BQ32000 VBACK Question

Hi Prescott,

I have forwarded your query to the applications engineer most familiar with this part. Expect a reply soon.

Regards

Arvind Sridhar

Hi Prescott,

The device will draw 1.5uA MAX from the backup supply and requires at least 1.4V to stay alive. You should be able to calculate the expected life time from the super capacitor with these values and the starting voltage.

Hi Nick,

Thanks for your reply
Is there any example that can show how to estimate the life time base on the 1.5uA MAX from the backup supply when I use the super capacitor?

Regards,
Prescott

We can calculate the charge stored in the super capacitor and then already know the discharge rate.

Q (available) = C * (V (initial) - V (final))

Q = charge, C = capacitance, V = voltage.

1 A = 1 C/s

Ex.

I = 1.5 uA

C = 1 mF, V (initial) = 3.3 V, V (final) = 1.4 V

Q = 1 mF * (3.3 V - 1.4 V) = 1.9 mC

1.9 mC / 1.5 uA = time = 1,267 seconds

Hi Nick,

I have a roughly test for the RTC life time without main power supply.

The CAP voltage dropped from 3.0V to 1.35V around 2 hours.

It is far away from the calculation.

Could you please help me to check if the problem is by I=1.5uA? 

I used 0.22F CAP to maintain RTC power.

From the formula, it should be 

(3.0-1.4) *0.22/1.5 = 65.185 hours

CAP data sheet is on attachment.

Also provide the schematic for your reference.

DCS Series_LELON.pdf

Hi Prescott,

Can you confirm the correct power up sequence is being followed? There is a mode which can cause the device to consume greater than expected current. This was added as section 8.2.2.3 in the current datasheet (rev E).

Hi Nick,

We follow the power-up sequence as section 8.2.2.3 in datasheet.

Fist we connect the Vcc to 3.3V and check the C82 is be charged to 3.3 V, then we disconnect the Vcc.

But the CAP (C82) voltage dropped from 3.0V to 1.35V around 2 hours.

Do you have any idea for this issue?

Thanks,

Prescott

Hi Prescott,

What is the status VCC during this period? Is VCC Hi-Z, tied to GND, etc?

Can you place a current meter between the super cap and VBACK to measure the device current? Are there any potential leakage paths on the capacitor?

Can you verify the value of the supercap capacitance?

Also, the datasheet for the supercap says that it has a leakage <330uA. Can you determine the actual value? This cap leakage spec is several orders of magnitude large that the BQ leakage.