BQ24401_15 Datasheet, PDF (11/27 Pages) Texas Instruments – Programmable NiCd/NiMH Fast-Charge Management IC

bq24401
www.ti.com
SLUS497A – SEPTEMBER 2001 – REVISED MARCH 2010
Being a hysteretic controller, the switching frequency of the bq24401 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 bq24401). 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 8 and Figure 9 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 bq24401 are between 100 and 200 kHz,
though it is possible to achieve switching frequencies in excess of 300kHz.
180
160
R_f = 748W
140
120
100
80
60
40
20
0
220 720 1220 1720 2220 2720 3220 3720 4220
C_f - pF
Figure 8. Switching Frequency vs Capacitance
210
190
C_f = 1000pF
170
150
130
110
90
70
50
200 300 400 500 600 700 800 900 1000 1100 1200
R_f - W
Figure 9. Switching Frequency vs Resistance
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