ADP3031

REV. PrB

–6–

PRELIMINARY TECHNICAL DATA

Capacitor Selection

The ADP3031 requires an input capacitor to reduce the

switching ripple and noise on the IN pin. The value of the

input capacitor will be dependent on the application. For

most applications, a minimum of 10 µF is required. For

applications that are running close to current limit or that

have large transient loads, input capacitors in the range of

22 µF to 47 µF are required. The selection of the output

capacitor is also dependent on the application. Given the

allowable output ripple voltage,

∆V

selecting the output capacitor can be calculated using the

following equations:

()

OUT

IN

OUT

OUT

sOUT

OUT

VV

CI

fV

V

8



−

≥×



×× ∆



(4)

()

OUT

OUT

C

L

V

ESR

IPEAK

∆

≤

(5)

When selecting an output capacitor, make sure that the

ripple current rating is sufficient to cover the rms switching

current of the ADP3031.

The ripple current in the output capacitor is given by the

following:

()

OUT

IN

RMS

OUT

OUT

IN

VV

IC

I

V

−

=

(6)

Multi-layer ceramic capacitors are a good choice, as they

have low ESR, high ripple current rating and a very small

package size. Tantalum or OS-CON capacitors can be used,

however they have a larger package size and have higher

ESR. Table 2 shows a list of several capacitor manufactur-

ers. Consult the manufacturer for more information.

Table 2. Capacitor Manufacturers

Vendor

Phone #

Web Address

AVX

408-573-4150

www.avxcorp.com

Murata

714-852-2001

www.murata.com

Sanyo

408-749-9714

www.sanyovideo.com

Taiyo-Yuden

408-573-4150

www.t-yuden.com

Diode Selection

In specifying a diode, consideration must be given to speed,

the forward current, the forward voltage drop, reverse

leakage current, and the breakdown voltage. The output

diode should be rated to handle the maximum output

current. If the output can be subjected to accidental short

circuits then the diode must be rated to handle currents up

to the current limit of the ADP3031. The breakdown rating

of the diode must exceed the output voltage. A high-speed

diode with low forward drop, and low leakage will help

improve the efficiency of the converter by lowering the

losses of the diode. Schottky diodes are recommended.

Loop Compensation

Like most current programmed PWM converters, the

ADP3031 needs compensation to maintain stability over the

operating conditions of the particular application. For

operation at duty cycles above 50%, the choice of inductor

is critical in maintaining stability. If the slope of the inductor

current is too small or too large, the circuit will be unstable.

See Inductor Selection for more information on choosing

the proper inductor.

The ADP3031 provides a pin (COMP) for compensating

the voltage feedback loop. This is done by connecting a

series RC network from the COMP pin to GND. See

Figure 2. For most applications, the compensation resistor,

R

compensation capacitor, C

C

, in the range of 100 pF < C

C

<

4 nF.

REF

COMP

FB

C2

CC

RC

gm

ERROR AMP

Figure 2. Compensation Components

Shutdown

The ADP3031 shuts down to reduce the supply current to

1 µA maximum when the shutdown pin is pulled low. In this

mode, the internal reference, error amplifier, comparator,

biasing circuitry, and the internal MOSFET switch are

turned off. Note that the output is still connected to the

input via the inductor and Schottky diode when in shut-

down.

Layout Procedure

In order to get high efficiency, good regulation and stability,

a good printed circuit board layout is required. It is strongly

recommended that the evaluation board layout be followed

as closely as possible. Use the following general guidelines

when designing printed circuit boards (refer to Figure 1):

1. Keep C

2. Keep the high current path from C

SW pin and PGND pin as short as possible.

3. Similarly, keep the high current path from C

L1, D1, and C

4. High current traces should be kept as short and as wide

as possible.

5. Place the compensation components as close to the

COMP pin as possible.

6. Place the feedback resistors as close to the FB pin as

possible to prevent noise pickup.

7. Avoid routing noise sensitive traces near the high current

traces and components.