ADP2323ACPZ-R7 datasheet(24/32 Pages) AD

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ADP2323

Data Sheet

Rev. A | Page 24 of 32

For the 1.2 V rail, the output capacitor ESR needs to be smaller

than 12 mΩ, and the output capacitance needs to be larger than

191 µF. It is recommend that three pieces of 100 µF/X5R/6.3 V

ceramic capacitor be used, such as the GRM32ER60J107ME20

from Murata, with an ESR = 2 mΩ.

For the 3.3 V rail, the ESR of the output capacitor must be

smaller than 32 mΩ and the output capacitance must be larger

than 54 µF. It is recommended that two pieces of 47 µF/X5R/6.3

V ceramic capacitor be used, such as the Murata

GRM32ER60J476ME20, with an ESR = 2 mΩ.

LOW-SIDE MOSFET SELECTION

A low RDSON N-channel MOSFET is selected for high efficiency

solutions. The MOSFET breakdown voltage needs to be greater

than 1.2 V × VIN, and the drain current needs to be greater than

1.2 V × ILIMIT.

It is recommended that a 30 V, N-channel MOSFET be used, such as

the FDS8880 from Fairchild. The RDSON of the FDS8880 at a 4.5 V

driver voltage is 12 mΩ, and the total gate charge is 12 nC.

COMPENSATION COMPONENTS

For better load transient and stability performance, set the cross

frequency, fC, to fSW/10. In this case, fSW is running at 500 kHz;

therefore, the fC is set to 50 kHz.

For the 1.2 V rail, the 100 µF ceramic output capacitor has

a derated value of 64 µF.

kΩ

A/V

μs

300

kHz

μF

(

)

957

kΩ

μF

001

kΩ

μF

001

CP1

Choose standard components, RC1 = 82 kΩ and CC1 = 1000 pF. No

CCP1 is needed.

Figure 48 shows the 1.2 V rail bode plot at 3 A. The cross

frequency is 49 kHz and the phase margin is 59°.

–60

–48

–36

–24

–12

–180

–144

–108

–72

–36

108

144

180

10k

100k

FREQUENCY (Hz)

Figure 48. Bode Plot for 1.2 V Rail

For the 3.3 V rail, the 47µF ceramic output capacitor has a

derated value of 32 µF.

kΩ

A/V

μs

300

kHz

μF

(

)

956

kΩ

μF

001

kΩ

μF

001

CP2

Choose standard component values of RC2 = 75 kΩ and

CC2 = 1000 pF. No CCP2 is needed.

Figure 49 shows the 3.3 V rail bode plot at 3 A. The cross

frequency is 59 kHz and phase margin is 61°.

–60

–48

–36

–24

–12

–180

–144

–108

–72

–36

108

144

180

10k

100k

FREQUENCY (Hz)

Figure 49. Bode Plot for 3.3 V Rail

SOFT START TIME PROGRAMMING

The soft start feature allows the output voltage to ramp up in a

controlled manner, eliminating output voltage overshoot during

soft start and limiting inrush current. The soft start time is set

to 3 ms.

μA

Choose a standard component value of CSS1 = CSS2 = 22 nF.

INPUT CAPACITOR SELECTION

A minimum 10 µF ceramic capacitor is required, placed near

the PVINx pin. In this application, one piece of 10 µF, X5R, 25

V ceramic capacitor is recommended.

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