LT1309 Linear Technology, LT1309 Datasheet - Page 6

• Manufacturer Part Number: LT1309
• Description: 500kHz Micropower DC/DC Converter for Flash Memory
• Manufacturer: Linear Technology

LT1309
APPLICATIONS
Inductor Selection
The low inductance value required with the LT1309 (10 H)
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 F or 2 F 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 F ceramic units. Lower ripple can be
6
U
INFORMATION
U
V
OUT
V
IN
W
L1
Figure 1. LT1309 Recommended Layout
D1
U
C1
1
2
3
4
C2
R1
C3
obtained by increasing the capacitance to the 5 F to 10 F
level. 2 F to 5 F 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.
GND
8
7
6
5
VPP VALID
1309 F01
ON/OFF