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BQ24400_16 Datasheet, PDF (11/27 Pages) Texas Instruments – Programmable NiCd/NiMH Fast-Charge Management IC
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bq24400
SLUS498A – SEPTEMBER 2001 – REVISED FEBRUARY 2010
DC+
D3
MMSD914LT
D4
S1A
C6
47 mF
Q1
FMMT718
Q2
MMBT3904LT1
L1
47 mH
D2
ZHCS1000
R10
1 KW
D5
MMSD914LT
C9
R12
120 W
1000 PF
VCC
D6
BZT52-C5V1
R2
2 KW
D1
RED
C2
0.1 mF
C3
10 mF
C7
C4
4.7 PF
0.0022 mF
R1
C8
0.33 mF
R4
12.4 KW
U1
1
2
3
4
SNS
VSS
LED
BAT
100 KW
MOD
VCC
RC
8
7
6
5
TS
bq24400
C1
0.1 mF
R8
6.81 KW
R5
20 KW
Q3
MMBT3904LT1
R11
220 W
R6
210 KW
C5
10 mF
R7
105 KW
BAT+
THERM
BAT-
R13
1.1 KW
DC-
NOTES: 1. DC input voltage: 9–16V
2. Charge current: 1A
3. L1: 3L Global P/N PKSMD-1005-470K-1A
Figure 8. 3-Cell NiCd/NiMH 1A Charger
R3
0.05 W
Being a hysteretic controller, the switching frequency of the bq24400 is determined by the values of several of
the external circuit components. The components that affect the switching frequency are: input voltage, RSNS
value, inductor value, hysteresis capacitor value (CHYS), and the value of the filter on the current sense signal (Rf
and Cf values). Rf and Cf have the most impact on the switching frequency and are also the components that are
easiest to change to adjust the frequency, as they do not affect anything else in the circuit (besides, of course,
the cleanliness and quality of the current sense signal being fed to the bq24400). In general, increasing the input
voltage and/or inductor value or decreasing CHYS and/or the Rf × Cf filter corner frequency will increase the
switching frequency. Figure 9 and Figure 10 show empirical data on the variation in switching frequency based
on adjusting Rf and Cf. This data was taken with an input voltage of 12V, inductor value of 220 µH, RSNS value of
50 mΩ, and CHYS value of 4.7 pF. Typical switching frequencies for the bq24400 are between 100 and 200 kHz,
though it is possible to achieve switching frequencies in excess of 300kHz.
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