Datasheet

ADP2381 Data Sheet
Rev. 0 | Page 20 of 28
DESIGN EXAMPLE
This section provides the procedures of selecting the external
components based on the example specifications listed in Table 10.
The schematic of this design example is shown in Figure 36.
Table 10. Step-Down DC-to-DC Regulator Requirements
Parameter Specification
Input Voltage V
IN
= 12.0 V ± 10%
Output Voltage V
OUT
= 3.3 V
Output Current I
OUT
= 6 A
Output Voltage Ripple ∆V
OUT_RIPPLE
= 33 mV
Load Transient ±5%, 1 A to 5 A, 2 A/μs
Switching Frequency f
SW
= 500 kHz
OUTPUT VOLTAGE SETTING
Choose a 10 kΩ resistor as the top feedback resistor (R
TOP
) and
calculate the bottom feedback resistor (R
BOT
) by using the
following equation:










6.0
6.0
OUT
TOPBOT
V
RR
To set the output voltage to 3.3 V, the resistors values are
R
TOP
= 10 kΩ, R
BOT
= 2.21 kΩ.
FREQUENCY SETTING
Connect a 100 kΩ resistor from RT pin to GND to set the
switching frequency at 500 kHz.
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 inductor value:
SW
L
OUT
IN
fI
DVV
L



)(
where:
V
IN
= 12 V.
V
OUT
= 3.3 V.
D = V
OUT
/V
IN
= 0.275.
Δ
I
L
= 1.8A.
f
SW
= 500 kHz.
This results in L = 2.659 μH. Choose the standard inductor
value of 2.2 μH.
The peak-to-peak inductor ripple current can be calculated by
the following equation:


SW
OUT
IN
L
fL
DVV
I



This results in ΔI
L
= 2.18 A.
The peak inductor current can be calculated using the following
equation:
2
L
OUT
PEAK
I
II


This results in I
PEAK
= 7.09 A.
The rms current flowing through the inductor can be calculated
by the following equation:
12
2
2
L
OUT
RMS
I
II


This results in I
RMS
= 6.03 A.
According to the calculated rms and peak inductor current
values, select an inductor with a minimum rms current rating of
6.03 A and a minimum saturation current rating of 7.09 A.
To protect the inductor from reaching its saturation limit, the
inductor should be rated for at least 9.6 A saturation current for
reliable operation.
Based on these requirements, select a 2.2 μH inductor, such as
the FDVE1040-2R2M from Toko, which has 6.8 mΩ DCR and
11.4 A saturation current.
OUTPUT CAPACITOR SELECTION
The output capacitor is required to meet both the output voltage
ripple requirement and the load transient response.
To meet the output voltage ripple requirement, use the
following equation to calculate the ESR and capacitance of the
output capacitor:
RIPPLEOUT
SW
L
RIPPLEOUT
Vf
I
C
_
_
8 


L
RIPPLEOUT
ESR
I
V
R



_
This results in C
OUT_RIPPLE
= 16.5 μF and R
ESR
= 15.1 mΩ.
To meet the ±5% overshoot and undershoot transient
requirements, use the following equations to calculate the
capacitance:
UVOUTOUT
IN
STEP
UV
UVOUT
OUTOVOUTOUT
STEP
OV
OVOUT
VVV
LIK
C
VVV
LIK
C
_
2
_
2
2
_
2
_
)(2
)(






where:
K
OV
= K
UV
= 2, the coefficients for estimation purposes.
ΔI
STEP
= 4 A, the load transient step.
ΔV
OUT_OV
= 5%V
OUT
, the overshoot voltage.
ΔV
OUT_UV
= 5%V
OUT
, the undershoot voltage.
This results in C
OUT_OV
= 63.1 μF and C
OUT_UV
= 24.5 μF.
According to the preceding calculation, the output capacitance
must be larger than 63 μF, and the ESR of the output capacitor
must be smaller than 15 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 = 2 mΩ.