BQ24705 Datasheet, PDF (21/30 Pages) Texas Instruments – Host-Controlled Multi-Chemistry Battery Charger With Low Input Power Detect

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bq24705
SLUS779 – DECEMBER 2007
For a typical notebook charger application, the total stray inductance of the adapter output wire and the PCB
connections is normally 5–12 µH, and the total effective resistance of the input connections is 0.15–0.5 Ω.
Figure 30(a) demonstrates that a higher Ci helps to damp the voltage spike. Figure 30(b) demonstrates the effect
of the input stray inductance Li on the input voltage spike. The dashed curve in Figure 30(b) represents the worst
case for Ci=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 mF
30
Li = 12 mF
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
Minimizing the input stray inductance, increasing the input capacitance and using high-ESR input capacitors
helps to suppress the input voltage spike.
Copyright © 2007, Texas Instruments Incorporated
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