bq2002T Fast Charge Management IC: Fast charge times out & goes to the top-off mode in less than one minute (about 55 seconds). If I momentarily disconnect and reconnect the charger input it charges for another 55 seconds and then again goes to top-off. The 55 second time is quite repeatable.
The measured voltage at the TM (timer mode select) pin is 2.41V when the measured supply (Vcc) is 5.09V, so the fast charge should last 320 minutes.
The measured TS (temperature sensor) voltage is 1.68V and does not change significantly, so it is not near the high or low cut-off values. The thermistor circuit is similar to the DV2002 evaluation board and includes a 100Kohm/0.027ufd filter on the TS pin.
The BATT pin measured voltage is typically around 1.3 volts, well short of the 2V cut-off value.
The circuit uses a 2N3904 to control an LM317 regulator (set up as a constant current source) in a circuit similar to the DV2002 board.
I have isolated the charger/battery circuit from all other circuitry on my board, so nothing else is powered, drawing current or generating electrical noise. I am charging three 1.2V NiMH AA cells in series.
I have run this test on two different circuit boards (two different bq2002T ICs) with the same result.
Does anyone have suggestions or have had a similar experience? (I did see one somewhat-similar post already in this forum, although in that post the cut-off time was less than an hour rather than less than a minute.)
I have been closely examining the bq2002 specs (see my posting earlier today describing my problem).
One of the criteria for exiting Fast Charge mode is rate of change of temperature. The bq2002T samples the temperature every 19 seconds, and compares it to the reading three samples earlier. If it has fallen by more than 25.6mV fast charge mode will terminate. Three samples at 19 seconds = 57 seconds. All timings are subject to a significant tolerance, so this could well be the 55 seconds you mention.
I would suspect, therefore, that the voltage on TS drops from its initial value. You say there is an RC filter on TS. Is it possible that there is a volt drop along the PCB track between the C and pin 4 (0V) that could reduce the voltage on TS when current is drawn to charge to battery?
In reply to Alan Rouse1:
Alan, thank you very much!!! It's amazing how obvious something can seem after someone else points it out. As an engineer with almost 50 years experience in analog signal conditioning and temperature measurement (including thermistors at YSI), I should have caught this.
The thermistor is heating up on start-up, not due to self-heat (it is dissipating only about 0.1 milliwatts) but due to poor thermal design. As it turns out, a modification we are already planning should solve the problem.
In our present advanced-prototype design the surface-mount thermistor is on the same board as the charger circuit, and not really close to the batteries. The charge current regulator (LM317T) and two diodes, which collectively dissipate about 1.5 watts, are also surface mount components on the same board, about 1-1/2 inches away from the thermistor. When I fan the board the problem goes away (until I stop fanning).
We're already planning to relocate the thermistor next to the batteries, off the circuit board, and have a thermistor on order. Thank you very much for your help.
Harry Trietley, JH Technology, Inc.
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