Using the BQ2000 in a standby mode

David,

bq2000 can only handle up to 6V on VIN

Understood. And, I have that handled with a 5.1V Zener as recommended in the application notes.

I'm referencing the external circuitry. In specific the output of the diode OR where the battery charging pulses may exceed the 'other' input to the ORing circuit.

I'll try to paste it here...

Note that testing has changed some of the component values, like R35, but the question is more on the output of the FMMT718 and the effect on the diode ORing circuit.

Note the WALL_5V is now set to 8V and the LM3671 and MCP1700 have been changed out with circuits that will handle the higher voltage.

Any questions, just ask...

Thanks

you can use rich edit and insert PDF file.

Sheet4.pdf

I should have stated that the Sheet4.pdf is the circuit as originally designed and not as modified. For example, i taking the WALL_5V to 8V, I cut the trace above C43 on the left and bridged it with a 845 Ohm resistor. Then I placed a 5.1V zener across C43. That held the voltage on the BQ2000 to 5.1V. That part works OK.

Again, any questions, just ask.

Thanks

That is the root of my question.

The circuit as per the schematic yields pulses of 12V (from J1) on the BAT+ line. Since this is higher than the 8V coming out of the first regulator, the ideal diode (U17) becomes forward biased and these pulses now appear on the MAIN_PWR bus. This causes current to flow into the circuit in addition to the battery cells.

Would it be a better design if instead of using the input from J1 to power the charging circuit (Q13, etc.) to use the output of the 8V circuit?

Since the pulses used to charge the battery cells would not exceed the 8V output, then the ideal diode would not become forward biased during the charging cycle.

To me, this seems like a cleaner design, but I'd like a second opinion from someone more familiar with battery back-up systems using rechargeable batteries.

Thanks

Sorry for the delay. I think it is better to use 8V. let me know how it goes.

No problem with the delay. The boards were out for modifications unrelated to the charging circuit. In the meantime I had more time to review the circuit from a different viewpoint.

Using the 8V for the charging circuit would take care of the pulses getting through the diode ORing circuit but also adds the benefit of reducing the probability of overcharging the NiMH cells. Since the 12V -> 8V circuit is designed to handle up to 3A and the charging circuit is set for 0.2C, the incoming regulator should be able to handle it without any problem

I should be able to test these modifications sometime in the next few days, but I don't anticipate any problems

Thanks.