I'm using the STC4054 Li-ion single cell charger in a design, the CHRG pin is supposed to have a weak pull-down when not charging but the current drawn is higher than specs

[PVaca.1]

In the STC4054 datasheet, it says the CHRG pin is supposed to have a weak pull-down of max 35uA when no battery is connected yet. However, with a 4.7Kohm pullup from that pin to 3.3v, I measure a 1.2v drop, which indicates about 250uA of current. With a 390 ohm pullup, the drop is about the same, which indicates almost 10x the current. Am I misunderstanding something on the datasheet? Thanks!

[Peter BENSCH]

Welcome, @Pablo Vacatello​, to the community!

The data sheet of the STC4054 can indeed be misunderstood. Perhaps the function of pin CHRG becomes clearer if one imagines two functions connected there internally:

• a MOSFET that switches through to GND when the charge is running (= low).
• a switched current source with 8...35µA, typ 20µA, which is switched on at the end of a charging cycle.

If both are switched off (=High), this signals the Undervoltage Condition.

By the way, there is a more detailed description of the CHRG pin in AN2370, section 7.

Hope that helps?

Regards

/Peter

[PVaca.1]

Hello Peter, thanks for your reply!

I do understand that the CHRG pin has both a weak pulldown and a strong ground depending on the state of the charger. From the AN: " A weak pull-down current of approximately 20 μA indicates that sufficient voltage is applied to VCC to begin charging. When a discharged battery is connected to the charger, the constant current portion of the charge cycle begins and the CHRG pin pulls to ground." What I'm experiencing is *before* I connect a battery, I expect a weak pulldown of 20uA, but the pulldown is much stronger than that.