DAC-Controlled Boost/Inverter

LCD Bias Supply with Internal Switch

12

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Selecting Diodes

The MAX686’s high switching frequency demands a

high-speed rectifier. Schottky diodes, such as the

1N5818 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.

Selecting Capacitors

Output Filter Capacitors

The primary selection criterion for the output filter

capacitor is low equivalent 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 appropriately

selecting the current limit, as discussed in the

Setting

the Peak Inductor Current Limit section. Table 1 lists

some low-ESR capacitor suppliers.

Bypass Capacitors

Although the output current of many MAX686 applica-

tions may be relatively small, the input supply must be

able to source current transients equal to the ISET cur-

rent limit. The input bypass capacitor reduces the peak

currents drawn from the voltage source and reduces

noise caused by the MAX686’s switching action. The

input source impedance determines the size of the

filter capacitor, low ESR is the primary consideration. A

15µF, low-ESR capacitor is adequate for most applica-

tions, although smaller bypass capacitors may also be

acceptable in light-load applications. Bypass the IC

separately with a 0.1µF ceramic capacitor placed as

Bypass REF to GND with a 0.1µF ceramic capacitor for

REF currents up to 25µA. REF can source up to 50µA of

current for external loads. For 25µA

bypass REF with a 0.47µF capacitor.

Table 1. Component Suppliers

MAX686

TO 5.5V

TO 27.5V

22

µH

MBR0530L

LX

DACOUT

15

µF

0.1

µF

FB

R3

R1

R2

Figure 4. Feed-Forward Capacitor

Figure 5. Using a Potentiometer to Adjust Output Voltage

MAX686

TO 5.5V

TO 27.5V

REF

LX

15

µF

22

µH

0.1

µF

FB

R3

R1

100k

POTENTIOMETER

R2

MBR0530L

SUPPLIER

PHONE

FAX

(603) 224-1430

(603) 224-1961

Sprague 595D series

(847) 639-1469

Murata-Erie: LQH4

series

(814) 237-1431

(814) 238-0490

(847) 956-0702

(847) 390-4428

(847) 390-4373

(847) 956-0666

Sumida: CD43, CD54,

and CD74 series

TDK: NLC565050 series

Coilcraft: DO1608 and

DT1608 series

(847) 639-6400

(803) 626-3123

(803) 946-0690

AVX: TPS series

(714) 960-6492

(714) 969-2491

Matsuo: 267 series

(602) 994-6430

(602) 303-5454

Motorola: MBR0530L

(805) 867-2698

(805) 867-2555

Nihon; EC11 FS1 series

CAPACITORS

DIODES

INDUCTORS