Datasheet

Data Sheet ADP2381
Rev. 0 | Page 21 of 28
LOW-SIDE MOSFET SELECTION
A low R
DSON
N-channel MOSFET is selected as a high efficiency
solution. The breakdown voltage of the MOSFET must be
higher than 1.2 × V
IN
, and the drain current must be larger than
1.2 × I
LIMIT
.
It is recommended that a 30 V, N -channel MOSFET, such as the
FDS6298 from Fairchild, be used. The R
DSON
of the FDS6298 at
a 4.5 V driver voltage is 9.4 mΩ, and the total gate charge at 5 V
is 10 nC.
COMPENSATION COMPONENTS
For a better load transient and stability performance, set the
cross frequency, f
C
, at f
SW
/10. In this case, f
C
= 1/500 kHz =
50 kHz.
))((
))((
_
_
_
0
_
ArCRB
CCRr
C
C
CRB
R
ArCRB
CCRr
gBC
0
CC
CPCC
0
EACP
EAC
CC
EAC
0
CC
CPCC
m
EAC
++
=
+
=
++
−×=
where:
kΩ3.37
/7.8μS500V6.0
kHz50μF943.32
2
=
××
××××
=
××
××××
=
VA
V
AgV
fCV
R
VI
m
REF
C
OUTOUT
C
π
π
nF39.1
kΩ3.37
μF94)002.0A6/V3.3(
)(
=
×Ω+
=
×+
=
C
OUT
ESR
C
R
CRR
C
pF04.5
kΩ3.37
μF94002.0
=
×Ω
=
×
=
C
OUT
ESR
CP
R
CR
C
( )
( )
7
1062.3MΩ40μS5001
kΩ21.2kΩ10
kΩ2.21kΩ10
1
×=×+
×
+
×
=×+
+
=
0
m
BOTTOP
BOTTOP
rg
RR
RR
A
6
7
1046.1
)MΩ401062.3(μS5001
)nF39.1pF04.5(MΩ40
)(1
)(
−
×
=
+××+
+×
=
++
+
=
0
m
CCP
0
rAg
CCr
B
This results in
R
C_EA
= 73.3 kΩ.
C
C_EA
= 727.6 pF.
C
CP_EA
= 2.56 pF.
Choose the standard values for R
C_EA
= 73.2 kΩ, C
C_EA
= 820 p F,
and C
CP_EA
= 2.2 pF.
Figure 35 shows the bode plot at 6 A. The cross frequency is
kHz, and the phase margin is 61°.
Figure 35. 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.
nF22
V6.0
μA3.3ms4
6.0
_
_
=
×
=
×
=
UPSS
EXTSS
SS
It
C
Choose a standard component value, C
SS
= 22 nF.
INPUT CAPACITOR SELECTION
A minimum 10 μF ceramic capacitor is required to be placed
near the PVIN pin. In this application, one 10 μF, X5R, 25 V
ceramic capacitor is recommended.
SCHEMATIC OF DESIGN EXAMPLE
See Figure 36 for a schematic of the design example.
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 (dB)
FREEQUENCY (Hz)
10209-035