Datasheet

Data Sheet ADP2370/ADP2371
Rev. C | Page 25 of 32
APPLICATIONS INFORMATION
ADIsimPower DESIGN TOOL
ADP2370/ADP2371 are supported by the ADIsimPower™ design
tool set. ADIsimPower is a collection of tools that produce
complete power designs optimized for a specific design goal.
The tools enable the user to generate a full schematic, bill of
materials, and calculate performance in minutes. ADIsimPower
can optimize designs for cost, area, efficiency, and parts count
taking into consideration the operating conditions and limita-
tions of the IC and all real external components. For more
information about, and to obtain ADIsimPower design tools,
visit www.analog.com/ADIsimPower. Users can also request
an unpopulated board through the ADIsimPower tool.
EXTERNAL COMPONENT SELECTION
Table 6 and Table 7 list external component selections for the
ADP2370/ADP2371 application circuit shown in Figure 82. The
selection of components is dependent on the input voltage, output
voltage, and load current requirements. Additionally, trade-offs
among performance parameters, such as efficiency and transient
response, are made by varying the choice of external components.
SELECTING THE INDUCTOR
The high frequency switching of the ADP2370/ADP2371 allows
for the use of small surface-mount power inductors. The inductor
value affects the transition from PWM to PSM, efficiency, output
ripple, and current-limit values. Use the following equation to cal-
culate the ideal inductance, which is derived from the inductor
current slope compensation, for a given output voltage and
switching frequency:
SW
OUT
f
V
L
×
×
=
478.0
2.1
The ripple current is calculated as follows:








−×
×
=∆
IN
OUT
SW
OUT
L
V
V
Lf
V
I 1
where:
f
SW
is the switching frequency in MHz (1.2 MHz typical).
L is the inductor value in μH.
The dc resistance (DCR) value of the selected inductor affects
efficiency; however, a decrease in this value typically means an
increase in root mean square (rms) losses in the core and skin.
A minimum requirement of the dc current rating of the inductor
is for it to be equal to the maximum load current plus half of
the inductor current ripple, as shown by the following equation:
)
2
(
)(
L
MAXLOAD
PK
I
II
∆
+
=
OUTPUT CAPACITOR
Output capacitance is required to minimize the voltage overshoot,
voltage undershoot, and the ripple voltage present on the output.
Capacitors with low equivalent series resistance (ESR) values
produce the lowest output ripple; therefore, use capacitors such as
the X5R dielectric. Do not use Y5V and Z5U capacitors. Y5V
and Z5U capacitors are unsuitable choices because of their large
capacitance variation over temperature and their dc bias voltage
changes. Because ESR is important, select the capacitor using
the following equation:
L
RIPPLE
COUT
I
V
ESR
Δ
≤
where:
ESR
COUT
is the ESR of the chosen capacitor.
V
RIPPLE
is the peak-to-peak output voltage ripple.
Use the following equations to determine the output
capacitance:
RIPPLE
SW
IN
OUT
VLf
V
C
××××
≥
2)2(
π
RIPPLE
SW
L
OUT
Vf
I
C
××
∆
≥
8
Increasing the output capacitor value has no effect on stability
and may reduce output ripple and enhance load transient response.
When choosing the output capacitor value, it is important to
account for the loss of capacitance due to output voltage dc bias.
INPUT CAPACITOR
An input capacitor is required to reduce input voltage ripple, input
ripple current, and source impedance. Place the input capacitor
as close as possible to the VIN pin. A low ESR X7R- or X5R-type
capacitor is highly recommended to minimize the input voltage
ripple. Use the following equation to determine the rms input
current:
IN
OUT
IN
OUT
MAXLOAD
CIN
V
VVV
II
)(
)(
−
≥
IN
OUT
IN
OUT
MAXLOAD
V
V
VV
IrmsI
)(
)(
−
≥
ADJUSTABLE OUTPUT VOLTAGE PROGRAMMING
The ADP2370/ADP2371 feature an adjustable output voltage range
from 0.8 V to 12 V. The output voltage is set by the ratio of two
external resistors, R2 and R3, as shown in Figure 83. The device
servos the output to maintain the voltage at the FB pin at 0.8 V,
referenced to ground; the current in R2 is then equal to 0.8 V/R3
plus the FB pin bias current. The bias current of the FB pin,
10 nA at 25°C, flows through R2 into the FB pin.
The output voltage is calculated using the equation
V
OUT
= 0.8 V(1 + R2/R3) + (FB
I-BIAS
)(R2)