CS51411G datasheet(11/16 Pages) ONSEMI

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CS51411, CS51412, CS51413, CS51414

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The total output ripple is the sum of the VRIPPLE(ESR) and

VRIPPLE(ESR).

Figure 12. The Output Voltage Ripple Using Two 10

Ceramic Capacitors in Parallel

Figure 13. The Output Voltage Ripple Using One 100

POSCAP Capacitor

Figure 14. The Output Voltage Ripple Using

One 100

mF OS–CON

Figure 15. The Output Voltage Ripple Using

One 100

mF Tantalum Capacitor

Figure 12 to Figure 15 show the output ripple of a 5.0 V

to 3.3 V/500 mA regulator using 22

µH inductor and various

capacitor types. At the switching frequency, the low ESR

and ESL make the ceramic capacitors behave capacitively

as shown in Figure 12. Additional paralleled ceramic

capacitors will further reduce the ripple voltage, but

inevitably increase the cost. “POSCAP”, manufactured by

SANYO, is a solid electrolytic capacitor. The anode is

sintered tantalum and the cathode is a highly conductive

polymerized organic semiconductor. TPC series, featuring

low ESR and low profile, is used in the measurement of

Figure 13. It is shown that POSCAP presents a good balance

of capacitance and ESR, compared with a ceramic capacitor.

In this application, the low ESR generates less than 5.0 mV

of ripple and the ESL is almost unnoticeable. The ESL of the

through–hole OS–CON capacitor give rise to the inductive

impedance. It is evident from Figure 14 which shows the

step rise of the output ripple on the switch turn–on and large

spike on the switch turn–off. The ESL prevents the output

capacitor from quickly charging up the parasitic capacitor of

the inductor when the switch node is pulled below ground

through the catch diode conduction. This results in the spike

associated with the falling edge of the switch node. The D

package tantalum capacitor used in Figure 15 has the same

footprint as the POSCAP, but doubles the height. The ESR

of the tantalum capacitor is apparently higher than the

POSCAP. The electrolytic and tantalum capacitors provide

a low–cost solution with compromised performance. The

reliability of the tantalum capacitor is not a serious concern

for output filtering because the output capacitor is usually

free of surge current and voltage.

Diode Selection

The diode in the buck converter provides the inductor

current path when the power switch turns off. The peak

reverse voltage is equal to the maximum input voltage. The

peak conducting current is clamped by the current limit of

the IC. The average current can be calculated from:

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