ADP2442-EVALZ Analog Devices, ADP2442-EVALZ Datasheet - Page 20

• Manufacturer Part Number: ADP2442-EVALZ
• Description: Power Management IC Development Tools Eval board
• Manufacturer: Analog Devices
• Type: DC/DC Converters, Regulators & Controllersr
• Series: ADP2442r

Specifications of ADP2442-EVALZ

• Rohs: yes
• Tool Is For Evaluation Of: ADP2442
• Factory Pack Quantity: 1

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
where:
V
R
I
D is the duty cycle.
f
For best practice, use a ceramic bypass capacitor because the
ESR associated with this type of capacitor is near zero, simplifying
the equation to
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.
ADP2442
SW
OUT
ESR
PP
is the switching frequency.
is the desired input ripple voltage.
is the maximum load current.
1200
1000
is the equivalent series resistance of the capacitor.
1100
C
C
900
800
700
600
500
400
300
200
IN
IN
50
_
_
MIN
MIN
=
=
100
(
I
V
OUT
PP
Figure 59. Frequency vs. Resistor
V
I
−
×
PP
OUT
I
D
OUT
150
×
×
×
RESISTANCE (kΩ)
f
1 (
SW
D
×
−
D
×
D
1 (
×
200
)
R
−
ESR
D
)
)
f
250
SW
300
350
Rev. 0 | Page 20 of 36
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Inductor Selection
The high switching frequency of the
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
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
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:
where:
V
V
f
L is the inductor value.
ΔI
L
For applications with a wide input (V
inductor based on the geometric mean (V
input voltage extremes.
where:
V
V
The inductor value is based on V
SW
IDEAL
IN
OUT
IN_MAX
IN_MIN
L
is the regulator switching frequency.
is the input voltage.
is the peak-to-peak inductor ripple current.
0.2 A ≤ ΔI
V
is the desired output voltage.
∆
L
L
2
is the ideal calculated inductor value.
I
IDEAL
IN
IDEAL
×
is the minimum input voltage.
is the maximum input voltage.
L
V
(GEOMETRIC
=
OUT
has internal slope compensation, which assumes that
V
V
=
=
ADP2442
OUT
IN
×
3
3
V
L
3 .
3 .
(
×
≤ 0.5 A
V
IN
×
×
×
)
IN
f
V
V
(
×
SW
=
V
OUT
OUT
−
IN
f
V
V
SW
V
uses the emulated peak current mode
V
IN
IN
−
OUT
×
×
IN
(
×
×
V
GEOMETRIC
(
(
_
V
V
L
OUT
f
MAX
)
IN
IN
SW
≤
(
GEOMETRIC
)
−
L
×
V
IN (GEOMETRIC)
≤
V
OUT
)
IN
ADP2442
5
×
_
×
IN
)
f
MIN
SW
V
) range, choose the
)
OUT
−
IN (GEOMETRIC)
V
V
IN
×
OUT
as follows:
allows for minimal
(
×
V
Data Sheet
)
IN
f
SW
−
V
) of the
OUT
)
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(8)
(9)