Datasheet

ADP2325 Data Sheet
Rev. A | 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 V
IN1
= 12.0 V ± 10%
Output Voltage V
OUT1
= 1.2 V
Output Current
I
OUT1
= 5 A
Output Voltage Ripple ΔV
OUT1_RIPPLE
= 12 mV
Load Transient ±5%, 1 A to 4 A, 1 A/µs
Channel 2
Input Voltage V
IN2
= 12.0 V ± 10%
Output Voltage V
OUT2
= 3.3 V
Output Current I
OUT2
= 5 A
Output Voltage Ripple ΔV
OUT2_RIPPLE
= 33 mV
Load Transient ±5%, 1 A to 4 A, 1 A/µs
Switching Frequency f
SW
= 500 kHz
OUTPUT VOLTAGE SETTING
Choose a 10 kΩ top feedback resistor (R
TOP
); calculate the
bottom feedback resistor using the following equation:








−
×=
6.0
6.0
OUT
TOPBOT
V
RR
To set the output voltage to 1.2 V, the resistor values are R
TOP1
=
10 kΩ and R
BOT1
= 10 kΩ. To set the output voltage to 3.3 V,
the resistors values are R
TOP2
= 10 kΩ and R
BOT2
= 2.21 kΩ.
CURRENT-LIMIT SETTING
For 5 A output current operation, the typical peak current
limit is 8 A. In this case, no R
ILIM
is required.
FREQUENCY SETTING
To set the switching frequency to 500 kHz, use the following
equation to calculate the resistor value, R
OSC
:
( )
( )
kHz
000,60
kΩ
SW
OSC
f
R =
Therefore, R
OSC
=120 kΩ.
INDUCTOR SELECTION
The peak-to-peak inductor ripple current, ΔI
L
, is set to 30%
of the maximum output current. Use the following equation
to estimate the value of the inductor:
( )
SW
L
OUT
IN
fI
DVV
L
×∆
×−
=
For V
OUT1
= 1.2 V, Inductor L1 = 1.4 µH, and for V
OUT2
= 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:
( )
SW
OUT
IN
L
fL
DVV
I
×
×−
=∆
For V
OUT1
= 1.2 V, ΔI
L1
= 1.44 A. For V
OUT2
= 3.3 V, ΔI
L2
= 1.45 A.
Find the peak inductor current using the following equation:
2
L
OUT
PEAK
I
II
∆
+=
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
12
2
2
L
OUT
RMS
I
II
∆
+=
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:
RIPPLEOUT
SW
L
OUT_RIPPLE
Vf
I
C
_
8 ∆××
∆
=
L
RIPPLEOUT
ESR
I
V
R
_
∆
=
For V
OUT1
= 1.2 V, C
OUT_RIPPLE1
= 30 µF and R
ESR1
= 8.3 mΩ. For
V
OUT2
= 3.3 V, C
OUT_RIPPLE2
= 11 µF and R
ESR2
= 23 mΩ.