BQ2970: Problem with initial "bootstrap"

Hi Andria and thank you for your answer.

Yes, I was aware of that.

But there's another problem: after the rest of the circuit has been powered up (by connecting a charger or "touching" the gate), sometimes QP1 MOSFET opens again, thus turning off the entire board; if I keep my finger on the magic point, everything works correctly. Seems like a "lack" of a pull-down resistor... don't know what to do

Hi Marco,

I don't think touching your finger to the gate of the DSG FET is having a pull-down resistor effect like the 5MOhm resistor.  The 5MOhm resistor is placed between the gate and source of the FET to help discharge the gate capacitance.  If anything, increasing the pull-down effect on the gate of the FET would keep the FET OFF, not turn the FET ON.

To help troubleshoot your issue, I would like you to probe the DOUT pin to see if:

1. The DOUT pin is ON during the FET opening--is it attempting to provide a HIGH voltage signal to the gate of the FET?
2. There is noise in the system.  There is a possibility that your finger is having a capacitive effect on the circuit and helping with noise.

A general rule of thumb: if your finger is wet, it is adding resistance, and if it is dry, it adds capacitance.(:

Best,

Andria

Hi Andria,

further testings showed that there was some faulty connections on the programming/testing rig - now the bq29700 + related MOSFETs seems to work as expected.

But another problem appeared: the OCD (discharge overcurrent protection) is jumpin' in too much, cutting the power to the microprocessor and causing a reset; I saw that there's a lot of bq297xx variants, how can I choose the one with the desired OCD current limit? I've taken a look at the datasheet but it's very complicated, there's no magic formula to pick the right chip based on OCD current?

Best,
marco

Hi Marco,

I am glad to hear that you discovered the cause of the issue!

If you let me know what threshold values you are looking for, then I could help recommend a device.

Best,

Andria

Hi Andria,

I need a OCD current limit of about 2.0A, 3.0A and 4.0A (three different devices).  Battery is a single Li-Ion 4.2V.

Thank you,

m

Hi Andria,

after several testings, the board's only problem is the bq29700 that's cutting connection when the current consumption rises.

Please give me some advice to choose the best bq297xx version depending on requested current (2, 3 and 4 A - a formula will be appreciated), let me know if you need some other parameters.

Best regards
md

Hi Andria,

thank you very much for your detailed and clear explanation!

Just to check out, using the CSD85301Q2 MOSFET as in the schematic:

Total MOSFETs resistance (at Vgs = 2.5V, the worst case in my circuit) = 0.033 x 2 = 0.066 ohm

at Ids = 3A: V = 0.066 x 3 = 0.099 V  <-- only 1 mV below bq29700 threshold!!! here's the problem!!!

I'll try to change the bq29700 with a bq29702 (OCD threshold = 0.160V) and post the results here.

BTW, there's no difference between bq297xxDSER and bq297xxDSET except packaging, right?

again, thank you!

Marco

Hi Andria,

just a quick note to confirm that everything now works correctly with a bq29702 (higher discharge current limit than bq29700).

Thank you very much for your assistance, best regards

md

Hi Marco,

Thanks for the update, and best of luck with your overall project!

Best,

Andria