Diode Selection

The MAX1626/MAX1627’s high switching frequency

demands a high-speed rectifier. Schottky diodes, such

as the 1N5817–1N5822 family or surface-mount equiva-

lents, are recommended. Ultra-high-speed rectifiers

with reverse recovery times around 50ns or faster, such

as the MUR series, are acceptable. Make sure that the

diode’s peak current rating exceeds the peak current

exceeds V+. Schottky diodes are preferred for heavy

loads due to their low forward voltage, especially in

low-voltage applications. For high-temperature applica-

tions, some Schottky diodes may be inadequate due to

their high leakage currents. In such cases, ultra-high-

speed rectifiers are recommended, although a Schottky

diode with a higher reverse voltage rating can often

provide acceptable performance.

Capacitor Selection

Choose filter capacitors to service input and output

peak currents with acceptable voltage ripple.

Equivalent series resistance (ESR) in the capacitor is a

major contributor to output ripple, so low-ESR capaci-

tors are recommended. Sanyo OS-CON capacitors are

best, and low-ESR tantalum capacitors are second

best. Low-ESR aluminum electrolytic capacitors are tol-

erable, but do not use standard aluminum electrolytic

capacitors.

Voltage ripple is the sum of contributions from ESR and

the capacitor value:

To simplify selection, assume initially that two-thirds of

the ripple results from ESR and one-third results from

capacitor value. Voltage ripple as a consequence of

ESR is approximated by:

Estimate input and output capacitor values for given

voltage ripple as follows:

where I

These equations are suitable for initial capacitor selec-

tion; final values should be set by testing a prototype or

evaluation kit. When using tantalum capacitors, use

good soldering practices to prevent excessive heat

from damaging the devices and increasing their ESR.

Also, ensure that the tantalum capacitors’ surge-current

ratings exceed the start-up inrush and peak switching

currents.

Pursuing output ripple lower than the error compara-

tor’s hysteresis (0.5% of the output voltage) is not prac-

tical, since the MAX1626/MAX1627 will switch as

needed, until the output voltage crosses the hysteresis

threshold. Choose an output capacitor with a working

voltage rating higher than the output voltage.

The input filter capacitor reduces peak currents drawn

from the power source and reduces noise and voltage

ripple on V+ and CS, caused by the circuit’s switching

action. Use a low-ESR capacitor. Two smaller-value

low-ESR capacitors can be connected in parallel for

lower cost. Choose input capacitors with working volt-

age ratings higher than the maximum input voltage.

Place a surface-mount ceramic capacitor very close to

V+ and GND, as shown in Figure 7. This capacitor

bypasses the MAX1626/MAX1627, and prevents spikes

and ringing on the power source from obscuring the

C

LI

VV

C

LI

VV

V

VV

IN

L

RIPPLE CIN IN

OUT

L

RIPPLE COUT OUT

IN

IN

OUT

=

=

−









1

2

2

1

2

2

∆

∆

,

,

)

,,

VV

RIPPLE ESR

RIPPLE C

5V/3.3V or Adjustable, 100% Duty-Cycle,

High-Efficiency, Step-Down DC-DC Controllers

______________________________________________________________________________________

11

Table 2. Component Suppliers

COMPANY

PHONE

FAX

(803) 946-0690

AVX

USA

or

(803) 626-3123

(800) 282-4975

Coilcraft

USA

(847) 639-6400

(847) 639-1469

Coiltronics

USA

(516) 241-7876

(516) 241-9339

Dale

USA

(605) 668-4131

(605) 665-1627

International

USA

(310) 322-3331

(310) 322-3332

Rectifier

IRC

USA

(512) 992-7900

(512) 992-3377

Motorola

USA

(602) 303-5454

(602) 994-6430

Nichicon

USA

(847) 843-7500

(847) 843-2798

Japan

81-7-5231-8461

81-7-5256-4158

Nihon

USA

(805) 867-2555

(805) 867-2698

Japan

81-3-3494-7411

81-3-3494-7414

Sanyo

USA

(619) 661-6835

(619) 661-1055

Japan

81-7-2070-6306

81-7-2070-1174

(408) 988-8000

Siliconix

USA

or

(408) 970-3950

(800) 554-5565

Sprague

USA

(603) 224-1961

(603) 224-1430

Sumida

USA

(847) 956-0666

(847) 956-0702

Japan

81-3-3607-5111

81-3-3607-5144

United

USA

(714) 255-9500

(714) 255-9400

Chemi-Con