LTC4040 Charging problem

[Najgel254]

Hi all,

i am building a LTC4040 backup system and i have a problem.

Backup mode works fine, battery connected Li-Ion 18650 cell, boosting to 5V at output tested at 1A, working well. PFO LED is ON, other LEDS are OFF. When i connect input to LTC4040, 5V from my onboard buck converter (4A capability tested), LTC is still boosting from battery, PFO LED is OFF and all other LEDS are OFF. Load is powered from Li-Ion Cell, and power drawn from Buck is 0W. Charging is not hapenning too.

In datasheet there is this part:
"When the power-fail comparator detects that input power is available, the OVP charge
pump starts to charge up the IGATE pin but the backup
boost converter continues to deliver system load until
IGATE is approximately 8V. This ensures that the forward
conduction path through the external NFET pass transistors has been established. At this point, the backup boost
gets deactivated and the charger turns back on to charge
the battery while the system load gets delivered directly
from the input to VSYS through the pass transistors."

Page 16.

I measured this, when LTC is in boost mode, PFO is ON ... Then i connect Input to the system, and measuring a IGATE pin to GND. There is sawtooth signal, from 0V to approx. 1V with frequency 5,7kHz. So i think LTC4040 cant charge Gate properly to 8V ? But why ? I tried 2 Mosfets ... One is PSMN6R0 and second is BSC0906 , same behaviour. When battery is disconnected and i connect input only, IGATE voltage is 8,7V and output is ON and powered from Input. But when battery is connected and then i connect input, IGATE voltage is stuck below 1V (oscillating).

Schematic included. I tried changing Rprog(R35) to 2k2 to lower charging current but no change in behaviour.

https://www.analog.com/media/en/technical-documentation/data-sheets/4040fb.pdf

Thank you.

[Najgel254]

Topic Update:

Now i tried other batteries, i connected a deep discharged good cell at BAT input. Datasheet says:

When a battery charge cycle begins, the battery charger first determines if the battery is deeply discharged. If
the battery voltage is below VLOWBAT, typically 2.85V, an
automatic trickle charge feature sets the charge current
to 1/8th or 12.5% of the programmed value. To improve
charge current accuracy at this low level, the buck switching charger is turned off and a secondary linear charger
is used to deliver charge to the battery.

After connecting Cell with voltage under 2,85V, i had 2,6V, LTC4040 started charging battery with current 113mA. Calculation says 12,5% from Iset = my case is 909mA. so 116mA.

After battery reaches 2,85V charging stops, and Load is powered from battery ( i measured negative current) and power from input is 0W 0A.

Someone who knows why my BUCK charger is not starting and charging properly?