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BQ77PL900 shutdown mode with unreliable behavior

Stefan Hittinger
Prodigy 40 points

i use shutdown mode by entering host mode and setting shutdown bit in register [State Control]. The voltage at pin PACK falls to 30mV.

Sometimes nothing happens, no shutdown is done, the voltage regulators are active and the discharge fet is on even if i disable it in register [Output Control].

Sometimes the voltage regulators change to 0.76V at the 5V pin and 1V at the 3.3V pin.

The schematic is similar to the demo board, the fets are changed to NP100P06, gate resistors are modified from 1M to 100k.

How can i solve this problem?

  • 0
    TI__Guru 54715 points

    Look for leakage paths on your schematic or circuit.  If it is one part or board perhaps the IC or assembly is damaged.

    Be aware of the input resistance of your measuring instruments.  Many have a 10M ohm input resistance. When probing the typical application circuit with a 1M gate pullup, the voltage divider between the gate resistor and the meter or scope probe to ground is sufficient to turn on the FET and  thus wake up the part.  You may have prevented this in your system by using the 100k resistor, but still if the diode to the BAT pin is biased, the DSG output will follow the BAT pin. 10M to ground with the 100k might add ~0.4V (at 40V) to the gate and possibly put you into the Vgs threshold region of the FET.  Measuring from gate to source of the FETs rather than gate to ground may be a more consistent way to tell if the part has the FETs off.

    You mention pack pin rather than system pack terminal, so you are likely measuring at the device pin which can be different than the system terminal.  You might see if the response is different leaving the measuring instrument connected from the pack pin of the device to ground. 

    Check for reliable communication and if the part is going to sleep and waking up again.  If nothing happens it would seem that the device never got the command or the pack pin never fell to the low voltage.  Monitoring the VREG pins or XRST may provide observation points that don't influence the FETs.  If these show the part begins to shut off and then comes back up, look for a leakage or other wake up trigger event.

    See if there is a significant voltage drop between the FET source and the BAT pin.  The device wants to keep the DSG pin at the BAT pin voltage when it commands the FET off. Current from the regulators will be drawn through the BAT pin circuit and can cause a drop, particularly if you have a heavy load or pulsed load on the regulators which cycles the charge pump on and off.  Note the BAT pin diode and resistor are indicated as optional on the data sheet, consider whether your system needs these to keep it awake during short circuit/heavy load and use a low forward drop diode if so.

    If you are using shutdown in-system, you may have a micro controlled wake up circuit.  Be sure it is staying off.

    Check your I2C interface.  If the pins are driven high or have a small value pullup, this could be the cause of the unexpected regulator voltages. A voltage applied to SCLK or SDATA can back drive the regulators.  If VLOG is selected from VREG2, VREG2 is ~ 0.25V above VREG1 in a quick test of applied voltage on SDATA.

     

     

     

  • 0 in reply to WM5295
    Prodigy 40 points

    Thanks for your detailed answer!

    After removal of the external pullups and setting of the right port options at the microcontroller the BQ changes correctly to shutdown mode.