Datasheet

ADP1612/ADP1613 Data Sheet
Rev. D | Page 14 of 28
CHOOSING THE INPUT AND OUTPUT CAPACITORS
The ADP1612/ADP1613 require input and output bypass capa-
citors to supply transient currents while maintaining constant
input and output voltages. Use a low equivalent series resistance
(ESR), 10 µF or greater input capacitor to prevent noise at the
ADP1612/ADP1613 input. Place the capacitor between VIN
and GND as close to the ADP1612/ADP1613 as possible.
Ceramic capacitors are preferred because of their low ESR
characteristics. Alternatively, use a high value, medium ESR
capacitor in parallel with a 0.1 µF low ESR capacitor as close
to the ADP1612/ADP1613 as possible.
The output capacitor maintains the output voltage and supplies
current to the load while the ADP1612/ADP1613 switch is on.
The value and characteristics of the output capacitor greatly
affect the output voltage ripple and stability of the regulator. A
low ESR ceramic dielectric capacitor is preferred. The output
voltage ripple (∆V
OUT
) is calculated as follows:
OUT
ONOUT
OUT
C
OUT
C
tI
C
Q
V
×
==∆
(8)
where:
Q
C
is the charge removed from the capacitor.
t
ON
is the on time of the switch.
C
OUT
is the output capacitance.
I
OUT
is the output load current.
SW
ON
f
D
t =
(9)
and
OUT
IN
OUT
V
VV
D
−
=
(10)
Choose the output capacitor based on the following equation:
OUTOUTSW
INOUTOUT
OUT
VVf
VVI
C
∆××
−×
≥
)(
(11)
Multilayer ceramic capacitors are recommended for this
application.
DIODE SELECTION
The output rectifier conducts the inductor current to the output
capacitor and load while the switch is off. For high efficiency,
minimize the forward voltage drop of the diode. For this reason,
Schottky rectifiers are recommended. However, for high voltage,
high temperature applications, where the Schottky rectifier
reverse leakage current becomes significant and can degrade
efficiency, use an ultrafast junction diode.
Ensure that the diode is rated to handle the average output
load current. Many diode manufacturers derate the current
capability of the diode as a function of the duty cycle. Verify
that the output diode is rated to handle the average output
load current with the minimum duty cycle. The minimum
duty cycle of the ADP1612/ADP1613 is
OUT
MAXIN
OUT
MIN
V
VV
D
)(
−
=
(12)
where V
IN(MAX)
is the maximum input voltage.
The following are suggested Schottky diode manufacturers:
• ON Semiconductor
• Diodes, Inc.
LOOP COMPENSATION
The ADP1612/ADP1613 use external components to
compensate the regulator loop, allowing optimization of
the loop dynamics for a given application.
The step-up converter produces an undesirable right-half plane
zero in the regulation feedback loop. This requires compensating
the regulator such that the crossover frequency occurs well
below the frequency of the right-half plane zero. The right-
half plane zero is determined by the following equation:
L
R
V
V
RHPF
LOAD
OUT
IN
Z
×π
×








=
2
)(
2
(13)
where:
F
Z
(RHP) is the right-half plane zero.
R
LOAD
is the equivalent load resistance or the output voltage
divided by the load current.
To stabilize the regulator, ensure that the regulator crossover
frequency is less than or equal to one-fifth of the right-half
plane zero.
The regulator loop gain is
OUTCSCOMPOUTMEA
OUT
IN
OUT
FB
VL
ZGZRG
V
V
V
V
A ×××××= ||
(14)
where:
A
VL
is the loop gain.
V
FB
is the feedback regulation voltage, 1.235 V.
V
OUT
is the regulated output voltage.
V
IN
is the input voltage.
G
MEA
is the error amplifier transconductance gain.
R
OUT
is 125 MΩ.
Z
COMP
is the impedance of the series RC network from COMP
to GND.
G
CS
is the current sense transconductance gain (the inductor
current divided by the voltage at COMP), which is internally
set by the ADP1612/ADP1613.
Z
OUT
is the impedance of the load in parallel with the output
capacitor.