BQ76940: bq76940 CHG fet drive

Ansul Tyagi

Part Number: BQ76940
Other Parts Discussed in Thread: BQ78350-R1, BQ78350

Hi,

I'm using BQ76940 with BQ78350-R1 for one og my projects.

One of the cells in battery stack is weak because of which undervoltage error is hitting. As per my understating, the DSG pin will be low in UV fault and the it will go high again only after fault/error is recovered.

But while the DSG FET is off, the CHG signal will not get its reference ground and the CHG fet will not turn on. What do I do to charge my battery once the UV fauly has occured?

over 5 years ago

0 Matt Sunna over 5 years ago

TI__Guru* 81034 points

Hi Ansul,

This is a good question. This app note is very helpful and has a section that discusses FET Drive for the BQ769x0 in detail. The last paragraph on Page 11 describes the scenario where the DSG FET is off and the CHG FET needs to be controlled: http://www.ti.com/lit/an/slua749a/slua749a.pdf

Best regards,

Matt

0 Ansul Tyagi over 5 years ago in reply to Matt Sunna

Intellectual 305 points

Hi Matt,

so essentially what i could understand from the article is that, when DSG fet is off and load is present, the CHG fet source will be pulled at PACK+ volts and CHG drive pin will be clamped at Vchg_clamp, so to turn on the CHG fet, one must turn ON the DSG fet and the other fet (CHG) will automatically get switched ON.

Let me know if there is something wrong in my understanding.

But the question is if UV fault has occurred and it has not recovered, bq78350 host will not allow to turn on the DSG fet, because of this chg fet will remain off and I cannot charge my pack. It seems to be a deadlock.

0 Matt Sunna over 5 years ago in reply to Ansul Tyagi

TI__Guru* 81034 points

Hi Ansul,

I'm not sure if that is a correct interpretation. I will check with my colleague who wrote this app note to verify and reply back on Monday.

Best regards,

Matt

0 Ansul Tyagi over 5 years ago in reply to Matt Sunna

Intellectual 305 points

Hi Matt,

any updates?

I'm waiting for your reply

0 Matt Sunna over 5 years ago in reply to Ansul Tyagi

TI__Guru* 81034 points

Hi Ansul,

You are correct - the CHG FET will not be able to turn on until PACK- is pulled low. When you connect the charger, it should pull PACK- low and allow the CHG FET to turn on.

Is the charger not applying a voltage across the PACK+/PACK- pins?

Matt

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BQ76940: bq76940 CHG fet drive