Datasheet

Data Sheet ADP2384
Rev. 0 | Page 19 of 24
OUTPUT CAPACITOR SELECTION
The output capacitor is required to meet both the output voltage
ripple and load transient response requirements.
To meet the output voltage ripple requirement, use the following
equation to calculate the ESR and capacitance value of the output
capacitor:
C
OUT_RIPPLE
=
RIPPLEOUTS
L
Vf
I
_
8 ∆××
∆
R
ESR
=
L
RIPPLEOUT
I
V
∆
∆
_
This calculation results in C
OUT_RIPPLE
= 7.6 μF, and R
ESR
= 27 mΩ.
To meet the ±5% overshoot and undershoot transient
requirements, use the following equations to calculate the
capacitance:
C
OUT_OV
=
2
2
_
2
)(
OUTOVOUTOUT
STEPOV
VVV
LIK
−∆+
×∆×
C
OUT_UV
=
UVOUTOUTIN
STEPUV
VVV
LIK
_
2
)(2 ∆×−×
×∆×
where:
K
OV
= K
UV
= 2 are the coefficients for estimation purposes.
∆I
STEP
= 3 A is the load transient step.
∆V
OUT_OV
= 5%V
OUT
is the overshoot voltage.
∆V
OUT_UV
= 5%V
OUT
is the undershoot voltage.
This calculation results in C
OUT_OV
= 53.2 μF, and C
OUT_UV
= 20.7 μF.
According to the calculation, the output capacitance must be
greater than 53 μF, and the ESR of the output capacitor must be
smaller than 27 mΩ. It is recommended that two pieces of 47
μF/X5R/6.3 V ceramic capacitors be used, such as the
GRM32ER60J476ME20 from Murata, with an ESR of 2 mΩ.
COMPENSATION COMPONENTS
For better load transient and stability performance, set the cross
frequency, f
C
, to f
SW
/10. In this case, f
SW
is running at 600 kHz;
therefore, the f
C
is set to 60 kHz.
The 47 µF ceramic output capacitor has a derated value of 32 µF.
R
C
=
A/V7.8μs470V6.0
kHz60μF322V3.32
××
×××××
π
= 32.5 kΩ
C
C
=
Ω
××Ω+Ω
k5.32
μF322)002.0825.0(
= 1629 pF
C
CP
=
Ω
××Ω
k5.32
μF322002.0
= 3.9 pF
Choose standard components, as follows: R
C
= 31.6 kΩ,
C
C
= 1500 pF, and C
CP
= 3.9 pF.
Figure 34 shows the bode plot at 4 A. The cross frequency is
59 kHz, and the phase margin is 55°.
60
48
36
24
12
0
–60
–48
–36
–24
–12
180
144
108
72
36
0
–180
–144
–108
–72
–36
1k 10k 100k 1M
MAGNITUDE (dB)
PHASE (Degrees)
FREEQUENCY (Hz)
10725-040
Figure 34. Bode Plot at 4 A
SOFT START TIME PROGRAM
The soft start feature allows the output voltage to ramp up in a
controlled manner, eliminating output voltage overshoot during
soft start and limiting the inrush current. Set the soft start time
to 4 ms.
C
SS
=
V6.0
μA2.3ms4
6.0
__
×
=
×
UP
SSEXTSS
IT
= 21.3 nF
Choose a standard component value, as follows: C
SS
= 22 nF.
INPUT CAPACITOR SELECTION
A minimum 10 μF ceramic capacitor must be placed near the
PVIN pin. In this application, it is recommended that one 10 μ F,
X5R, 25 V ceramic capacitor be used.