ADP2325
Data Sheet
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.
Calculate the peak-to-peak inductor ripple current as follows:
VIN −VOUT × D
(
)
∆IL =
L × fSW
Table 11. Dual Step-Down DC-to-DC Regulator Requirements
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:
∆IL
Parameter
Specification
Channel 1
Input Voltage
VIN1 = 12.0 V 10%
VOUT1 = 1.2 V
IOUT1 = 5 A
ΔVOUT1_RIPPLE = 12 mV
5%, 1 A to 4 A, 1 A/µs
IPEAK = IOUT
+
Output Voltage
Output Current
Output Voltage Ripple
Load Transient
Channel 2
2
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
Input Voltage
VIN2 = 12.0 V 10%
VOUT2 = 3.3 V
IOUT2 = 5 A
ΔVOUT2_RIPPLE = 33 mV
5%, 1 A to 4 A, 1 A/µs
fSW = 500 kHz
2
∆IL
12
2
IRMS
=
IOUT
+
Output Voltage
Output Current
Output Voltage Ripple
Load Transient
Switching Frequency
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.
OUTPUT VOLTAGE SETTING
Choose a 10 kΩ top feedback resistor (RTOP); calculate the
bottom feedback resistor using the following equation:
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.
0.6
VOUT − 0.6
RBOT = RTOP
×
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.
=
OUTPUT CAPACITOR SELECTION
CURRENT-LIMIT SETTING
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:
For 5 A output current operation, the typical peak current
limit is 8 A. In this case, no RILIM is required.
FREQUENCY SETTING
∆IL
COUT_RIPPLE
=
To set the switching frequency to 500 kHz, use the following
equation to calculate the resistor value, ROSC
8× fSW ×∆VOUT _ RIPPLE
:
∆VOUT _ RIPPLE
60,000
RESR
=
ROSC kΩ =
( )
IL
fSW
kHz
( )
For VOUT1 = 1.2 V, COUT_RIPPLE1 = 30 µF and RESR1 = 8.3 mΩ. For
OUT2 = 3.3 V, COUT_RIPPLE2 = 11 µF and RESR2 = 23 mΩ.
Therefore, ROSC =120 kΩ.
V
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:
(
VIN −VOUT × D
)
L =
∆IL × fSW
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.
Rev. 0 | Page 24 of 32
ADI [ ADI ]
