Datasheet

Data Sheet ADP5050
Rev. 0 | Page 35 of 60
SELECTING THE OUTPUT CAPACITOR
The output capacitor must meet the output voltage ripple and
load transient requirements. To meet the output voltage ripple
requirement, use the following equations to calculate the ESR
and capacitance:
RIPPLEOUT
SW
L
RIPPLEOUT
Vf
I
C
_
_
8 ∆××
∆
=
L
RIPPLEOUT
ESR
I
V
R
∆
∆
=
_
The calculated capacitance, C
OUT_RIPPLE
, is 20.8 µF, and the
calculated R
ESR
is 10 mΩ.
To meet the ±5% overshoot and undershoot requirements,
use the following equations to calculate the capacitance:
( )
UVOUTOUT
IN
STEP
UV
UVOUT
VVV
LIK
C
_
2
_
2 ∆×−×
×∆×
=
( )
2
2
2
_
OUTOUT_OVOUT
STEP
OV
OVOUT
VVV
LIK
C
−∆+
×∆×
=
For estimation purposes, use K
OV
= K
UV
= 2; therefore,
C
OUT_OV
= 117 µF and C
OUT_UV
= 13.3 µF.
The ESR of the output capacitor must be less than 13.3 mΩ,
and the output capacitance must be greater than 117 µF. It is
recommended that three ceramic capacitors be used (47 µF,
X5R, 6.3 V), such as the GRM21BR60J476ME15 from Murata
with an ESR of 2 mΩ.
SELECTING THE LOW-SIDE MOSFET
A low R
DSON
N-channel MOSFET must be selected for high
efficiency solutions. The MOSFET breakdown voltage (V
DS
)
must be greater than 1.2 × V
IN
, and the drain current must be
greater than 1.2 × I
LIMIT_MAX
.
It is recommended that a 20 V, dual N-channel MOSFET—such
as the Si7232DN from Vishay—be used for both Channel 1 and
Channel 2. The R
DSON
of the Si7232DN at 4.5 V driver voltage is
16.4 mΩ, and the total gate charge is 12 nC.
DESIGNING THE COMPENSATION NETWORK
For better load transient and stability performance, set the cross
frequency, f
C
, to f
SW
/10. In this example, f
SW
is set to 600 kHz;
therefore, f
C
is set to 60 kHz.
For the 1.2 V output rail, the 47 µF ceramic output capacitor has
a derated value of 40 µF.
Ω=
×µ×
×µ×××π×
= k4.14
A/V10S470V8.0
kHz60F403V2.12
C
R
( )
nF51.2
k4.14
F403001.03.0
=
Ω
µ××Ω+Ω
=
C
C
pF3.8
k4.14
F403001.0
=
Ω
µ××Ω
=
CP
C
Choose standard components: R
C
= 15 kΩ and C
C
= 2.7 nF.
C
CP
is optional.
Figure 62 shows the Bode plot for the 1.2 V output rail.
The cross frequency is 62 kHz, and the phase margin is 58°.
Figure 63 shows the load transient waveform.
100
–100
–80
–60
–40
–20
0
20
40
60
80
120
–180
–150
–120
–90
–60
–30
0
30
60
90
1k 10k 100k 1M
MAGNITUDE (dB)
PHASE (Degrees)
FREQUENCY (Hz)
CROSS FREQUENCY: 62kHz
PHASE MARGIN: 58°
10899-161
Figure 62. Bode Plot for 1.2 V Output
CH1 50.0mV
B
W
CH4 2.00A Ω
B
W
M200µs A CH4 2.32A
1
4
V
OUT
I
OUT
10899-162
Figure 63. 0.8 A to 3.2 A Load Transient for 1.2 V Output
SELECTING THE SOFT START TIME
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.
The SS12 pin can be used to program a soft start time of 2 ms,
4 ms, or 8 ms and can also be used to configure parallel opera-
tion of Channel 1 and Channel 2. For more information, see the
Soft Start section and Table 10.
SELECTING THE INPUT CAPACITOR
For the input capacitor, select a ceramic capacitor with a mini-
mum value of 10 µF; place the input capacitor close to the PVIN1
pin. In this example, one 10 µF, X5R, 25 V ceramic capacitor is
recommended.