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LTC4015 will not start charging

Category: Hardware
Product Number: LTC4015

We have a battery charger circuit based on the LTC4015.  It is strapped for 6 cell Lead Acid, programmed mode.  I connect a battery, which is showing 12.5 V open circuit.  Then I connect input power, a benchtop supply set to 15 V, and current limit 10A.

There is only about 20 mA load on the SYS side.  The charger is not charging, the battery current is 4 or 5 mA drawn from the battery, that is, - 5 mA charge current, and the SW is not switching. 

If I power up the circuit with the same 15 V DC at Vin with the battery disconnected, I see about 7 V at the battery terminals. 

I am looking at the telemetry over the I2C bus.  If I read back CHARGER_STATE I see 0x0200 (yes, I know its little-endian), so absorb_charge is set.  CHARGE_STATUS is 0x0000, so its not in input undervoltage, nor input overcurrent, nor constant current limit, nor constant voltage limit. 

I read back NTC_RATIO, which is around 0x21E4, which seems a bit off from the actual room temp, but not too warm as to cause temperature compensation foldback.

The measurement system can read back the battery voltage correctly, VBAT is 0x3F92, which scales to 12.51 V.

If I set suspend_charger, then clear it, the charger still does not start.

Since with no battery, I see 7 V, I assume its in limit for some other reason?

Anyone have ideas on how can I find out why the charger is shut down?



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  • Sure, I can share the schematic.  How would I do that?  I don't see a way to attach a file to this conversation.

    In the meantime, I have been comparing my circuit to the working EVK circuit and I find that the Top FET gate is not going high enough to turn on the top FET.  I think it is supposed to be held at (Vin)+(DVcc)-(Boot diode forward drop) during the top FET on time.   The Top FET source and gate are oscillating when the chip tries to turn top FET on.  Here's a scope shot:

    Yellow trace 1 is TG at top FET gate, Blue trace 2 is SW at top FET Source, Purple trace 3 is BG at bottom FET gate.  Cursor math shows the Vgs for teh top FET is about 3.2 V.  Vgs On (max) for this FET is about 3.7V, so its turning on partly, I think.

    It looks as if the TG is decaying and letting the FET turn off again, perhaps?

    I'd love to know your opinion.

  • To close this out, to help the next person: I found that the root cause here was that the Top FET was not turning on fully.  I replaced the FET with a logic-level-gate device (I used IRLR2705) and the charger is now running.  This may not be the optimum device for this application, but it is working, and I can optimize from here for production design.  Note that the boosted gate drive at TG is at best (DRVcc - Vf of the BOOST diode).  Even with careful choice of the diode, and using the full 5V from INTVcc gate drive voltage is an important spec for the Top FET.  Happy Charging!