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ADP2389 Datasheet(PDF) 15 Page - Analog Devices

Part # ADP2389
Description  18 V, 12 A Step-Down Regulator with Programmable Current Limit
PDF  23 Pages
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Manufacturer  AD [Analog Devices]
Direct Link  http://www.analog.com
Logo AD - Analog Devices

ADP2389 Datasheet(HTML) 15 Page - Analog Devices

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Data Sheet
ADP2389/ADP2390
OUTPUT CAPACITOR SELECTION
The output capacitor selection affects both the output ripple
voltage and the loop dynamics of the regulator.
During a step load transient, for instance, when the load is
suddenly increased, the output capacitor supplies the load until
the control loop ramps up the inductor current. The delay caused
by the control loop causes the output to undershoot. Use the
following equation to calculate the output capacitance required
to satisfy the voltage droop requirement:
(
)
UV
OUT
OUT
IN
STEP
UV
UV
OUT
V
V
V
L
I
K
C
_
2
_
2
∆
×
−
×
×
∆
×
=
where:
KUV is a factor; the typical setting is KUV = 2.
ΔISTEP is the load step.
ΔVOUT_UV is the allowable undershoot on the output voltage.
When a load is suddenly removed from the output and the
energy stored in the inductor rushes into the output capacitor,
the output overshoots. Calculate the output capacitance required to
meet the overshoot requirement using the following equation:
(
)
OUT
OV
OUT
OUT
STEP
OV
OV
OUT
V
V
V
L
I
K
C
−
∆
−
×
×
∆
×
=
_
2
_
2
where:
KOV is a factor; the typical setting is KOV = 2.
ΔISTEP is the load step.
ΔVOUT_OV is the allowable overshoot on the output voltage.
The output ripple is determined by the ESR and the value of the
capacitance. Use the following equations to select a capacitor
that can meet the output ripple requirements:
RIPPLE
OUT
SW
L
RIPPLE
OUT
V
f
I
C
_
_
8
∆
×
×
∆
=
where ΔVOUT_RIPPLE is the allowable output ripple voltage.
L
RIPPLE
OUT
ESR
I
V
R
∆
∆
=
_
where RESR is the equivalent series resistance of the output
capacitor in ohms.
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 rms current rating of the output
capacitor must be greater than the value calculated using the
following equation:
12
_
L
RMS
COUT
I
I
∆
=
PROGRAMMING INPUT VOLTAGE UVLO
The ADP2389/ADP2390 have a precision enable input that
programs the UVLO threshold of the input voltage, as shown in
Figure 30.
Figure 30. Programming the Input Voltage UVLO
Use the following equations to calculate RTOP_EN and RBOT_EN:
__
_
1.1 V
– 1.2 V
1.1 V 6.1μΑ – 1.2 V 1μA
IN RISING
IN FALLING
TOP EN
VV
R
××
=
×
×
where:
VIN_RISING is the VIN rising threshold.
VIN_FALLING is the VIN falling threshold.
_
_
__
1.2 V
–
6.1μΑ – 1.2 V
TOP EN
BOT EN
IN RISING
TOP EN
R
R
VR
×
=
×
COMPENSATION 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 with the
following equations:


×
×
+


×
×
+
×
×
=
=
P
Z
VI
COMP
OUT
VD
f
s
f
s
R
A
s
V
s
V
s
G
π
π
2
1
2
1
)
(
)
(
)
(
where:
GVD is the control to output transfer function.
AVI = 20 A/V.
R is the load resistance.
fZ is the zero of GVD.
fP is the domain pole of GVD.
OUT
ESR
Z
C
R
f
×
×
×
=
π
2
1
where:
RESR is the equivalent series resistance of the output capacitor.
COUT is the output capacitance.
(
)
OUT
ESR
P
C
R
R
f
×
+
×
×
=
π
2
1
EN
EN CMP
ADP2389/
ADP2390
1.2V
4.8µA
1.3µA
PVIN
RTOP_EN
RBOT_EN
Rev. 0 | Page 15 of 23



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