I have designed the following circuit based around the LTC4010 battery charger IC.
This circuit is essentially a direct copy of an example circuit from the datasheet.
The target battery is an 18V 1.5Ah 15 cell NiCd cordless drill battery.
I've built it and it's exhibiting strange behaviour that I've been unable to debug, explain or fix.
When a 24V DC source and battery are connected to their respective terminals, the following sequence repeats:
- ready and charge LEDs illuminate for ~2ms
- fault LED illuminates for ~2.5us
Occasionally the ready/charge LEDs will remain on for 2-4 seconds before the fault LED comes on. From this I believe the IC attempts to start charging, detects a fault, stops and restarts.
My two main observations are:
The datasheet says that any fault state should latch until power is removed, unless the fault occurred due to an over-temperature condition. I don't believe this to be the case. I am using a 10K resistor to GND in place of a thermistor, as per the datasheet. There is a 68n filter capacitor in parallel that is not on the above schematic. There is a steady 1.41V on the VTEMP pin. The IC itself is not warm or hot when touched.
The IC does not appear to be producing any switching waveform on BGATE (for the N-channel FET). I believe this to be because the charger is operating in soft-start mode until it restarts.
Everything else looks sensible, as far as I can ascertain.
- TGATE is producing a 550kHz PWM waveform to drive Q1
- Vcell is 1.28V, which is the correct per-cell voltage and within the IC's requirements.
- I have replaced the IC and both FETs. At no point did the behaviour change.
- I'm not a professional PCB designer, but I believe the layout to be sensible in terms of component placement, routing, track widths etc. I can post additional pictures if it would help.
I am at a loss to explain the behaviour that I am seeing. Why is the IC going into fault and resetting?
I know that "please debug my entire circuit for me" questions are slightly frowned upon. I hope I've done enough debugging of my own to help.