Datasheet

Data Sheet ADP2325
Rev. A | Page 25 of 32
To meet the ±5% overshoot and undershoot requirement, use
the following equation to calculate the capacitance:
( )
2
2
_
2
OUTOVOUTOUT
STEP
OV
OUT_OV
VVV
LIK
C
−∆+
×∆×
=
( )
UVOUTOUT
IN
STEP
UV
OUT_UV
VVV
LIK
C
_
2
2 ∆×−×
×∆×
=
For estimation purposes, use K
OV
= K
UV
= 2. For V
OUT1
= 1.2 V,
use C
OUT_OV1
= 188 µF and C
OUT_UV1
= 21 µF. For V
OUT2
= 3.3 V,
use C
OUT_OV2
= 55 µF and C
OUT_UV2
= 21 µF.
For the 1.2 V rail, ESR of the output capacitor must be smaller
than 8.3 mΩ, and the output capacitance must be larger than
188 µF. It is recommend that three 100 µF, X5R, 6.3 V ceramic
capacitors be used, such as the GRM32ER60J107ME20 from
Murata, with an ESR = 2 mΩ.
For the 3.3 V rail, the ESR of the output capacitor must be
smaller than 23 mΩ, and the output capacitance must be
larger than 55 µF. It is recommended that two 47 µF, X5R,
6.3 V ceramic capacitors be used, such as the Murata
GRM32ER60J476ME20, with an ESR = 2 mΩ.
LOW-SIDE MOSFET SELECTION
A low R
DSON
N-channel MOSFET is selected for high efficiency
solutions. The MOSFET breakdown voltage must be greater
than 1.2 V × V
IN
, and the drain current must be greater than
1.2 V × I
LIMIT
.
It is recommended that a 30 V, N-channel MOSFET be used, such
as the FDS8880 from Fairchild. The R
DSON
of the FDS8880 at a
4.5 V driver voltage is 12 mΩ, and the total gate charge is 12 nC.
COMPENSATION COMPONENTS
For better load transient and stability performance, set the
cross frequency, f
C
, to f
SW
/10. In this case, f
SW
runs at 500 kHz;
therefore, the f
C
is set to 50 kHz.
For the 1.2 V rail, the 100 µF ceramic output capacitor has
a derated value of 64 µF.
kΩ9.28
A/V8.33μS500V0.6
kHz50μF643V2.12
=
××
×××××
=
π
C1
R
( )
pF1598
kΩ9.28
μF643Ω001.0Ω24.0
=
××+
=
C1
C
pF6.6
k
Ω9.28
μF643Ω001.0
=
××
=
CP1
C
By choosing standard components where R
C1
= 28 kΩ and C
C1
=
1500 pF, no C
CP1
is needed.
Figure 52 shows the 1.2 V rail bode plot at 5 A. The cross
frequency is 42 kHz and the phase margin is 50°.
Figure 52. Bode Plot for 1.2 V Rail
For the 3.3 V rail, the 47 µF ceramic output capacitor has a
derated value of 32 µF.
kΩ5.26
A/V8.33μS500V0.6
kHz50μF232V3.32
=
××
×××××
=
π
C2
R
( )
pF1594
kΩ5.26
μF322Ω001.0Ω66.0
=
××+
=
C2
C
pF4.2
kΩ5.26
μF322Ω001.0
=
××
=
CP2
C
By using standard component values of R
C2
= 27 kΩ and
C
C2
= 1500 pF, no C
CP2
is needed.
Figure 53 shows the 3.3 V rail bode plot at 5 A. The cross
frequency is 55 kHz and phase margin is 67°.
Figure 53. Bode Plot for 3.3 V Rail
1k
–60
60
–48
48
–36
–24
–12
12
24
36
MAGNITUDE (dB)
PHASE (Degrees)
FREQUENCY (Hz)
0
–180
180
–144
144
–108
–72
–36
36
72
108
0
10k 100k
1 2
1M
10036-061
1k
–60
60
–48
48
–36
–24
–12
12
24
36
MAGNITUDE (dB)
PHASE (Degrees)
FREQUENCY (Hz)
0
–180
180
–144
144
–108
–72
–36
36
72
108
0
10k 100k
1 2
1M
10036-062