bq24401

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SLUS497A – SEPTEMBER 2001 – REVISED MARCH 2010

to tell which state the IC is in (top-off or trickle charge). The best way to tell if the bq24401 is in top-off or trickle

charge is to look at the RC pin when the temperature is between the LTF and HTF. In top-off, the RC pin is

counting and has a sawtooth waveform on it. In trickle charge, there is no timer and the RC pin is at a DC value.

The RC pin contains valuable information in determining what state the bq24401 is in, since it always operates in

brief instants during fast charge, the bq24401 is sampling the TS pin for

ΔT/Δt). If the RC pin is at some DC

value (usually around 1-2V), then the IC has paused the timer or the timer is inactive. If the RC pin is a sawtooth

waveform (similar to Figure 14), then the timer is running and the RC pin is considered “active.” Lastly, the RC

pin can be loaded by too large of a C or too small of an R. This will sometimes make the usual sawtooth

waveform look like a triangle waveform on an oscilloscope (the rise time is lengthened), or the RC signal could

have the appearance of being clipped (flat top or bottom). The timer is unreliable under these conditions and the

bq24401 should not be operated in this manner. Table 2 summarizes the different states of the RC pin.

Table 2. RC Pin Status

bq24401 CHARGE STATE

TS PIN STATE

RC PIN BEHAVIOR

Battery absent

N/A

1-2V DC level

Sleep mode

N/A

Ground (Vss)

Charge qualification (including battery

conditioning (trickle charge) and charge

N/A

1-2V DC level

suspended)

Active

Fast charge (current regulation)

1-2V DC level (timer is paused and will

state)

Top-off charge

Active

Active (timer is still counting, even though

charging is suspended)

Trickle maintenance charge (after fast charge)

N/A

1-2V DC level

Charge complete

N/A

Active

Both top-off and trickle maintenance charge are terminated and the pack never receives any more charge (until a

maintenance charge forever (excluding the two faults just mentioned) in order to keep the Nickel pack full.

Charge Current Control

The bq24401 implements a hysteretic control loop that regulates the current being delivered to the battery pack

The bq24401 controls the MOD pin to regulate the current and voltage of the pack. The bq24401 monitors

battery pack. See Figure 6 for a typical current-sensing circuit.

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