BQ24740_14 Datasheet, PDF (25/33 Pages) Texas Instruments – Host-Controlled Multi-Chemistry Battery Charger

bq24740
www.ti.com............................................................................................................................................... SLUS736C – DECEMBER 2006 – REVISED MARCH 2009
Figure 32(a) demonstrates a higher Ci helps dampen the voltage spike. Figure 32(b) demonstrates the effect of
the input stray inductance Li upon the input voltage spike. Figure 32(c) shows how increased resistance helps to
suppress the input voltage spike.
35
C_i = 20 mF
30
C_i = 40 mF
25
R_i = 0.21 W
L_i = 9.3 mH
35
L_i = 5 mH
30
L_i = 12 mH
25
R_i = 0.15 W
C_i = 40 mF
20
20
15
15
10
10
5
5
0
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
Time - ms
(a) V_c with various C_i values
0
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
Time - ms
(b) V_c with various L_i values
35
R_i = 0.15 W
30
R_i = 0.5 W
25
L_i = 9.3 mH
C_i = 40 mF
20
15
10
5
0
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
Time - ms
(c) V_c with various R_i values
Figure 32. Parametric Study Of The Input Voltage
As shown in Figure 32, minimizing the input stray inductance, increasing the input capacitance, and adding
resistance (including using higher ESR capacitors) helps suppress the input voltage spike. However, a user often
cannot control input stray inductance and increasing capacitance can increase costs. Therefore, the most
efficient and cost-effective approach is to add an external resistor.
Figure 33 depicts the recommended input filter design. The measured input voltage and current waveforms are
shown in Figure 34. The input voltage spike has been well damped by adding a 2-Ω resistor, while keeping the
capacitance low.
V_IN
V_PVCC
2W
(0.5 W, 1210 anti-surge) ^Rext
2.2 mF
C1
(25 V, 1210)
C2 0.1 mF
(50 V, 0805, very close to PVCC)
Figure 33. Recommended Input Filter Design
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