LT1109 Datasheet, PDF (4/8 Pages) Linear Technology – Micropower Low Cost DC/DC Converter Adjustable and Fixed 5V, 12V

LT110 9
TYPICAL PERFOR A CE CHARACTERISTICS
Minimum/Maximum
Oscillator Frequency vs t_ON
160
150
0°C ≤ T_A ≤ 70°C
140
130
120
T_A = 25°C
110
100
90
80
2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0
T_ON (µs)
LT1109 • TPC08
Quiescent Current
450
T_A = 25°C
400
350
300
250
200
0 2 4 6 8 10 12 14 16 18 20
INPUT VOLTAGE (V)
LT1109 • TPC09
Quiescent Current
400
380
360
340
320
300
280
260
240
220
200
–50 –25 0
25 50 75 100
TEMPERATURE (°C)
LT1109 • TPC10
BLOCK DIAGRA S
LT1109-5, -12 Z Package
V_OUT
1.25V
REFERENCE
R2
250k
SW
+
A1
120kHz
OSCILLATOR
Q1
–
COMPARATOR
DRIVER
R1
GND
LT1109-5: R1 = 83k
LT1109-12: R1 = 29k
LT1109 • TA03
LT1109-5, -12 N8, S8 Package
V_IN
FB SENSE
R2
250k
1.25V
REFERENCE
COMPARATOR
+
A1
–
R1
SW
120kHz
OSCILLATOR
Q1
DRIVER
GND
SHUTDOWN
LT1109 • TA04
ON FIXED VERSION PIN 8 IS SENSE
ON ADJUSTABLE VERSION PIN 8 IS FB AND R1 AND R2 ARE DISCONNECTED
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
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.
4