Data Sheet
ADP2325
To meet the 5% overshoot and undershoot requirement, use
the following equation to calculate the capacitance:
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°.
60
180
144
108
72
K
OV ×∆ISTEP 2 ×L
COUT_OV
=
=
48
36
VOUT + ∆VOUT _OV
2−VOUT
2
KUV × ∆ISTEP 2 × L
24
COUT_UV
2×
(
VIN −VOUT × ∆VOUT _UV
)
12
36
For estimation purposes, use KOV = KUV = 2. For VOUT1 = 1.2 V,
use COUT_OV1 = 188 µF and COUT_UV1 = 21 µF. For VOUT2 = 3.3 V,
use COUT_OV2 = 55 µF and COUT_UV2 = 21 µF.
0
0
–12
–24
–36
–48
–60
–36
–72
–108
–144
–180
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.
1
2
1k
10k
100k
FREQUENCY (Hz)
1M
Figure 52. Bode Plot for 1.2 V Rail
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.
For the 3.3 V rail, the 47 µF ceramic output capacitor has a
derated value of 32 µF.
2×π ×3.3V×2×32 μF×50 kHz
RC2
CC2
CCP2
=
= 26.5 kΩ
0.6 V×500 μS×8.33 A/V
0.66Ω + 0.001Ω ×2×32μF
26.5kΩ
LOW-SIDE MOSFET SELECTION
(
)
=
=
1594pF
A low RDSON N-channel MOSFET is selected for high efficiency
solutions. The MOSFET breakdown voltage must be greater
than 1.2 V × VIN, and the drain current must be greater than
0.001 Ω×2×32μF
=
= 2.4 pF
1.2 V × ILIMIT
.
26.5kΩ
It is recommended that a 30 V, N-channel MOSFET be used, such
as the FDS8880 from Fairchild. The RDSON of the FDS8880 at a
4.5 V driver voltage is 12 mΩ, and the total gate charge is 12 nC.
By using standard component values of RC2 = 27 kΩ and
C2 = 1500 pF, no CCP2 is needed.
C
Figure 53 shows the 3.3 V rail bode plot at 5 A. The cross
frequency is 55 kHz and phase margin is 67°.
COMPENSATION COMPONENTS
60
180
144
108
72
For better load transient and stability performance, set the
cross frequency, fC, to fSW/10. In this case, fSW runs at 500 kHz;
therefore, the fC is set to 50 kHz.
48
36
For the 1.2 V rail, the 100 µF ceramic output capacitor has
a derated value of 64 µF.
24
12
36
2×π ×1.2V ×3×64 μF×50 kHz
0
0
RC1
CC1
=
=
= 28.9 kΩ
0.6 V
0.24Ω + 0.001Ω
28.9kΩ
×
500 μS
×
8.33 A/V
–12
–24
–36
–48
–60
–36
–72
–108
–144
–180
(
)
×3×64μF
= 1598pF
0.001Ω×3×64μF
CCP1
=
= 6.6pF
1
2
1k
10k
100k
FREQUENCY (Hz)
1M
28.9kΩ
By choosing standard components where RC1 = 28 kΩ and CC1
1500 pF, no CCP1 is needed.
=
Figure 53. Bode Plot for 3.3 V Rail
Rev. 0 | Page 25 of 32
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