Datasheet

BAT+
Q1
FMMT718
D2
ZHCS1000
L1
47 Hm
C9
1000PF
R12
120 W
Q2
MMBT3904LT1
C6
47 Fm
D5
MMSD914LT
D3
MMSD914LT
D4
S1A
DC-
R10
1KW
D1
RED
R2
2KW
C3
10 Fm
D6
BZT52-C5V1
R1
100KW
RC
6
LED
3
BAT
4
VSS
2
TS
5
VCC
7
SNS
1
MOD
8
U1
bq24400
C4
0.0022 Fm
R4
12.4KW
VCC
C7
4.7PF
C8
0.33 Fm
Q3
MMBT3904LT1
R11
220 W
R6
210KW
C5
10 Fm
BAT-
THERM
R7
105KW
R5
20KW
C1
0.1 Fm
R8
6.81KW
C2
0.1 Fm
NOTES: 1. DCinputvoltage:9–16V
3.L1:3L GlobalP/NPKSMD-1005-470K-1A
2.Chargecurrent:1A
R13
1.1KW
R3
0.05 W
DC+
bq24400
www.ti.com SLUS498A –SEPTEMBER 2001–REVISED FEBRUARY 2010
Figure 8. 3-Cell NiCd/NiMH 1A Charger
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, R
SNS
value, inductor value, hysteresis capacitor value (C
HYS
), and the value of the filter on the current sense signal (R
f
and C
f
values). R
f
and C
f
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 C
HYS
and/or the R
f
× C
f
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 R
f
and C
f
. This data was taken with an input voltage of 12V, inductor value of 220 µH, R
SNS
value of
50 mΩ, and C
HYS
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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