Datasheet

ADP2442 Data Sheet
Rev. 0 | Page 20 of 36
Figure 59. Frequency vs. Resistor
EXTERNAL COMPONENT 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(
_
××−
−××
=
(4)
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.
D is the duty cycle.
f
SW
is the switching frequency.
For best practice, 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(
_
(5)
In addition, 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 because of their
poor temperature and dc bias characteristics. Table 10 shows a list
of recommended MLCC capacitors.
For large step load transients, add more bulk capacitance by
using electrolytic or polymer capacitors. Ensure 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 ADP2442 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; however, it degrades efficiency. Because of the high
switching frequency of the ADP2442, use shielded ferrite core
inductors for their low core losses and low EMI.
The inductor ripple current also affects the stability of the loop
because the ADP2442 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
ADP2442 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 ADP2442, 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. Calculate the inductor value as follows:
LfV
VVV
I
SW
IN
OUT
IN
OUT
L
××
−×
=∆
)(
(6)
0.2 A ≤ ΔI
L
≤ 0.5 A
SW
IN
OUT
IN
OUT
SW
IN
OUT
IN
OUT
fV
VVV
L
fV
VVV
×
−××
≤≤
×
−×× )(5)(2
SW
IN
OUT
IN
OUT
IDEAL
fV
VVV
L
×
−××
=
)(3.3
(7)
where:
V
IN
is the input voltage.
V
OUT
is the desired output voltage.
f
SW
is the regulator switching frequency.
L is the inductor value.
ΔI
L
is the peak-to-peak inductor ripple current.
L
IDEAL
is the ideal calculated inductor value.
For applications with a wide input (V
IN
) range, choose the
inductor based on the geometric mean (V
IN (GEOMETRIC)
) of the
input voltage extremes.
MININ
MAXIN
)(GEOMETRIC
IN
VVV
_
_
×=
(8)
where:
V
IN_MAX
is the maximum input voltage.
V
IN_MIN
is the minimum input voltage.
The inductor value is based on V
IN (GEOMETRIC)
as follows:
SW
GEOMETRICIN
OUT
GEOMETRICIN
OUT
IDEAL
fV
VVV
L
×
−××
=
)(
)(
)(3.3
(9)
200
300
400
500
600
700
800
900
1000
1100
1200
50 100 150 200 250 300 350
FREQUENCY (kHz)
RESISTANCE (kΩ)
10667-153