LT1109
U W
TYPICAL PERFOR A CE CHARACTERISTICS
Minimum/Maximum
Oscillator Frequency vs tON
Quiescent Current
Quiescent Current
450
400
350
300
250
200
160
150
140
130
120
110
100
90
400
380
360
340
320
300
280
260
240
220
200
T
= 25°C
0°C ≤ T ≤ 70°C
A
A
T
= 25°C
A
80
0
2
4
6
8
10 12 14 16 18 20
2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0
–50 –25
0
25
50
75
100
INPUT VOLTAGE (V)
T
(µs)
TEMPERATURE (°C)
ON
LT1109 • TPC09
LT1109 • TPC08
LT1109 • TPC10
W
BLOCK DIAGRA S
LT1109-5, -12 N8, S8 Package
LT1109-5, -12 Z Package
V
OUT
V
FB SENSE
IN
R2
250k
SW
R2
250k
COMPARATOR
A1
SW
1.25V
REFERENCE
+
–
1.25V
REFERENCE
+
–
120kHz
OSCILLATOR
Q1
120kHz
OSCILLATOR
A1
Q1
DRIVER
DRIVER
COMPARATOR
R1
R1
LT1109 • TA04
GND
SHUTDOWN
LT1109-5: R1 = 83k
LT1109-12: R1 = 29k
ON FIXED VERSION PIN 8 IS SENSE
ON ADJUSTABLE VERSION PIN 8 IS FB AND R1 AND R2 ARE DISCONNECTED
GND
LT1109 • TA03
U
LT1109Z OPERATIO
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
VOUT pin, a technique known as “bootstrapping.” Circuit
operation can be best understood by referring to the block
diagram. VOUT, 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 outputvoltage. WhenA1’s nega-
tive input rises above 1.25V, it turns off the oscillator. A
small amount of hysteresis in A1 obviates the need for
frequencycompensationcircuitry.WhenQ1isoff,current
into the VOUT pin drops to just 320µA. Quiescent current
from the battery will be higher because the device oper-
ates off the stepped-up voltage.
4
Linear [ Linear ]
