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Original question:

BQ25071: TS input oscillating and disabling charger

BQ25071: LDO poor choice for TS voltage divider

user4839477
Intellectual 265 points
Part Number: BQ25071

Related to an earlier inquiry regarding the oscillation seen in the BQ25071, I have finally identified the issue.

When supplying the BQ25071 with output from a solar harvester, the load introduced when the BQ25071 turns on (VIN rises to 3.5 or above) results in the MPP reducing the current to maintain the voltage on the harvester output.  Nonetheless, the LDO, specified to maintain 4.9 volts, is unable to maintain 4.9 volts when the voltage input drops.  This results in an oscillation at the LDO and hence the TS divider.  When the TS voltage falls out of range the device turns off.  This results in oscillating load to the harvester.

This seems to occur when the battery voltage (Lithium iron phosphate) is below 2.9 volts.  Everything works well above this voltage. Cannot understand this battery voltage impact on the charging circuit other than increased charging current at low voltage and hence, larger load for the harvester.  I have disabled the Enable Iset function by leaving this open circuit high impedance.  Is the USB500 standard 500 milliamps of input current?

Questions:

a) Is there an easy way of disabling the TS monitoring.  I have followed the datasheet with 490/100 kohms for the divider; this does not work.

b)  Can I use a stable voltage source elsewhere on the board and divider to bring 1 volt to TS (12.5-25 % of LDO) OR !!! is the threshold 12.5-25% of the instantaneous LDO voltage?

c) The TS threshold is 25milliseconds out of range.  Can a large capacitor across the divider smooth the on-off waveform to lengthen the time in range until battery voltage rises?

d) If larger resistors are chosen for the TS divider might this, in combination with a parallel capacitor, keep the TS voltage in range?

Thanks for your assistance.

Jim

  • 0
    TI__Mastermind 23400 points

    Hi Jim,

    user4839477 said:
    Is the USB500 standard 500 milliamps of input current?

    Yes, USB500 refers to an input current limit of 500mA.

    user4839477 said:
    Is there an easy way of disabling the TS monitoring.  I have followed the datasheet with 490/100 kohms for the divider; this does not work.

    If the LDO and TS voltage oscillation is still occurring, this may be the result. The TS monitoring is intended to work with a stable LDO voltage output. 

    user4839477 said:
    The TS threshold is 25milliseconds out of range.  Can a large capacitor across the divider smooth the on-off waveform to lengthen the time in range until battery voltage rises?

    This could be a possibility to solve the issue but again, the TS monitoring is based on the LDO output and if the LDO output is not stable, there is no guarantee. We also do not specifying the affects of adding a capacitor on the TS pin. This may be better placed at the LDO output if this has not been tested already.

    user4839477 said:
    If larger resistors are chosen for the TS divider might this, in combination with a parallel capacitor, keep the TS voltage in range?

    I do not believe there would be an issue with the resistors but the capacitor is out of datasheet specifications. Can you try increasing the capacitor at the LDO pin?

    I noticed a similar recommendation in the following thread with us: BQ25071: TS input oscillating and disabling charger