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We are designing device that will use BQ25790.
This charger allows user to connect external N-FET according to the datasheet. We will need that feature.
But there is no clue how to select right N-FET that will work for this application. What parameters apart from current rating are important for this ship FET to work. As I understand when trying to enable ship FET from shutdown state, some sort of charge pump is used, and very low current is provided to the gate. That probably requires "sensitive" FET to work, not everyone will do.
That is why I'm asking for an advice how to select one.
Also, example FETs that will work well as Ship FET's for BQ25790 will be a very helpful.
Regards Mateusz K.
Hi Mateusz,
The NFET should have low RDSon, have low gate leakage and have VDS voltage this is high enough for the max battery voltage (plus voltage spikes if hot plugging the battery). The output drive current from SDRV is very low so turn on/off time will depend on gate capacitance but that shouldn't be an issue in most applications. The output drive voltage is 5V higher than the battery voltage. The EVM user's guide at https://www.ti.com/lit/ug/sluuc51d/sluuc51d.pdf has a BOM that lists TI's CSD17575Q3. It was chosen to accommodate up to a 4S battery pack.
Regards,
Jeff
I was also curious about this, as I've noticed that the EVM selects CSD17581Q3A for the AC_FETs but the slightly different CSD17575Q3 for the ship FET.
I'm assuming the CSD17581Q3A is also suitable for the ship FET, which would simplify the design?
Or is there a specific reason besides the Rdson for selecting a different part?
Hi Sjors,
I don't have the engineer's reasoning for why he picked those FETs. I suspect he choose the 7575 for lower Rdson.
Regards,
Jeff
Thank You.
In terms of ship FET for BQ25792, there is some iinconsistency. In chapter 9.3.12.1 of datasheet (Shutdown Mode) it says: The device can only be woken up by plugging in an adapter.
Is that really true?
In the TI video (time 20:50), presenter says that it can be woken up either from adapter or a button press. Button press - I believe he means !QON shorted to GND.
So Can You wake up BQ25792 from shutdown mode, using only !QON shorted to GND, with no adapter? Could You check that for me please?
Yes. In shutdown mode, only adapter plug in can wake the charger. In ship mode, either adapter plug in or /QON wakes the charger.
Regards,
Jeff