LT1108 Datasheet, PDF (4/12 Pages) Linear Technology – Micropower DC/DC Converter Adjustable and Fixed 5V, 12V

LT1108
TYPICAL PERFOR A CE CHARACTERISTICS
Oscillator Frequency
22
21
20
19
18
17
16
15
14
13
–50 –25 0
25 50 75 100
TEMPERATURE (°C)
LT1108 • TPC07
Minimum/Maximum Frequency
vs ON-Time
28
26
24
22
20
18
16
14
12
10
0
25
30
35
40
45
50
ON-TIME (µs)
LT1108 • TPC10
Duty Cycle
80
75
70
65
60
55
50
– 50
–25 0 25 50
TEMPERATURE (°C)
75 100
LT1108 • TPC08
Switch Saturation Voltage
Step-Up Mode
0.8
I_SW = 650mA
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
–50 –25 0
25 50 75 100
TEMPERATURE (°C)
LT1108 • TPC11
Switch-ON Time
44
42
40
38
36
34
32
30
–50 –25
0 25 50
TEMPERATURE (˚C)
75 100
LT1108 • TPC09
Switch Saturation Voltage
Step-Down Mode
1.8
I_SW = 650mA
1.7
1.6
1.5
1.4
1.3
1.2
1.1
1.0
0.9
0.8
–50 –25 0
25 50 75 100
TEMPERATURE (°C)
LT1108 • TPC12
PI FU CTI S
I_LIM (Pin 1): Connect this pin to V_IN for normal use. Where
lower current limit is desired, connect a resistor between
I_LIM and V_IN. A 220Ω resistor will limit the switch current
to approximately 400mA.
V_IN (Pin 2): Input supply voltage.
SW1 (Pin 3): Collector of power transistor. For step-up
mode connect to inductor/diode. For step-down mode
connect to V_IN.
SW2 (Pin 4): Emitter of power transistor. For step-up
mode connect to ground. For step-down mode connect to
inductor/diode. This pin must never be allowed to go more
than a Schottky diode drop below ground.
GND (Pin 5): Ground.
AO (Pin 6): Auxiliary gain block (GB) output. Open collector,
can sink 100µA.
SET (Pin 7): GB input. GB is an op amp with positive input
connected to SET pin and negative input connected to
1.245V reference.
FB/SENSE (Pin 8): On the LT1108 (adjustable) this pin
goes to the comparator input. On the LT1108-5 and
LT1108-12, this pin goes to the internal application resistor
that sets output voltage.
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