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ADP2323ACPZ-R7 数据表(PDF) 21 Page - Analog Devices

部件名 ADP2323ACPZ-R7
功能描述  Dual 3 A, 20 V Synchronous Step-Down Regulator with Integrated High-Side MOSFET
PDF  32 Pages
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制造商  AD [Analog Devices]
网页  http://www.analog.com
标志 AD - Analog Devices

ADP2323ACPZ-R7 数据表(HTML) 21 Page - Analog Devices

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Data Sheet
ADP2323
Rev. A | Page 21 of 32
where:
ΔVOUT_RIPPLE is the allowable output voltage ripple.
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 must be greater
than the output voltage. The minimum rms current rating of
the output capacitor is determined by the following equation:
12
_
L
rms
C
I
I
OUT
∆
=
LOW-SIDE POWER DEVICE SELECTION
The ADP2323 has integrated low-side MOSFET drivers, which
can drive the low-side N-channel MOSFETs (NFETs). The
selection of the low-side N-channel MOSFET 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 the 1.2 × ILIMIT_MAX,
where ILIMIT_MAX is the selected maximum current-limit
threshold.
The ADP2323 low-side gate drive voltage is 5 V. Make sure that
the selected MOSFET can be fully turned on at 5 V.
Total gate charge (Qg at 5 V) must be less than 30 nC. Lower Qg
characteristics constitute higher efficiency.
When the high-side MOSFET is turned off, the low-side
MOSFET carries the inductor current. For low duty cycle
applications, the low-side MOSFET carries the current for most
of the period. To achieve higher efficiency, it is important to
select a low on-resistance MOSFET. The power conduction loss
for the low-side MOSFET can be calculated 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.
In some cases, efficiency is not critical for the system; therefore,
the diode can be selected as the low-side power device. The
average current of the diode can be calculated using the
following equation:
IDIODE(AVG) = (1 − D) × IOUT
The reverse breakdown voltage rating of the diode must be
greater than the input voltage with an appropriate margin to
allow for ringing, which may be present at the SWx node. A
Schottky diode is recommended because it has low forward
voltage drop and fast switching speed.
If a diode is used for the low-side device, the ADP2323 must
enable the PFM mode by connecting the MODE pin to ground.
Table 10. Recommended MOSFETs
Vendor
Part No.
V
DS
I
D
R
DSON
Qg
Fairchild
FDS8880
30 V
10.7 A
12 mΩ
12 nC
Fairchild
FDMS7578
25 V
14 A
8 mΩ
8 nC
Fairchild
FDS6898A
20 V
9.4 A
14 mΩ
16 nC
Vishay
Si4804CDY
30 V
7.9 A
27 mΩ
7 nC
Vishay
SiA430DJ
20 V
10.8 A
18.5 mΩ
5.3 nC
AOS
AON7402
30 V
39 A
15 mΩ
7.1 nC
AOS
AO4884L
40 V
10 A
16 mΩ
13.6 nC
PROGRAMMING UVLO INPUT
The precision enable input can be used to program the UVLO
threshold and hysteresis of the input voltage as shown in Figure 46.
ENx
1.2V
EN CMP
4µA
1µA
PVINx
RTOP_EN
RBOT_EN
Figure 46. Programming UVLO Input
Use the following equation to calculate RTOP_EN and RBOT_EN:
μA
1
V
2
.
1
μA
5
V
1
.
1
V
2
.
1
V
1
.
1
_
_
_
×
−
×
×
−
×
=
FALLING
IN
RISING
IN
EN
TOP
V
V
R
V
2
.
1
μ
5
V
2
.
1
_
_
_
_
−
Α
×
−
×
=
EN
TOP
RISING
IN
EN
TOP
EN
BOT
R
V
R
R
where:
VIN_RISING is the VIN rising threshold.
VIN_FALLING is the VIN falling threshold.
COMPENSATION COMPONENTS DESIGN
For peak current-mode control, the power stage can be
simplified as a voltage controlled current source supplying
current to the output capacitor and load resistor. It is composed of
one domain pole and a zero contributed by the output capacitor
ESR. The control-to-output transfer function is shown in the
following equations:
×
π
×
+


×
π
×
+
×
×
=
=
p
z
VI
COMP
OUT
vd
f
s
f
s
R
A
s
V
s
V
s
G
2
1
2
1
)
(
)
(
)
(
OUT
ESR
z
C
R
f
×
×
π
×
=
2
1
(
)
OUT
ESR
p
C
R
R
f
×
+
×
π
×
=
2
1
where:
AVI = 5 A/V
R is the load resistance.
COUT is the output capacitance.



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