ADP2325ACPZ-R7 datasheet(24/32 Pages) AD

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ADP2325

Data Sheet

Rev. 0 | Page 24 of 32

DESIGN EXAMPLE

This section describes the design procedure and component

selection for the example application shown in Figure 54, and

Table 11 provides a list of the required settings.

Table 11. Dual Step-Down DC-to-DC Regulator Requirements

Parameter

Specification

Channel 1

Input Voltage

VIN1 = 12.0 V ± 10%

Output Voltage

VOUT1 = 1.2 V

Output Current

IOUT1 = 5 A

Output Voltage Ripple

ΔVOUT1_RIPPLE = 12 mV

Load Transient

±5%, 1 A to 4 A, 1 A/µs

Channel 2

Input Voltage

VIN2 = 12.0 V ± 10%

Output Voltage

VOUT2 = 3.3 V

Output Current

IOUT2 = 5 A

Output Voltage Ripple

ΔVOUT2_RIPPLE = 33 mV

Load Transient

±5%, 1 A to 4 A, 1 A/µs

Switching Frequency

fSW = 500 kHz

OUTPUT VOLTAGE SETTING

Choose a 10 kΩ top feedback resistor (RTOP); calculate the

bottom feedback resistor using the following equation:













−

OUT

TOP

BOT

To set the output voltage to 1.2 V, the resistor values are RTOP1 =

10 kΩ and RBOT1 = 10 kΩ. To set the output voltage to 3.3 V,

the resistors values are RTOP2 = 10 kΩ and RBOT2 = 2.21 kΩ.

CURRENT-LIMIT SETTING

For 5 A output current operation, the typical peak current

limit is 8 A. In this case, no RILIM is required.

FREQUENCY SETTING

To set the switching frequency to 500 kHz, use the following

equation to calculate the resistor value, ROSC:

( )

(

)

kHz

000

kΩ

OSC

Therefore, ROSC =120 kΩ.

INDUCTOR SELECTION

The peak-to-peak inductor ripple current, ΔIL, is set to 30%

of the maximum output current. Use the following equation

to estimate the value of the inductor:

(

)

OUT

∆

−

For VOUT1 = 1.2 V, Inductor L1 = 1.4 µH, and for VOUT2 = 3.3 V,

Inductor L2 = 3.2 µH.

Select the standard inductor value of 1.5 µH and 3.3 µH for

the 1.2 V and 3.3 V rails.

Calculate the peak-to-peak inductor ripple current as follows:

(

)

OUT

−

∆

For VOUT1 = 1.2 V, ΔIL1 = 1.44 A. For VOUT2 = 3.3 V, ΔIL2 = 1.45 A.

Find the peak inductor current using the following equation:

OUT

PEAK

∆

For the 1.2 V rail, the peak inductor current is 5.73 A, and for

the 3.3 V rail, the peak inductor current is 5.73 A.

The rms current through the inductor can be estimated by

OUT

RMS

∆

The rms current of the inductor for both the 1.2 V and 3.3 V

rails is approximately 5.02 A.

For the 1.2 V rail, select an inductor with a minimum rms

current rating of 5.01 A and a minimum saturation current

rating of 5.73 A. For the 3.3 V rail, select an inductor with a

minimum rms current rating of 5.02 A and a minimum

saturation current rating of 5.73 A.

Based on these requirements, for the 1.2 V rail, select a

1.5 µH inductor, such as the Sumida CDRH105RNP-1R5N,

with a DCR = 5.8 mΩ; for the 3.3 V rail, select a 3.3 µH

inductor, such as the Sumida CDRH105RNP-3R3N, with a

DCR = 10.4 mΩ.

OUTPUT CAPACITOR SELECTION

The output capacitor is required to meet the output voltage

ripple and load transient requirements. To meet the output

voltage ripple requirement, use the following equation to

calculate the capacitance and ESR:

RIPPLE

OUT

OUT_RIPPLE

∆

RIPPLE

OUT

ESR

∆

For VOUT1 = 1.2 V, COUT_RIPPLE1 = 30 µF and RESR1 = 8.3 mΩ. For

VOUT2 = 3.3 V, COUT_RIPPLE2 = 11 µF and RESR2 = 23 mΩ.

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