i

t

i

C

i

L

R

R + R

> 2

C

=

(7)

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

0

5

10

15

20

25

30

35

Ci=20 F

m

Ci=40 F

m

Ri=0.21

,

Li=9.3 H

W

m

(a)VcwithvariousCivalues

Time-100 s/div

m

Li=12 H

m

Ri=0.15

,

Ci=40 F

W

m

(b)VcwithvariousLivalues

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

0

5

10

15

20

25

30

35

Li=5 H

m

Ri=0.50 W

Li=9.3 H,

Ci=40 F

m

m

(c)VcwithvariousRivalues

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

0

5

10

15

20

25

30

35

Ri=0.15 W

Time-100 s/div

m

Time-100 s/div

m

bq24705

Damping Conditions:

Figure 31. Parametric Study Of The Input Voltage

As shown in Figure 31, 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 32 depicts the recommended input filter design. The measured input voltage and current waveforms are

shown in Figure 33. The input voltage spike has been well damped by adding a 2

Ω resistor, while keeping the

capacitance low.

22

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