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LT1309

APPLICATIONS INFORMATION

Inductor Selection

The low inductance value required with the LT1309 (10

µH)

allows the use of very small units such as the Murata-Erie

LQH3C100. Requirements for the inductor include low

DCR, ability to perform efficiently at 500kHz and a satura-

tion current rating of 700mA to 900mA. Often inductor

manufacturers rate maximum current for a small inductor

based on self-heating considerations. For use in switching

regulators where the inductor current is not constant the

maximum specified DC current can be safely exceeded.

Capacitor Selection

The LT1309 needs very little input and output capacitance

to function. Output capacitance should be 1

µF or 2µF for

acceptable output ripple voltage. Flash memory tolerates

higher ripple voltage than might be suitable for a low noise

system. The capacitor can be either a single high capaci-

tance ceramic unit such as Tokin 1E105ZY5U-203 or

Murata-Erie GRM230Y5V105Z016, or can be distributed

as 10 to 20 0.1

µF ceramic units. Lower ripple can be

obtained by increasing the capacitance to the 5

µF to 10µF

level. 2

µF to 5µF of tantalum (low-Q) capacitance is

recommended on the input side to reduce resonance

effects which can otherwise cause hundreds of millivolts

of ripple voltage at the input.

Diode Selection

The LT1309’s high switching speed demands a high speed

rectifier. Schottky diodes are preferred for their low for-

ward drop and fast recovery. A suitable choice is the

Motorola MBR0520. This is a 0.5A, 20V Schottky in a very

small package that is 1.35mm high.

PC Board Layout

The component placement shown in Figure 1 is recom-

mended for PC board layouts. The high speed current

paths are kept to a minimum distance and the switch node

copper is minimized to keep radiated noise low. Notice the

placement of the input decoupling capacitor next to the IC.

This is necessary for best performance.

ON/OFF

VIN

VOUT

GND

1309 F01

VPP VALID

Figure 1. LT1309 Recommended Layout

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