UV.