ADP2325ACPZ-R7 datasheet(25/32 Pages) AD

25 / 32 page

Data Sheet

ADP2325

Rev. 0 | Page 25 of 32

To meet the ±5% overshoot and undershoot requirement, use

the following equation to calculate the capacitance:

(

)

OUT

STEP

OUT_OV

−

∆

(

)

OUT

STEP

OUT_UV

∆

−

∆

For estimation purposes, use KOV = KUV = 2. For VOUT1 = 1.2 V,

use COUT_OV1 = 188 µF and COUT_UV1 = 21 µF. For VOUT2 = 3.3 V,

use COUT_OV2 = 55 µF and COUT_UV2 = 21 µF.

For the 1.2 V rail, ESR of the output capacitor must be smaller

than 8.3 mΩ, and the output capacitance must be larger than

188 µF. It is recommend that three 100 µF, X5R, 6.3 V ceramic

capacitors 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 23 mΩ, and the output capacitance must be

larger than 55 µF. It is recommended that two 47 µF, X5R,

6.3 V ceramic capacitors 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 must be greater

than 1.2 V × VIN, and the drain current must 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 runs 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

8.33

μS

500

0.6

kHz

μF

(

)

1598

kΩ

μF

001

kΩ

μF

001

CP1

By choosing standard components where RC1 = 28 kΩ and CC1 =

1500 pF, no CCP1 is needed.

Figure 52 shows the 1.2 V rail bode plot at 5 A. The cross

frequency is 42 kHz and the phase margin is 50°.

Figure 52. Bode Plot for 1.2 V Rail

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

derated value of 32 µF.

kΩ

A/V

8.33

μS

500

0.6

kHz

μF

(

)

1594

kΩ

μF

001

kΩ

μF

001

CP2

By using standard component values of RC2 = 27 kΩ and

CC2 = 1500 pF, no CCP2 is needed.

Figure 53 shows the 3.3 V rail bode plot at 5 A. The cross

frequency is 55 kHz and phase margin is 67°.

Figure 53. Bode Plot for 3.3 V Rail

–60

–48

–36

–24

–12

FREQUENCY (Hz)

–180

180

–144

144

–108

–72

–36

108

10k

100k

2 1M

–60

–48

–36

–24

–12

FREQUENCY (Hz)

–180

180

–144

144

–108

–72

–36

108

10k

100k

2 1M

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