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bq77908a dominant signal, CHGST or Shutdown

Konstantin Nickel
Prodigy 90 points
Other Parts Discussed in Thread: BQ77908A

Hi community,

I am using a bq77908a and want to drive the shutdown by pulling the TS signal down to ground and having ZEDE high at the same time (1 sec shutdown). Now what I don't understand is what will happen if the CHGST signal would be still high while the shutdown pulldown is given (excluding the cell undervoltage case). Will the device keep itself in active mode or go to sleep? Which of these signals is the dominant one?

  • 0
    TI__Guru 54715 points

    CHGST will win forcing the part on.

    ZEDE is a logic input, having it high with the part shut down may back feed into the regulator applying voltages to things, but it should not wake up if CHGST is low.

    If you have CHGST high and ZEDE high while TS is shorted low, the part will stay on and you can read the status registers to see the shorted thermistor status bit.

  • 0 in reply to WM5295
    Prodigy 60 points

    Hello community,

    because i read about CHGST pin, maybe someone has the same problem:

    i use a BQ77908A for battery protection. It are LiFePo4 batteries with 60 Ah.

    I used parallel MOSFET mode (nearly the same schematic like Page 30, Figure 18 in SLUU854 PDF). Plus battery pack connected with plus charger,

    and separate minus for CHG and DSG. CHGST is pulled down with R33 1k and D4. I use a shunt R50 1mOhm  like the schematic. For charger short circuit

    i choosed -10mV  to test this function. If i increase charge current slowly higher than 10A (-10mV at 1 mOhm), the charge FET is turning on and off nearly

    according to the choosen SCCD of 960µs. I thought if FET is switched off, the BQ77908 goes in shutdown or fault mode, but it oszillates.

    When i make the same with CHGST pulled up to VBAT with 15kOhms, the BQ77908 enters to fault mode.

    Does it have effects if CHGST is always pulled up, or how i can reach the fault mode at SCCD with not pulled up CHGST?

    Thanks in advance

    Roland Stark

  • 0 in reply to Roland Stark1
    TI__Guru 54715 points

    Hi Roland,

    The bq77908A & '910A were designed to have CHGST signal the presence of a charger.  CHGST wakes the part, is used for configuration, controls some balance modes and controls some fault recovery as well as any other description in the datasheet. Table 2 on page 20 summarizes fault detection and recovery.

    Recovery from SCC is one condition which does not have options in configuration. To prevent SCC from automatically recovering, you will need to have CHGST high when it goes into the fault and not removed until you want recovery.

    CHGST is conceptually easy to provide with a terminal on the pack and a resistor or resistor divider to PACK+ in the charger.  If you don't have this you must provide some method to manipulate CHGST as needed and accept the resulting performance limits. There is a 3 transistor latching circuit which normally keeps CHGST high until CHG goes low.   It has performance limits also. 

    As you ask, other community users may have good suggesitons.