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Ci=20 F

m

Ci=40 F

m

( c ) C i = 4 9

( 4 7

e l e c t r o l y t i c a n d 2 x

ceramic)

m

m

m

F

F

F

bq24740

SLUS736 – DECEMBER 2006

APPLICATION INFORMATION (continued)

Figure 31 shows the measured input voltages and currents with different input capacitances. The voltage spike

drops by about 5 V after increasing Ci from 20

µF to 40 µF. The input voltage spike has been dramatically

damped by using a 47 F electrolytic capacitor.

Figure 31. Adapter DC Side Hot Plug-In With Various Input Capacitances

Since the input voltage to the IC is PVCC which is 0.7 V (diode voltage drop) lower than Vc during the adapter

insertion, a 40-

µF input capacitance is normally adequate to keep the PVCC voltage well below the maximum

voltage rating under normal conditions. In case of a higher input stray inductance, the input capacitance may be

increased accordingly. An electrolytic capacitor will help reduce the input voltage spike due to its high ESR.

25

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