ADP2370ACPZ-5.0-R7 datasheet(25/32 Pages) AD

25 / 32 page

Data Sheet

ADP2370/ADP2371

Rev. A | 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:

OUT

478

The ripple current is calculated as follows:













−

∆

OUT

where:

fSW 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:

)

(

)

(

MAX

LOAD

∆

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:

RIPPLE

COUT

ESR

≤

where:

ESRCOUT is the ESR of the chosen capacitor.

VRIPPLE is the peak-to-peak output voltage ripple.

Use the following equations to determine the output

capacitance:

RIPPLE

OUT

≥

)

( π

RIPPLE

OUT

∆

≥

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:

OUT

MAX

LOAD

CIN

)

(

)

(

−

≥

OUT

MAX

LOAD

rms

)

(

)

(

−

≥

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

VOUT = 0.8 V(1 + R2/R3) + (FBI-BIAS)(R2)

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