LT1309 Linear Technology, LT1309 Datasheet - Page 5

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

BLOCK DIAGRA
The LT1309 is a current limited, Burst Mode (gated
oscillator) type switching regulator that produces a 12V
output from a 3.3V or 5V input. Operation can best be
understood by referring to the Block Diagram. When the
voltage at the Sense pin (Pin 7) is less than 12V, compara-
tor C1 enables the oscillator, turning switches Q1 and Q2
on. The switch turns off when current limit is reached or
when the oscillator maximum on-time is reached. When
OPERATIO
U
SENSE
7
864k
4k
100k
V
W
REF
ON/OFF
8
1.24V
POWER ON
–
+
–
+
C2
C1
V
CC
200k
6
VPP VALID
OSCILLATOR
Q2 turns off, current built up in the inductor flows into the
output capacitor and load. This action occurs until the
output voltage reaches 12V. During start-up, the Sense pin
voltage is either 3.3V or 5V, depending on the input. The
VPP VALID signal remains high until the output voltage
reaches 12V, signaling correct voltage level to program
flash memory. Connecting a 1 F/0.1 F parallel RC to the
Soft Start pin limits inrush current during start-up.
500kHz
OFF
C3
+
–
SOFT START
–
36mV
Q3
1
GND
+
5
700
R2
BIAS
V
2
CC
Q2
R1
3
1
PGND
SW
1309 BD
4
3
Q1
50
LT1309
5