28V, Low-Power, High-Voltage,

Boost or Inverting DC-DC Converter

8

_______________________________________________________________________________________

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 pre-

ferred due to their low forward voltage. However, ultra-

high-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

capacitor is low effective series resistance (ESR). The

product of the peak inductor current and the output fil-

ter capacitor’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 S

etting

the Current Limit section. 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 applica-

tions 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 currents drawn from the voltage

source, and reduces noise caused by the MAX629’s

switching action. The input source impedance deter-

mines the size of the capacitor required at the input

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

is adequate for most applications, although smaller

bypass capacitors may also be acceptable. Bypass the

IC separately with a 0.1µF ceramic capacitor placed as

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

bypass REF with a 0.47µF capacitor.

Feed-Forward Capacitor

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

Values up to 270pF are recommended for most appli-

cations. Choose the lowest capacitor value that

ensures stability; high capacitance values may

degrade line regulation.

__________Applications Information

Adjusting the Output Voltage

Many biasing applications require an adjustable output

voltage, which is easily obtained using the configura-

tion 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

the section

provides a wide range of available output voltages

Input Voltage Range

Although, in many cases, the MAX629 and the inductor

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

geous in battery-powered applications to power the

device from an available regulated supply and to

power the inductor directly from a battery. The MAX629

tor can be powered from as low as +0.8V, significantly

V

V

R1

R

(V

V

)

OUT

INIT

B

FB

BIAS

=

+

−

Table 1. Component Suppliers

Coilcraft: DO1608 and

DT1608 series

(847) 639-6400

Sumida: CD43, CD54,

and CDRH62B series

(847) 956-0666

TDK: NLC565050 series

(847) 390-4373

(847) 390-4428

(847) 956-0702

Murata-Erie: LQH4 series

(814) 237-1431

(814) 238-0490

(847) 639-1469

Nihon: EC11 FS1 series

(805) 867-2555

(805) 867-2698

Motorola: MBR0530L

(602) 303-5454

(602) 994-6430

Matsuo: 267 series

(714) 969-2491

Sprague: 595D series

(603) 224-1961

(603) 224-1430

(714) 960-6492

AVX: TPS series

(803) 946-0690

(803) 626-3123

PHONE

SUPPLIER

FAX

CAPACITORS

DIODES

INDUCTORS