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ADP2381AREZ-R7 数据表(PDF) 17 Page - Analog Devices

部件名 ADP2381AREZ-R7
功能描述  20 V, 6 A Synchronous Step-Down
PDF  28 Pages
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制造商  AD [Analog Devices]
网页  http://www.analog.com
标志 AD - Analog Devices

ADP2381AREZ-R7 数据表(HTML) 17 Page - Analog Devices

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Data Sheet
ADP2381
Rev. 0 | Page 17 of 28
OUTPUT CAPACITOR SELECTION
The output capacitor selection affects both the output ripple
voltage and the loop dynamics of the regulator.
During a load step transient on the output, for example, when
the load is suddenly increased, the output capacitor supplies the
load until the control loop has a chance to ramp up the inductor
current, which causes the output to undershoot. The output
capacitance required to satisfy the voltage droop requirement
can be calculated using the following equation:
(
)
UV
OUT
OUT
IN
STEP
UV
UV
OUT
V
V
V
L
I
K
C
_
2
_
2
∆
×
−
×
×
∆
×
=
where:
KUV is a factor typically of 2.
ΔISTEP is the load step.
ΔVOUT_UV is the allowable undershoot on the output voltage.
Another case occurs when a load is suddenly removed from the
output. The energy stored in the inductor rushes into the
capacitor, which causes the output to overshoot. The output
capacitance required to meet the overshoot requirement can be
calculated using the following equation:
(
)
2
2
_
2
_
OUT
OV
OUT
OUT
STEP
OV
OV
OUT
V
V
V
L
I
K
C
−
∆
+
×
∆
×
=
where:
KOV is a factor typically of 2.
ΔVOUT_OV is the allowable undershoot on the output voltage.
The output ripple is determined by the ESR and the capaci-
tance. Use the following equation to select a capacitor that can
meet the output ripple requirements:
RIPPLE
OUT
SW
L
RIPPLE
OUT
V
f
I
C
_
_
8
∆
×
×
∆
=
L
RIPPLE
OUT
ESR
I
V
R
∆
∆
=
_
where:
ΔVOUT_RIPPLE is the allowable output ripple voltage.
RESR is the equivalent series resistance of the output capacitor.
Select the largest output capacitance given by COUT_UV, COUT_OV,
and COUT_RIPPLE to meet both load transient and output ripple
performance.
The selected output capacitor voltage rating should be greater
than the output voltage. The rms current rating of the output
capacitor should be larger than the following equation:
12
_
L
RMS
C
I
I
OUT
∆
=
LOW-SIDE POWER DEVICE SELECTION
The ADP2381 has an integrated low-side MOSFET driver that
drives the low-side NFET. The selection of the low-side NFET
affects the dc-to-dc regulator performance.
The selected MOSFET must meet the following requirements:
•
Drain-source voltage (VDS) must be higher than
1.2 × VIN.
•
Drain current (ID) must be greater than 1.2 × ILIMIT_MAX,
which is the selected maximum current-limit threshold.
•
The ADP2381 low-side gate drive voltage is 8 V. Make sure
that the selected MOSFET can fully turn on at 8 V. Total
gate charge (Qg at 8 V) must be less than 50 nC. Lower Qg
characteristics constitute higher efficiency.
•
The low-side MOSFET carries the inductor current when
the high-side MOSFET is turned off. For low duty cycle
application, the low-side MOSFET carries the output
current during most of the period. To achieve higher
efficiency, it is important to select a low on-resistance
MOSFET. The power conduction loss of the low-side
MOSFET can be calculated by using the following
equation:
PFET_LOW = IOUT2 × RDSON × (1 – D)
where RDSON is the on resistance of the low-side MOSFET.
•
Make sure that the MOSFET can handle the thermal
dissipation due to the power loss.
Some recommended MOSFETs are listed in Table 8.
Table 8. Recommended MOSFETs
Vendor
Part No.
V
DS (V)
I
D (A)
R
DSON (mΩ)
Q
g (nC)
Fairchild
FDS6298
30
13
12
10
Fairchild
FDS8880
30
10.7
12
12
Fairchild
FDM7578
25
14
8
8
Vishay
SiA430DJ
20
10.8
18.5
5.3
AOS
AON7402
30
39
15
7.1
AOS
AO4884L
40
10
16
13.6



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