ADP2387ACPZN-R7 datasheet(20/25 Pages) AD

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ADP2387

Data Sheet

Rev. C | Page 20 of 25

Based on the requirements previously described, select a 2.2 µH

inductor, such as the FDVE1040-2R2M from Toko, which has

a 6.1 mΩ DCR and an 11.4 A saturation current.

OUTPUT CAPACITOR SELECTION

The output capacitor is required to meet both the output voltage

ripple and load transient response requirements.

To meet the output voltage ripple requirement, use the following

equation to calculate the ESR and capacitance value of the

output capacitor:

COUT_RIPPLE =

RIPPLE

OUT

∆

RESR =

RIPPLE

OUT

∆

This calculation results in COUT_RIPPLE = 11.4 µF, and RESR = 18 mΩ.

To meet the ±5% overshoot and undershoot transient

requirements, use the following equations to calculate the

capacitance:

COUT_OV =

)

(

OUT

STEP

−

∆

COUT_UV =

OUT

STEP

)

(

∆

−

∆

where:

KOV = KUV = 2 are the coefficients for estimation purposes.

∆ISTEP = 4 A is the load transient step.

∆VOUT_OV = 5% VOUT is the overshoot voltage.

∆VOUT_UV = 5% VOUT is the undershoot voltage.

This calculation results in COUT_OV = 63.1 µF, and COUT_UV = 24.5 µF.

According to the calculation, the output capacitance must be

greater than 63 µF, and the ESR of the output capacitor must be

smaller than 18 mΩ. It is recommended that one 100 µF/X5R/

6.3 V ceramic capacitor and one 47 µF/X5R/6.3 V ceramic

capacitor be used, such as the GRM32ER60J107ME20 and

GRM32ER60J476ME20 from Murata, with an ESR of 2 mΩ.

COMPENSATION COMPONENTS

For better load transient and stability performance, set the cross

frequency (fC) to fSW/10. In this case, fSW is running at 600 kHz;

therefore, fC is set to 60 kHz.

The 100 µF and 47 µF ceramic output capacitors have a derated

value of 62 µF and 32 µF, respectively.

RC =

A/V

8.7

μS

480

0.6

kHz

μF

3.3

= 46.7 kΩ

CC =

Ω

46.7

μF

)

0.002

(0.55

= 1111 pF

CCP =

Ω

46.7

μF

0.002

= 4.0 pF

Choose standard components, as follows: RC = 44.2 kΩ,

CC = 1200 pF, and CCP = 4.7 pF.

Figure 35 shows the bode plot at 6 A. The cross frequency is

58 kHz, and the phase margin is 62°.

10k

100k

FREQUENCY (Hz)

–12

–24

–36

–48

–60

180

144

108

–36

–72

–108

–144

–180

Figure 35. Bode Plot at 6 A

SOFT START TIME PROGRAM

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

controlled manner, eliminating output voltage overshoot during

soft start and limiting the inrush current. Set the soft start time

to 4 ms.

CSS =

μA

EXT

= 20.7 nF

Choose a standard component value, as follows: CSS = 22 nF.

INPUT CAPACITOR SELECTION

Place a minimum 10 µF ceramic capacitor near the PVIN pin.

In this application, it is recommended that one 10 µF, X5R, 25 V

ceramic capacitor be used.

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