Datasheet

Data Sheet ADP2380
Rev. 0 | Page 21 of 28
LOW-SIDE MOSFET SELECTION
A low R
DSON
N-channel MOSFET is chosen 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 to use a 30 V, N -channel MOSFET, such as
the FDS6298 from Fairchild. 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 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
0CC
CPCC0
EACP
EAC
CC
EAC
0CC
CPCC
mEAC
++
=
+
=
++
×=
where:
R
C
=
VImREF
COUTOUT
AgV
fCV
××
××××
π
2
=
A/V7.8μS470V6.0
kHz50μF322V3.32
××
×××××
π
= 27.1 kΩ
C
C
=
( )
C
OUTESR
R
CRR ×+
=
kΩ1.27
μF322)002.0A4V/3.3( ××+
= 1.96 nF
C
CP
=
kΩ1.27
μF322002.0 ××
=
×
C
OUTESR
R
CR
= 4.73 pF
A =
( )
kΩ21.2kΩ10
kΩ21.2kΩ10
1
+
×
=×+
+
0m
BOTTOP
BOTTOP
rg
RR
RR
×
(1 + 470 µS × 40 MΩ) = 3.4 × 10
7
B =
MΩ40104.3(μS4701
nF)96.1pF73.4(MΩ40
)(1
)(
7
+××+
+×
=
++
+
0m
CCP0
rAg
CCr
=
2.26 × 10
−6
This results in the following:
R
C_EA
= 52.3 kΩ.
C
C_EA
= 1055 pF.
C
CP_EA
= 2.45 pF.
Choose the standard values for R
C_EA
= 49.9 kΩ, C
C_EA
= 1000 p F,
and C
CP_EA
= 2.2 pF.
Figure 35 shows the Bode plot at 4 A. The cross frequency is
43 kHz, and the phase margin is 59°.
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)
FREQUENCY (Hz)
09939-035
Figure 35. 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
V6.0
__
×
=
×
UPSSEXTSS
It
= 21.3 nF
Choose a standard component value, C
SS
= 22 nF.
INPUT CAPACITOR SELECTION
A minimum 10 μF ceramic capacitor must be placed near the
PVIN pin. In this application, one 10 μF, X5R, 25 V ceramic
capacitor is recommended.
ADP2380
1
PVIN
PVIN
UVLO
PGOOD
RT
SYNC
EN/SS
COMP
BST
SW
SW
LD
VREG
PGND
GND
FB
2
3
4
5
6
7
8
16
15
14
13
12
11
10
9
R
TOP
10kΩ
1%
R
BOT
2.21kΩ
1%
C
SS
22nF
R
OSC
100kΩ
C
IN
10µF
25V
C
OUT1
47µF
6.3V
C
OUT2
47µF
6.3V
V
OUT
= 3.3V
C
BST
0.1µF
C
VREG
1µF
L1
4.7µH
V
IN
= 12V
C
C_EA
1000pF
C
CP_EA
2.2pF
R
C_EA
49.9kΩ
M1
FDS6298
09939-036
Figure 36. Design Example Schematic