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TPS65217: TPS65217 MUX_OUT voltage out of range (>1.8V) when measuring battery charge current

Part Number: TPS65217
Other Parts Discussed in Thread: AM3352

Hello,

When charging a LI-PO battery with a custom tps65217 + am3352 board, I'm measuring high voltages (~2V) at MUX_OUT output when the analog multiplexer is configured to MUX[2:0]= 101 (VICH, Voltage proportional to charge current) and ICHRG[1:0] is set to 10 (500mA).

According to the datasheet the voltage should be arounf 4.5 / 0.5 = 2.25V, so this reading makes sense somehow.

But the TPS65217 is designed to work specifically with the am335x processor series and the typical application schematics within the tps65217 datasheet shows MUX_OUT directly connected to the CPU AIN4 pin which can only handle voltages up to 1.8V.

Is there anything I did wrong or is this a design flaw?

  • Bastian,

    In my experience, you would only be interested in measuring the charging current if it is a Low, dynamic value.

    When the voltage of the battery is between VLOWV = 2.9V and VOREG = 4.1V, there can only be one value for the charging current because the charger is in constant current mode, ICHG = 500mA (by default when ICHG[1:0] = 10b)

    -In most cases, you can simply set MUX_OUT to BAT, or MUX[2:0] = 001

    It is only important to determine the charging current when VBAT < VLOWV or when VBAT > VOREG, and it is most critical when VBAT > VOREG because a low voltage will always use the IPRECHG value (50mA by default).

    -This is a rare case when you may want to set MUX_OUT to VICHARGE, or MUX[2:0] = 100

    If for some reason you need to be sure that 500mA is being delivered to the battery, you will need to implement a MUX_OUT scaling circuit, shown on page 11 of the User's Guide titled "Powering the AM335x with the TPS65217x"

    Note: there is no Gain (or voltage scaling) on the MUX_IN to MUX_OUT path either, but MUX_IN is rated up to 3.3V

    If MUX_IN could reach 3.3V, this is another reason the MUX_OUT scaling circuit would be needed.