Other Parts Discussed in Thread: BQ24702Part Number: BQ24702
Question type: Development and troubleshooting
Description of the problem: We have used the bq24702 Battery Charger for the latest project, and we've found some discrepancies between datasheet and application note, as well as build of the EVK, and to top it off, strange behaviour of the charge circuitry as well as AC/DC to BAT change over. We've been trying the last couple of days to understand this chip, and the associated problems, but cannot resolve the thermal runaway, and seeing inconsistent operation when switching the digital control inputs...
I've attached the circuit, and this was copied as much as possible from the datasheet example. Please note that we've corrected the ACP and ACN connections to the shunt as per R13 and R15 on datasheet, as well as added D7 as per EVK.
We're seeing the following symptoms:
1) The device is working well under 18.5V, but around 18.8V (when charging supposed to kick in), the device heats up excessively. We cannot find any reason for this thermal runaway, and to avoid blown chip, we discontinue the operation above this voltage.
2) The VHSP voltage does not behave as per the datasheet, since we're measuring on both sides of Zener 15V +/- 0.5V. This does not make any sense - Could be due to damaged IC from thermal problems?
3) We cannot get any PWM signal of 300kHz for the BAT charging circuit immaterial of the modes on the ACSET and ENABLE. This may be due to non-functional front end of the AC monitoring, but we cannot be sure here? We briefly spotted a 100mV noise on this pin, but we had to reduce the charging voltage otherwise device would have blown up due to the excessive current draw and heat dissipation.
Now, as per the EVK guide, additional diode D7 was added across MOSFET U4 for the battery charger, probably to 'unload' U4 body diode. The body diode is also used on the U2 for powering the bq24702 when no AC/DC power available - The datasheet and user guide do not mention this though explicitly, but as VCC for the bq24702 must come from somewhere, that is the only logical place.
On the datasheet, R19 = R21 for the SRP / SRN inputs, but on the EVK, R21 is not listed on the assembly drawing, and is different package (0805 vs 2512) compared to R19.
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