Datasheet

f -SwitchingFrequency-kHz
s
0
20
40
60
80
100
120
140
160
180
220 720 1220 1720 2220 2720 3220 3720 4220
C -pF
f
R =748
f
W
f -SwitchingFrequency-kHz
s
50
70
90
110
130
150
170
190
210
200 300 400 500 600 700 800 900 1000 1100 1200
R -
f
W
C =1000pF
f
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.
Figure 8. Switching Frequency vs Capacitance
Figure 9. Switching Frequency vs Resistance
Copyright © 2001–2010, Texas Instruments Incorporated Submit Documentation Feedback 11
Product Folder Link(s): bq24401