BQ24400_16 Datasheet, PDF (9/27 Pages) Texas Instruments – Programmable NiCd/NiMH Fast-Charge Management IC

bq24400
www.ti.com
SLUS498A – SEPTEMBER 2001 – REVISED FEBRUARY 2010
Following top-off, the bq24400 trickle-charges the battery by enabling the MOD pin to charge at a rate of once
every 1.0 second. The trickle pulse-width is user-selectable and is set by the value of the resistor R_MTO,
connected between the RC pin and V_SS. Figure 6 shows the relationship between the trickle pulse-width and the
value of R_MTO. The typical tolerance of the pulsewidth below 150kΩ is ±10%.
200
180
160
140
120
100
80
60
40
20
4
3
2
1
2 4 6 8 10
Shows Tolerance
50 100 150 200 250
R_MTO - kW
Figure 6. Relationship Between Trickle Pulse-Width and Value of R_MTO
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 bq24400 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 bq24400 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 bq24400 is sampling the BAT pin for PVD). 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
bq24400 should not be operated in this manner. Table 2 summarizes the different states of the RC pin.
bq24400 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
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