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ADP2442ACPZ-R7 Datasheet(PDF) 20 Page - Analog Devices

No. de pieza ADP2442ACPZ-R7
Descripción Electrónicos  36 V,1 A, Synchronous, Step-Down, DC-to-DC Regulator with External Clock Synchronization
PDF  36 Pages
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Fabricante Electrónico  AD [Analog Devices]
Página de inicio  http://www.analog.com
Logo AD - Analog Devices

ADP2442ACPZ-R7 Datasheet(HTML) 20 Page - Analog Devices

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ADP2442
Data Sheet
Rev. B | 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
approximately 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
SW
ESR
OUT
PP
OUT
MIN
IN
f
R
D
I
V
D
D
I
C
)
(
)
1
(
_
×
×
−
−
×
×
=
(4)
where:
VPP is the desired input ripple voltage.
RESR is the equivalent series resistance of the capacitor.
IOUT is the maximum load current.
D is the duty cycle.
fSW 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
MIN
IN
f
V
D
D
I
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:
L
f
V
V
V
V
I
SW
IN
OUT
IN
OUT
L
×
×
−
×
=
∆
)
(
(6)
0.2 A ≤ ΔIL ≤ 0.5 A
SW
IN
OUT
IN
OUT
SW
IN
OUT
IN
OUT
f
V
V
V
V
L
f
V
V
V
V
×
−
×
×
≤
≤
×
−
×
×
)
(
5
)
(
2
SW
IN
OUT
IN
OUT
IDEAL
f
V
V
V
V
L
×
−
×
×
=
)
(
3
.
3
(7)
where:
VIN is the input voltage.
VOUT is the desired output voltage.
fSW is the regulator switching frequency.
L is the inductor value.
ΔIL is the peak-to-peak inductor ripple current.
LIDEAL is the ideal calculated inductor value.
For applications with a wide input (VIN) range, choose the
inductor based on the geometric mean (VIN(GEOMETRIC)) of the
input voltage extremes.
MIN
IN
MAX
IN
GEOMETRIC
IN
V
V
V
_
_
)
(
×
=
(8)
where:
VIN_MAX is the maximum input voltage.
VIN_MIN is the minimum input voltage.
The inductor value is based on VIN(GEOMETRIC) as follows:
SW
GEOMETRIC
IN
OUT
GEOMETRIC
IN
OUT
IDEAL
f
V
V
V
V
L
×
−
×
×
=
)
(
)
(
)
(
3
.
3
(9)
200
300
400
500
600
700
800
900
1000
1100
1200
50
100
150
200
250
300
350
RESISTANCE (kΩ)



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