BQ24704 Datasheet, PDF (22/31 Pages) Texas Instruments – Low Cost, Host-Controlled Li-Ion and Li-Polymer Battery Charger With Low Iq

bq24704
SLUS838 – MAY 2009........................................................................................................................................................................................................ www.ti.com
Damping Conditions:
R_t = R_i + R_C
>2
L_i
C_i
(7)
Figure 30(a) demonstrates a highr C_i which helps dampen the voltage spike. Figure 30(b) demonstrates the
effect of the input stray inductance (L_i) on the input voltage spike. The dashed curve in Figure 30(b) represents
the worst case for C_i = 40 µF. Figure 30(c) shows how the resistance helps to suppress the input voltage spike.
35
Ci = 20 mF
30
Ci = 40 mF
25
Ri = 0.21 W,
Li = 9.3 mH
35
Li = 5 mH
30
Li = 12 mH
25
Ri = 0.15 W,
Ci = 40 mF
20
20
15
15
10
10
5
0
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
Time - 100 ms/div
(a) Vc with various Ci values
5
0
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
Time - 100 ms/div
(b) Vc with various Li values
35
Ri = 0.15 W
30
Ri = 0.50 W
25
Li = 9.3 mH,
Ci = 40 mF
20
15
10
5
0
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
Time - 100 ms/div
(c) Vc with various Ri values
Figure 30. Parametric Study Of The Input Voltage
As shown in Figure 30, minimizing the input stray inductance, increasing the input capacitance, and adding
resistance (including using higher ESR capacitors) helps supress the input voltage spike. However, a user often
cannot control input srtay inductance, and increasing capacitance can increase costs. therefore, the most
efficient and cost-effective approach is to add an external resistor.
Figure 31 depicts the recommended input filter design. The measured input voltage and current waveforms are
shown in Figure 32. The input voltage spike has been well damped by adding a 2 Ω resistor, while keeping the
capacitance low.
22
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