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

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

bq2000T

SLUS149D – MAY 1999 – REVISED JANUARY 2010

www.ti.com

Being a hysteretic controller, the switching frequency of the bq2000T 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 bq2000T). In general, increasing the input

voltage and/or inductor value or decreasing CHYS and/or the Rf × Cf filter corner frequency increases 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 12 V, inductor value of 220 µH, RSNS value

of 50 m

Ω, and CHYS value of 4.7 pF. Typical switching frequencies for the bq2000T are between 100 and 200

kHz, though it is possible to achieve switching frequencies in excess of 300 kHz.

Figure 9. Switching Frequency vs Capacitance

Figure 10. Switching Frequency vs Resistance

12

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