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bq25570: driving enable pins

Danilo Porcarelli
Prodigy 100 points
Other Parts Discussed in Thread: BQ25570

Hello,

I'm using the bq25570 to recharge a small lithium-ion battery exploiting indoor light. Secondarily, the integrated buck converter is used to step-down the battery voltage to 2.2V to supply a MSP430 microcontroller.

Now, the microcontroller should drive the enable pins due to power management constraints. My problem is how to drive these pins maintaining an overall quiescent current lower than 2uA.

The high logic level of the gpio pins of the microcontroller is 2.2V. Otherwise, the minimum voltage required by the bq25570 is VSTORE (or VBAT) for the high level of the enable pins.  At the moment I've tried the following solutions:

_ A pmos to pull-up the enable pin and a nmos to pull-down it. Both transistors are drived by the same gpio of the MCU (they are connected as a cmos inverter). This solution fails because the gpio high level can't switch-off the pmos 

_ Load switch: a pmos is used as pass element between VSTOR and the enable pin, a nmos is used to drive the gate of the pass element. In addition a resistor is used to pull-up the pmos gate when the nmos is off. This solution fails for two reasons: first, the enable pin is never pulled-down. Second when the nmos is ON there is a current flowing through the resistor and the nmos.

Any suggestions?

Thanks to all,

Danilo

  • 0
    TI__Guru**** 166916 points

    I am confused by your operating modes.  The MSP430 will not be operating unless VOUT_EN=high.  So, I would expect VOUT_EN would need to be always be high (tied to VBAT) for the MSP430 to always operate.  If you shut it down, how would you ever turn it back on (unless maybe you are using the 570's VBAT_OK signal?)  To enable and disable the boost charger, can you connect a 10Meg resistor between /EN and VBAT with a NMOS from /EN to ground.  The MSP430 could drive the gate of that FET 

  • 0 in reply to Jeff F
    Prodigy 100 points

    Hi Jeff, thanks for reply

    Jeff F said:

    I am confused by your operating modes.  The MSP430 will not be operating unless VOUT_EN=high.  So, I would expect VOUT_EN would need to be always be high (tied to VBAT) for the MSP430 to always operate.  If you shut it down, how would you ever turn it back on (unless maybe you are using the 570's VBAT_OK signal?)

    Right question! The system operates in duty-cycle mode as follows (this is still an idea, I have to prove it):

    _ The node is normally in LPM3 and it wakes up once a minute

    _ During the activity time it measures temperature and Vcc voltage, blink a led and next it comes back to LPM3

    During the sleep period (LPM3) the power consumption is dominated by the quiescent current of the bq25570 (as the MSP430 in LPM3 is negligible). So, putting a proper large capacitor to the integrated buck output allows to switch-off the buck converter maintaining the msp430 supplied  until the next wake-up event.

    Jeff F said:

    To enable and disable the boost charger, can you connect a 10Meg resistor between /EN and VBAT with a NMOS from /EN to ground.  The MSP430 could drive the gate of that FET 

    Yeah. This sounds good. How did I not think of it before? Do you think it can also work with 15meg or 20meg pull-up resistor?