Data Sheet

ADP2389/ADP2390

DESIGN EXAMPLE

This section provides the procedures of selecting the external

components based on the example specifications listed in Table 7.

The schematic of this design example is shown in Figure 32.

Table 7. Step-Down DC-to-DC Regulator Requirements

Parameter

Specification

Input Voltage

12.0 V ± 10%

Output Voltage

1.2 V

Output Current

12 A

Output Voltage Ripple

12 mV

Load Transient

±5%, 3 A to 9 A, 2 A/µs

Switching Frequency

500 kHz

OUTPUT VOLTAGE SETTING

following equation:

0.6

0.6

BOT

TOP

OUT

R

R

V





=

×



−





FREQUENCY SETTING

67,000

(k )

– 12

(kHz)

T

SW

R

f

Ω=

INDUCTOR SELECTION

the maximum output current. Use the following equation to

estimate the inductor value:

(

)

SW

L

OUT

IN

f

I

D

V

V

L

×

∆

×

=

–

where:

D = 10%

This results in L = 0.54 µH. Select the standard inductor value

of 0.68 µH.

Calculate the peak-to-peak inductor ripple current using the

following equation:

(

)

SW

OUT

IN

L

f

L

D

V

V

I

×

×

=

∆

–

Calculate the peak inductor current with the following equation:

2

L

PEAK

OUT

I

II

∆

=

+

Calculate the rms current flowing through the inductor by the

following equation:

2

2

12

L

RMS

OUT

I

II

∆

=

+

According to the calculated current value, select an inductor

with a minimum rms current rating of 12.035 A and a minimum

saturation current rating of 13.588 A.

However, to protect the inductor from reaching its saturation

point under a current-limit condition, the inductor must be

rated for at least a 20 A saturation current for reliable operation.

Based on these requirements, select a 0.68 µH inductor, such as

the 7443330068 from Würth Elektronik, which has 1.35 mΩ dc

resistance (DCR) and a 38 A saturation current.

OUTPUT CAPACITOR SELECTION

The output capacitor must meet both the output voltage ripple

requirement and load transient response.

To meet the output voltage ripple requirement, use the following

equation to calculate the ESR and capacitance value of the

output capacitor:

RIPPLE

OUT

SW

L

RIPPLE

OUT

V

f

I

C

_

_

8

∆

×

×

∆

=

L

RIPPLE

OUT

ESR

I

V

R

∆

∆

=

_

To meet the ±5% overshoot and undershoot transient

requirements, use the following equations to calculate the

capacitance:

(

)

OUT

OV

OUT

OUT

STEP

OV

OV

OUT

V

V

V

L

I

K

C

−

∆

−

×

×

∆

×

=

_

2

_

2

(

)

UV

OUT

OUT

IN

STEP

UV

UV

OUT

V

V

V

L

I

K

C

_

2

_

2

∆

×

−

×

×

∆

×

=

where:

Rev. 0 | Page 17 of 23