Datasheet

Data Sheet ADP2441
Rev. A | Page 19 of 32
SOFT START
The soft start function limits the input inrush current and
prevents output overshoot at startup. The soft start time is
programmed by connecting a small ceramic capacitor between
the SS/TRK and AGND pins, with the value of this capacitor
defining the soft start time, t
SS
, as follows:
SS
SS
SS
REF
C
I
t
V
 (5)
where:
V
REF
is the internal reference voltage and equals 0.6 V.
I
SS
is the soft start current and equals 1 A.
C
SS
is the soft start capacitor value.
Table 7. Soft Start Time Selection
Soft Start Capacitor (nF) Soft Start Time (ms)
5 3
10 6
20 12
Alternatively, the user can float the SS/TRK pin and use the
internal soft start time of 2 ms.
EXTERNAL COMPONENTS SELECTION
Input Capacitor Selection
The input current to a buck regulator is pulsating in nature. The
current is zero when the high-side switch is off and is approxi-
mately equal to the load current when the switch is on. Because
switching occurs at reasonably high frequencies (300 kHz to
1 MHz), the input bypass capacitor usually supplies most of
the high frequency current (ripple current), allowing the input
power source to supply only the average (dc) current. The input
capacitor needs a sufficient ripple current rating to handle the
input ripple and needs an ESR that is low enough to mitigate the
input voltage ripple. In many cases, different types of capacitors
are placed in parallel to minimize the effective ESR and ESL.
The minimum input capacitance required for a particular load is
SWESR
OUT
PP
OUT
MININ
fRDIV
DDI
C
)(
)1(
_



(6)
where:
V
PP
is the desired input ripple voltage.
R
ESR
is the equivalent series resistance of the capacitor.
I
OUT
is the maximum load current.
It is recommended to use a ceramic bypass capacitor because
the ESR associated with this type of capacitor is near zero,
simplifying the equation to
SW
PP
OUT
MININ
fV
DDI
C



)1(
_
(7)
In addition, it is recommended to use a ceramic capacitor with a
voltage rating that is 1.5 times the input voltage with X5R and X7R
dielectrics. Using Y5V and Z5U dielectrics is not recommended
due to their poor temperature and dc bias characteristics. Table 10
shows a list of recommended MLCC capacitors from Murata
and Taiyo Yuden.
For large step load transients, add more bulk capacitance by, for
example, using electrolytic or polymer capacitors. Make sure
that the ripple current rating of the bulk capacitor exceeds the
minimum input ripple current of a particular design.
Inductor Selection
The high switching frequency of the ADP2441 allows for
minimal output voltage ripple even when small inductors are used.
Selecting the size of the inductor involves considering the trade-off
between efficiency and transient response. A smaller inductor
results in larger inductor current ripple, which provides excellent
transient response but degrades efficiency. Due to the high
switching frequency of the ADP2441, using shielded ferrite core
inductors is recommended because of their low core losses and
low EMI.
The inductor ripple current also affects the stability of the loop
because the ADP2441 uses the emulated peak current mode
architecture. In the traditional approach of slope compensation,
the user sets the inductor ripple current and then sets the slope
compensation using an external ramp resistor. In most cases, the
inductor ripple current is typically set to be 1/3 of the maximum
load current for optimal transient response and efficiency. The
ADP2441 has internal slope compensation, which assumes that
the inductor ripple current is set to 0.3 A (30% of the maximum
load of 1 A), eliminating the need for an external ramp resistor.
For the ADP2441, choose an inductor such that the peak-to-
peak ripple current of the inductor is between 0.2 A and 0.5 A
for stable operation.
Therefore, calculate the inductor value as follows:
LfV
VVV
I
SW
IN
OUT
IN
OUT
L




)(
(8)
0.2 A ≤ I
L
≤ 0.5 A
SW
IN
OUT
IN
OUT
SW
IN
OUT
IN
OUT
fV
VVV
L
fV
VVV









)(5)(2
S
W
IN
OUT
IN
OUT
IDEAL
fV
VVV
L





)(3.3
(9)
where:
V
IN
is the input voltage.
V
OUT
is the desired output voltage.
f
SW
is the regulator switching frequency.
For applications with a wide input (V
IN
) range, choose the
inductor based on the geometric mean of the input voltage
extremes.
MININ
MAXIN
GEOMETRICIN
VVV
_
_
)(

where:
V
IN_MAX
is the maximum input voltage.
V
IN_MIN
is the minimum input voltage.