Datasheet

LT1109
4
W
IDAGRA
B
L
O
C
K
S
LT1109-5, -12 Z Package
The LT1109Z-5 and LT1109Z-12 are fixed output voltage
step-up DC/DC converters in a 3-pin TO-92 package.
Power for internal regulator circuitry is taken from the
V
OUT
pin, a technique known as “bootstrapping.” Circuit
operation can be best understood by referring to the block
diagram. V
OUT
, attenuated by R1 and R2, is applied to the
negative input of comparator A1. When this voltage falls
below the 1.25V reference voltage, the oscillator is turned
on and the power switch Q1 cycles at the oscillator
frequency of 120kHz. Switch cycling alternately builds
current in the inductor, then dumps it into the output
capacitor, increasing the output voltage. When A1’s nega-
tive input rises above 1.25V, it turns off the oscillator. A
small amount of hysteresis in A1 obviates the need for
frequency compensation circuitry. When Q1 is off, current
into the V
OUT
pin drops to just 320µA. Quiescent current
from the battery will be higher because the device oper-
ates off the stepped-up voltage.
LT1109-5, -12 N8, S8 Package
CCHARA TERIST
ICS
UW
AT
Y
P
I
CA
LPER
F
O
R
C
E
LT1109Z
U
ATIOOPER
Minimum/Maximum
Oscillator Frequency vs t
ON
Quiescent Current Quiescent Current
INPUT VOLTAGE (V)
0
200
QUIESCENT CURRENT (µA)
300
450
20
LT1109 • TPC09
210
400
18
250
4 8 12 14
350
T
A
= 25°C
616
TEMPERATURE (°C)
–50
200
QUIESCENT CURRENT (µA)
280
400
100
LT1109 • TPC10
25
360
75
220
–25 0 50
320
240
260
300
340
380
IN
V
LT1109 • TA04
GND
1.25V
REFERENCE
A1
DRIVER
SW
120kHz
OSCILLATOR
COMPARATOR
R2
250k
R1
+
–
SENSE
SHUTDOWN
Q1
FB
ON FIXED VERSION PIN 8 IS SENSE
ON ADJUSTABLE VERSION PIN 8 IS FB AND R1 AND R2 ARE DISCONNECTED
OUT
V
LT1109 • TA03
GND
1.25V
REFERENCE
A1
DRIVER
SW
120kHz
OSCILLATOR
COMPARATOR
R2
250k
R1
+
–
Q1
LT1109-5: R1 = 83k
LT1109-12: R1 = 29k

T
ON
(µs)
2.0
80
OSCILLATOR FREQUENCY (kHz)
110
160
6.0
LT1109 • TPC08
2.5 4.0
140
120
5.5
150
90
3.0 3.5 4.5 5.0
130
100
T
A
= 25°C
0°C ≤ T
A
≤ 70°C