BQ24401_15 Datasheet, PDF (9/27 Pages) Texas Instruments – Programmable NiCd/NiMH Fast-Charge Management IC

bq24401
www.ti.com
SLUS497A – SEPTEMBER 2001 – REVISED MARCH 2010
Note that with an R_MTO value around 150 kΩ, the trickle charge pulse width is nearly identical to the top-off pulse
width of 62.5 ms (1/16 of a second for a 1A fast charge current). With R_MTO values near 150 kΩ, it can be difficult
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
one of three modes. If the RC pin is low (around V_SS potential), the IC is in sleep mode. (If the RC pin is low for
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.
bq24401 CHARGE STATE
Battery absent
Sleep mode
Charge qualification (including battery
conditioning (trickle charge) and charge
suspended)
Fast charge (current regulation)
Top-off charge
Trickle maintenance charge (after fast charge)
Charge complete
Table 2. RC Pin Status
TS PIN STATE
N/A
N/A
RC PIN BEHAVIOR
1-2V DC level
Ground (Vss)
N/A
1-2V DC level
V_TS < V_LTF
V_TS > V_LTF (in battery conditioning state)
V_TS > V_LTF (in trickle maintenance charge
state)
V_LTF > V_TS > V_HTF
V_HTF > V_TS > V_TCO
Active
1-2V DC level (timer is paused and will
resume when V_TS < V_LTF)
1-2V DC level (timer is paused and will
resume when V_TS < V_LTF)
Active
Active (timer is still counting, even though
charging is suspended)
N/A
1-2V DC level
N/A
Active
Both top-off and trickle maintenance charge are terminated and the pack never receives any more charge (until a
charge initialization occurs) if the voltage on the BAT pin reaches V_MCV. During trickle maintenance charge,
charging is suspended if V_TS < V_HTF. It resumes when V_TS > V_HTF. The bq24401 is designed to remain in trickle
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
to a user programmable value that is set by the value of the R_SNS resistor. A second, outer control loop reduces
the average current delivered to the pack in order to clamp the voltage at the BAT pin to a maximum of V_MCV.
The bq24401 controls the MOD pin to regulate the current and voltage of the pack. The bq24401 monitors
charge current at the SNS input by sensing the voltage drop across a sense-resistor, R_SNS, in series with the
battery pack. See Figure 6 for a typical current-sensing circuit.
Copyright © 2001–2010, Texas Instruments Incorporated
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