Datasheet

ADP5050 Data Sheet
Rev. 0 | Page 30 of 60
SOFT START SETTING
The buck regulators in the ADP5050 include soft start circuitry
that ramps the output voltage in a controlled manner during
startup, thereby limiting the inrush current. To set the soft start
time to a value of 2 ms, 4 ms, or 8 ms, connect a resistor divider
from the SS12 or SS34 pin to the VREG pin and ground (see the
Soft Start section).
INDUCTOR SELECTION
The inductor value is determined by the switching frequency,
input voltage, output voltage, and inductor ripple current. Using
a small inductor value yields faster transient response but degrades
efficiency due to the larger inductor ripple current. Using a large
inductor value yields a smaller ripple current and better efficiency
but results in slower transient response. Thus, a trade-off must be
made between transient response and efficiency. As a guideline,
the inductor ripple current, ΔI
L
, is typically set to a value from
30% to 40% of the maximum load current. The inductor value
can be calculated using the following equation:
L = [(V
IN
− V
OUT
) × D]/(ΔI
L
× f
SW
)
where:
V
IN
is the input voltage.
V
OUT
is the output voltage.
D is the duty cycle (D = V
OUT
/V
IN
).
ΔI
L
is the inductor ripple current.
f
SW
is the switching frequency.
The ADP5050 has internal slope compensation in the current
loop to prevent subharmonic oscillations when the duty cycle
is greater than 50%.
The peak inductor current is calculated using the following
equation:
I
PEAK
= I
OUT
+ (ΔI
L
/2)
The saturation current of the inductor must be larger than the
peak inductor current. For ferrite core inductors with a fast
saturation characteristic, make sure that the saturation current
rating of the inductor is higher than the current-limit threshold
of the buck regulator to prevent the inductor from becoming
saturated.
The rms current of the inductor can be calculated using the
following equation:
12
2
2 L
OUT
RMS
I
II
∆
+=
Shielded ferrite core materials are recommended for low core
loss and low EMI. Table 13 lists recommended inductors.
Table 13. Recommended Inductors
Vendor Part No.
Value
(µH)
I
SAT
(A)
I
RMS
(A)
DCR
(mΩ)
Size
(mm)
Coilcraft XFL4020-102 1.0 5.4 11 10.8 4 × 4
XFL4020-222 2.2 3.7 8.0 21.35 4 × 4
XFL4020-332 3.3 2.9 5.2 34.8 4 × 4
XFL4020-472 4.7 2.7 5.0 52.2 4 × 4
XAL4030-682
6.8
3.6
3.9
67.4
4 × 4
XAL4040-103 10 2.8 2.8 84 4 × 4
XAL6030-102 1.0 23 18 5.62 6 × 6
XAL6030-222 2.2 15.9 10 12.7 6 × 6
XAL6030-332 3.3 12.2 8.0 19.92 6 × 6
XAL6060-472 4.7 10.5 11 14.4 6 × 6
XAL6060-682 6.8 9.2 9.0 18.9 6 × 6
TOKO FDV0530-1R0 1.0 11.2 9.1 9.4 6.2 × 5.8
FDV0530-2R2 2.2 7.1 7.0 17.3 6.2 × 5.8
FDV0530-3R3
3.3
5.5
5.3
29.6
6.2 × 5.8
FDV0530-4R7 4.7 4.6 4.2 46.6 6.2 × 5.8
OUTPUT CAPACITOR SELECTION
The selected output capacitor affects both the output voltage
ripple and the loop dynamics of the regulator. For example,
during load step transients on the output, when the load is
suddenly increased, the output capacitor supplies the load until
the control loop can ramp up the inductor current, causing an
undershoot of the output voltage.
The output capacitance required to meet the undershoot
(voltage droop) requirement can be calculated using the
following equation:
( )
UVOUTOUT
IN
STEP
UV
UVOUT
VVV
LIK
C
_
2
_
2 ∆×−×
×∆×
=
where:
K
UV
is a factor (typically set to 2).
ΔI
STEP
is the load step.
ΔV
OUT_UV
is the allowable undershoot on the output voltage.
Another example of the effect of the output capacitor on the loop
dynamics of the regulator is when the load is suddenly removed
from the output and the energy stored in the inductor rushes into
the output capacitor, causing an overshoot of the output voltage.
The output capacitance required to meet the overshoot require-
ment can be calculated using the following equation:
( )
2
2
2
_
OUTOUT_OVOUT
STEP
OV
OVOUT
VVV
LIK
C
−∆+
×∆×
=
where:
K
OV
is a factor (typically set to 2).
ΔI
STEP
is the load step.
ΔV
OUT_OV
is the allowable overshoot on the output voltage.