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BQ24259: Charging current in constant current phase

Part Number: BQ24259

Hi,

I have a project with single cell 2A USB charger BQ24259.

My IC setting: PSEL = Low, OTG = Low, RILIM = 200 Ohm and it is supply from external 5V voltage source.

In datasheet I can see that fast charge current is on constant value in constant current phase (by default 2048mA). I noted that this current in my circuit is reduced below 2A when battery voltage is closer to 4,4V.

Should fast charging current be contatnt during whole constant current phase?

Waiting for reply.

  • Hello Michael,

    The device will transition from Constant Current mode to Constant Voltage mode once the charge voltage threshold (set in REG04) is reached. This value can be set between 3.504-4.400 volts and defaults to 4.204 V.

    The transition from CC to CV will occur once the charge voltage threshold is detected at the BAT pin. Note that if there is a high impedance in the circuit/cables connecting the BAT pin to the battery, there may be a difference between the BAT voltage and the actual battery voltage, which can cause CV mode to enter early. For instance, if there is 100 mOhm impedance between BAT pin and battery, then a 4.4V will be reached at BAT pin at about (4.4V - 0.1 Ohm * 2.0 A) = 4.2V of actual battery voltage which will cause the BQ24259 to enter CV mode and regulate 4.4V at the BAT pin, which will cause the charge current to drop as the battery voltage rises.

    I believe that you will find that your current is dropping when BAT pin voltage reaches the charge voltage value set in REG04. If this is not the case, please read the value of REG08 (system status register) once you see unexpected current limiting and reply to this thread with the value. There are a number of system conditions that can lead to charge current limiting including Dynamic Power Management (DPM) input current limiting, thermal limiting, etc., and these are indicated in REG08.

    Regards,
    Steve