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I have designed the following circuit based around the LTC4010 battery charger IC.

Schematic

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:

  1. 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.

  2. 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.

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  • \$\begingroup\$ Try drawing a circuit that is readable in the picture frame above. \$\endgroup\$ – Andy aka May 5 '16 at 9:03
  • \$\begingroup\$ The physical layout of any switchmode supply is critical; please post a picture (annotated if possible) of the layout. \$\endgroup\$ – Peter Smith May 5 '16 at 11:41
  • \$\begingroup\$ I can see one big difference between your schematic; the low pass noise reduction filter from C3 and R7 cutoff frequency is much lower than the reference design. Although the data sheet states <500Hz, yours is at about 70Hz and the reference design is about 480Hz assuming a large value of R6 . With an on state of about 2msec, this is a bit of a clue. Try replacing C3 with a 33nF or so capacitor. If this works, I will go into what I think is causing the issue. \$\endgroup\$ – Peter Smith May 5 '16 at 13:00
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    \$\begingroup\$ I decided to take a look at this: linear.com/docs/30164 Question 3 is very close to what you are seeing. \$\endgroup\$ – Peter Smith May 5 '16 at 13:19
  • \$\begingroup\$ In addition to what @PeterSmith said, seeing if your layout has clear separation between VCC for the LTC4010 and for Q1 would be important. You need that 1uF of extra capacitance at the LTC and you appear to have omitted the large ceramic capacitance next to the FETs of the buck converter that this design probably needs. Lacking the additional capacitance may result in excessive radiated noise or capacitor lifetime degradation due to ripple current. \$\endgroup\$ – user2943160 May 5 '16 at 15:04

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