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ADP2442ACPZ-R7 Datasheet(PDF) 24 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) 24 Page - Analog Devices

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ADP2442
Data Sheet
Rev. B | Page 24 of 36
DESIGN EXAMPLE
Consider an application with the following specifications:
•
VIN: 24 V ± 10%
•
VOUT: 5 V ± 1%
•
Switching frequency: 700 kHz
•
Load: 800 mA typical
•
Maximum load current: 1 A
•
Overshoot ≤ 2% under all load transient conditions
CONFIGURATION AND COMPONENTS SELECTION
Resistor Divider
The first step in selecting the external components is to
calculate the resistance of the resistor divider that sets the
output voltage.
Using Equation 1 and Equation 2,
kΩ
10
μA
60
6
.
0
=
=
=
STRING
REF
BOTTOM
I
V
R


−
×
=
REF
REF
OUT
BOTTOM
TOP
V
V
V
R
R
kΩ
3
.
73
V
6
.
0
V
6
.
0
V
5
kΩ
10
=
−
×
=
TOP
R
Switching Frequency
Choosing the switching frequency involves consideration of the
trade-off between efficiency and component size. Low frequency
improves the efficiency by reducing the gate losses but requires
a large inductor. The choice of high frequency is limited by the
minimum and maximum duty cycle.
Table 11. Duty Cycle
VIN
Duty Cycle
24 V (Nominal)
DNOMINAL = 20.8%
26 V (10% Above Nominal)
DMIN = 19%
22 V (10% Less than Nominal)
DMAX = 23%
Based on the estimated duty cycle range, choose the switching
frequency according to the minimum and maximum duty cycle
limitations, as shown in Figure 58. For example, a 700 kHz,
frequency is well within the maximum and minimum duty
cycle limitations.
Using Equation 3,
SW
FREQ
f
R
500
,
92
=
RFREQ = 132 kΩ
Inductor Selection
Select the inductor by using Equation 7.
SW
IN
OUT
IN
OUT
IDEAL
f
V
V
V
V
L
×
−
×
×
=
)
(
3
.
3
μH
3
.
18
μH
66
.
18
kHz
700
V
24
V
)
5
24
(
V
5
3
.
3
≈
=
×
−
×
×
=
IDEAL
L
In Equation 7, VIN = 24 V, VOUT = 5 V, ILOAD (MAX) = 1 A, and fSW =
700 kHz, which results in L = 18.66 µH. When L = 18 μH (the
closest standard value) in Equation 6, ΔIL = 0.314 A. Although
the maximum output current that is required is 1 A, the maximum
peak current is 1.6 A. Therefore, the inductor must be rated for
higher than 1.6 A current.
Input Capacitor Selection
The input filter consists of a small 0.1 µF ceramic capacitor
placed as close as possible to the IC.
The minimum input capacitance required for a particular load is
SW
PP
OUT
MIN
IN
f
V
D
D
I
C
×
−
×
×
=
)
1
(
_
where:
VPP = 50 mV.
IOUT = 1 A.
D = 0.23.
fSW = 700 kHz.
Therefore,
μF
9
.
4
kHz
700
V
05
.
0
)
22
.
0
1
(
22
.
0
A
1
_
≈
×
−
×
×
=
MIN
IN
C
Choosing an input capacitor of 10 µF with a voltage rating of
50 V ensures sufficient capacitance over voltage and temperature.
Output Capacitor Selection
Select the output capacitor by using Equation 12 and Equation 13
)
(
8
)
(
ESR
I
V
f
I
C
L
RIPPLE
SW
L
MIN
OUT
×
∆
−
∆
×
×
∆
≅
Equation 12 is based on the output voltage ripple (ΔVRIPPLE),
which is 1% of the output voltage.


∆
×
∆
≅
DROOP
SW
STEP
OUT
MIN
OUT
V
f
I
C
3
)
(
)
(
Equation 13 calculates the capacitor selection based on the
transient load performance requirement of 2%. Perform these
calculations, then use the equation that yields the larger capacitor
size to select a capacitor.



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