MAX629CSLASHD datasheet(8/12 Pages) MAXIM

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Diode Selection

The MAX629’s high switching frequency demands a

high-speed rectifier. Schottky diodes, such as the 1N5819

or MBR0530L, are recommended. Make sure that the

diode’s peak current rating exceeds the peak current

set by ISET, and that its breakdown voltage exceeds the

output voltage. Schottky diodes are preferred due to their

low forward voltage. However, ultrahigh-speed silicon

rectifiers are also acceptable. Table 1 lists Schottky diode

suppliers.

Capacitor Selection

Output Filter Capacitor

The primary criterion for selecting the output filter capaci-

tor is low effective series resistance (ESR). The product

of the peak inductor current and the output filter capaci-

tor’s ESR determines the amplitude of the high-frequency

ripple seen on the output voltage. These requirements

can be balanced by appropriate selection of the current

limit, as discussed in the Setting the Current Limit sec-

tion. Table 1 lists some low-ESR capacitor suppliers. See

the Output Voltage Ripple graph in the Typical Operating

Characteristics section.

Input Bypass Capacitor

Although the output current of many MAX629 applications

may be relatively small, the input must be designed to

withstand current transients equal to the inductor current

limit. The input bypass capacitor reduces the peak cur-

rents drawn from the voltage source and reduces noise

caused by the MAX629’s switching action. The input

source impedance determines the size of the capaci-

tor required at the input (VIN). As with the output filter

capacitor, a low-ESR capacitor is recommended. A 10μF,

low-ESR capacitor is adequate for most applications,

although smaller bypass capacitors may also be accept-

able. Bypass the IC separately with a 0.1μF ceramic

capacitor placed as close as possible to the VCC and

GND pins.

Reference Capacitor

Bypass REF to GND with a 0.1μF ceramic capacitor for

REF currents up to 10μA. REF can source up to 100μA

of current for external loads. For 10μA ≤ IREF ≤ 100μA,

bypass REF with a 0.47μF capacitor.

Feed-Forward Capacitor

Parallel a capacitor (CF) across R1 to compensate the

feedback loop and ensure stability (Figure 2 and Figure 3).

Values up to 270pF are recommended for most applica-

tions. Choose the lowest capacitor value that ensures

stability; high capacitance values may degrade line regu-

lation.

Applications Information

Adjusting the Output Voltage

Many biasing applications require an adjustable output

voltage, which is easily obtained using the configuration

in Figure 4. In this circuit, an external bias voltage (which

may be generated by a potentiometer, a DAC, or other

means) is coupled to FB through the resistor RB. The

output voltage of this circuit is given by:

(

)

OUT

INIT

BIAS

+−

where VINIT is the fixed output voltage as calculated in

the section Setting the Output Voltage, and VFB is equal

to either VREF (1.25V) for the positive configuration or

0V for the negative configuration. Proper choice of RB

provides a wide range of available output voltages using

simple external components to generate VBIAS.

Input Voltage Range

Although, in many cases, the MAX629 and the inductor

are powered from the same source, it is often advan-

tageous in battery-powered applications to power the

device from an available regulated supply and to power

the inductor directly from a battery. The MAX629 requires

a +2.7V to +5.5V supply at VCC, but the inductor can be

powered from as low as +0.8V, significantly increasing

Table 1. Component Suppliers

SUPPLIER

PHONE

FAX

CAPACITORS

AVX: TPS series

(803) 946-0690 (803) 626-3123

Matsuo: 267 series

(714) 969-2491 (714) 960-6492

Sprague: 595D series

(603) 224-1961 (603) 224-1430

DIODES

Motorola: MBR0530L

(602) 303-5454 (602) 994-6430

Nihon: EC11 FS1 series (805) 867-2555 (805) 867-2698

INDUCTORS

Coilcraft: DO1608 and

DT1608 series

(847) 639-6400 (847) 639-1469

Murata-Erie: LQH4

series

(814) 237-1431 (814) 238-0490

Sumida: CD43, CD54,

and CDRH62B series

(847) 956-0666 (847) 956-0702

TDK: NLC565050 series (847) 390-4373 (847) 390-4428

www.maximintegrated.com

Maxim Integrated │ 8

MAX629

28V, Low-Power, High-Voltage,

Boost or Inverting DC-DC Converter

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