Hi Team,
Why can the discharge current of BATFET be so large? The charging current is only 3A?
The current limit value should be to prevent thermal breakdown of the BATFET, right? What is the maximum power consumption of the BATFET before the thermal breakdown of the BATFET occurs? Thanks.
Regards,
Charlie
Hi Charlie,
The max charging current is set by the expected (designed for) input power and converter efficiency. For example, if the max input power is 5V*3A = 15W then max output current at 93% efficiency is 15W/4.2V*.93 = 3.35A. The BATFET is slightly overdesigned with lower RDSon to keep efficiency high.
Regarding the max discharge current before thermal breakdown, I do not have that data. Many factors, like bumps if WCSP, wirebonds if leaded QFN as well as FET silicon (Rdson), come into play when determining max discharge current. You can make a very rough estimate for max Idchg by computing die heating as JunctionTemprise = Pdiss*RthetaJA where Pdiss = Idchg^2*Rdson. If the junction plus ambient expected ambient temperature rises close to max allowed junction temperature (120-150 C), then the Idchg is too high. Most of our NVDC chargers have OCP protection after a deglitch time. The BQ25601 datasheet specifically says:
When the system is overloaded or shorted (IBAT > IBATFET_OCP=6A), the device turns off BATFET immediately and set BATFET_DIS bit to indicate BATFET is disabled.
There will be a deglitch time to allow for transient spikes > 6A. For newer products, we have begun spec'ing peak discharge current for a max time as shown below for the BQ25620. This gives some insight into the package thermal transient time as well as the allowed deglitch time.
Regards,
Jeff
