Datasheet

Data Sheet ADP2386
Rev. A | 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
=
RIPPLEOUTSW
L
Vf
I
_
8 ∆××
∆
R
ESR
=
L
RIPPLEOUT
I
V
∆
∆
_
This calculation results in C
OUT_RIPPLE
= 11.4 μF, and R
ESR
= 18 mΩ.
To meet the ±5% overshoot and undershoot transient
requirements, use the following equations to calculate the
capacitance:
C
OUT_OV
=
2
2
_
2
)(
OUTOVOUT
OUT
STEPOV
VV
V
LIK
−∆+
×
∆
×
C
OUT_UV
=
UV
OUTOUTIN
STEPUV
VVV
LIK
_
2
)(2 ∆×−×
×∆×
where:
K
OV
= K
UV
= 2 are the coefficients for estimation purposes.
∆I
STEP
= 4 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
= 63.1 μF, and C
OUT_UV
= 24.5 μF.
According to the calculation, the output capacitance must be
greater than 63 μF, and the ESR of the output capacitor must be
smaller than 18 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 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 100 µF and 47 µF ceramic output capacitors have a derated
value of 62 µF and 32 µF.
R
C
=
2 3.3V 94 μF 60kHz
0.6V 480 μs 8.7A/ V
π ×× × ×
××
= 46.7 kΩ
C
C
=
(0.55 0.002 ) 94 μF
46.7 kΩ
Ω+ Ω ×
= 1111 pF
C
CP
=
0.002 94 μF
46.7 kΩ
Ω×
= 4.0 pF
Choose standard components, as follows: R
C
= 44.2 kΩ,
C
C
= 1200 pF, and C
CP
= 4.7 pF.
Figure 34 shows the bode plot at 6 A. The cross frequency is
58 kHz, and the phase margin is 61°.
1k
10k 100k
FREQUENCY (Hz)
1M
60
48
36
24
12
0
–12
–24
–36
–48
–60
180
144
108
72
36
0
–36
–72
–108
–144
–180
PHASE (Degrees)
MAGNITUDE (dB)
10211-134
Figure 34. Bode Plot at 6 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
SS
EXTSS
It
= 21.3 nF
Choose a standard component value, as follows: C
SS
= 22 nF.
INPUT CAPACITOR SELECTION
Place a minimum 10 μF ceramic capacitor near the PVIN pin.
In this application, it is recommended that one 10 μF, X5R, 25 V
ceramic capacitor be used.