Capacitors

Output Filter Capacitor

A 22µF, 30V surface-mount (SMT) tantalum output filter

capacitor typically maintains 100mVp-p output ripple

when generating -24V at 40mA from a 5V input. Smaller

capacitors, down to 10µF, may be used for light loads

in applications that can tolerate higher output ripple.

Surface-mount capacitors are generally preferred

because they lack the inductance and resistance of the

leads of their through-hole equivalents.

Input Bypass Capacitor

A 22µF tantalum capacitor in parallel with a 0.1µF

ceramic normally provides sufficient bypassing. Mount

the 0.1µF capacitor very close to the IC. Larger

capacitors may be needed if the incoming supply has

high impedance. Less bypass capacitance is accept-

able if the circuit is run off a low-impedance supply.

Begin prototyping with a large bypass capacitor; when

the circuit is working, reduce the bypass to the smallest

value that gives good results. Although bench power

supplies have low impedance at DC, they often have

high impedance at the frequencies used by switching

DC-DC converters.

The effective series resistance (ESR) of both the

bypass and filter capacitors affects efficiency. Best per-

formance is obtained by doubling up on the filter

capacitors or using low-ESR types.

The smallest low-ESR SMT capacitors currently avail-

able are Sprague 595D series, which are about half the

size of competing products. Sanyo OS-CON organic

semiconductor through-hole capacitors also exhibit low

ESR, and are especially useful when operation below

0°C is required. Table 2 lists the phone numbers of

these and other manufacturers.

Compensation Capacitor

The high value of the feedback resistor makes the feed-

back loop susceptible to phase lag if parasitic capaci-

tance is present at the FB pin. To compensate for this, it

ues range from 0pF to 220pF.

PC Layout and Grounding

Due to high current levels and fast switching wave-

forms, proper PC board layout is essential. In particular,

keep all leads short, especially the lead connected to

the FB pin and those connecting Q1, L1, and D1

Use a star ground configuration: Connect the ground

lead of the input bypass capacitor, the output capaci-

tor, and the inductor at a common point next to the

GND pin of the MAX749. Additionally, connect the posi-

tive lead of the input bypass capacitor as close as pos-

sible to the V+ pin of the IC.

Digitally Adjustable LCD Bias Supply

______________________________________________________________________________________

11

___________________Chip Topography

TRANSISTOR COUNT: 521;

SUBSTRATE CONNECTED TO GND.

ADJ

CTRL

0.070"

(0.1178mm)

0.808"

(0.2032mm)

FB

GND

V+

V+

CS

DLOW

DHI