ADP3050

–12–

REV. 0

short). This is usually only a problem when the input voltage to out-

put voltage ratio is greater than 2.5. Under this condition, the load

current needed is slightly more than the regulator can provide. The

output voltage then droops slightly, and the switch stays on every

cycle until the internal current limit is reached. Under this condition,

the load current can reach around 1.2 A. For example: a system with

an input voltage of 24 V and an output voltage of 5 V, if a gradual

overload causes the output voltage to droop to 4 V, the average

diode current would be:

IA

DIODE(AVG)

–

=×

=

12

24

4

24

10

..

If the system must survive such gradual overloads for a prolonged

period of time, ensure that the diode chosen can survive these

conditions. A larger 2 A or 3 A diode can be used if necessary.

Choosing a Catch Diode

Use the following steps to pick an appropriate catch diode.

Table V shows several Schottky rectifiers with different reverse

voltage and forward current ratings.

1. The average diode current rating must be sufficient to provide

the required load current (see above calculations). Diodes

rated below 1 A should not be used, even if the average diode

current is much lower.

2. The reverse voltage rating of the catch diode should be at

least the maximum input voltage. Often a higher rating is

chosen (1.2 times the maximum input voltage) to provide a

safety margin.

INPUT CAPACITOR SELECTION

The input bypass capacitor plays an important role in proper

regulator operation, minimizing voltage transients at the input

and providing a short local loop for the switching current. Place

using short, wide traces. This input capacitor should have an rms

ripple current rating of at least:

II

V

V

V

V

CI

OUT

OUT

IN

OUT

IN

−









2

(6)

This rating is crucial, as the input capacitor must be able to

withstand the large current pulses present at the input of a step-

down regulator. Values of 20

µF to 50 µF are typical, but the

main criteria for capacitor selection should be the ripple current

and voltage ratings.

Ceramics are an excellent choice for input bypassing due to their

low ESR and high ripple current rating. Ceramics are especially

suited for high input voltages, and are available from many differ-

ent manufacturers. Tantalums are often used for input bypassing,

but precautions must be taken, as they occasionally fail when

subjected to large inrush currents during power-up. These

surges are common when the regulator input is connected to a

battery or high-capacitance supply. Several manufacturers now

offer surface-mount solid tantalum capacitors that are surge

tested (AVS TPS series, Sprague 593D series, Kemet T495 series,

and NEMCO LSR series), but even these devices can fail if the

current surge occurs when the capacitor voltage is near its maxi-

mum rating. For this reason, a 2:1 derating is suggested for

tantalum capacitors used in applications where large inrush currents

are present (i.e., a 20 V tantalum should only be used for an input

voltage up to 10 V). Aluminum electrolytics are the cheapest choice,

but it takes several in parallel to get a good rms current rating.

OS-CON capacitors have a good ESR and ripple current rating,

but they are typically larger and more costly. Refer to Table IV

for a list of capacitor manufacturers.

DISCONTINUOUS MODE RINGING

When operating in discontinuous mode, high frequency ringing

will appear at the switch node when the inductor current has

decreased to zero. This ringing is normal, and is not a result of

loop instability. It is caused by the switch and diode capacitance

reacting with the inductor to form a damped sinusoidal ringing.

This ringing is usually in the range of several megahertz, and is

not harmful to normal circuit operation.

SETTING THE OUTPUT VOLTAGE

The fixed voltage versions of the ADP3050 (3.3 V and 5 V) have

the feedback resistor divider included on-chip. For the adjustable

version, the output voltage is set using two external resistors.

Referring to Figure 23, pick a value for R1 between 10 k

Ω and

20 k

Ω, then calculate the appropriate value for R2 using:

RR

.

21

120

1

=×

−









V

OUT

(7)

It is important to note that the accuracy of these resistors directly

affects the accuracy of the output voltage. The FB pin threshold

variation is

±3%, and the tolerances of R1 and R2 will add to this

Table V. Schottky Diode Selection Guide

1 A Diodes

2 A Diodes

3 A Diodes

15 V

10BQ15

30BQ15

20 V

1N5817

B220

SK32

SK12

B320

B120

NSQ03A02

EC10QS02

30 V

V1N5818

B230

SK33

SK13

EC21QS03

B330

B130

NSQ03A03

MBRS130

EC10QS03

40 V

1N5819

B240

SK34

SK14

EC21QS04

B340

B140

MBRS340

MBRS140

30BQ040

10BQ040

NSQ03A04

EC10QS04

Table VI. Schottky Diode Manufacturers

Company

Phone

Fax

Motorola

(602) 244-3576

(602) 244-4015 www.mot.com

MBRSxxx

Diodes, Inc.

(805) 446-4800

(805) 446-4850 www.diodes.com

1N58xx, Bxxx, SKxxx

International Rectifier

(310) 322-3331

(310) 322-3332 www.irf.com

10BQxxx, 30BQxxx

Nihon Inter Electronic (USA)

(661) 867-2555

(661) 867-2698

Nihon Inter Electronic (Japan)

81-33-343-3411

81-33-343-7377

ECxxxxxx, NSQxxxx