Datasheet

4
LT1108
CCHARA TERIST
ICS
UW
AT
Y
P
I
CA
LPER
F
O
R
C
E
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.
Oscillator Frequency Duty Cycle Switch-ON Time
TEMPERATURE (°C)
13
FREQUENCY (kHz)
14
16
17
18
22
LT1108 • TPC07

15
19
20
21
–50
25
75
–25 0 50
100
TEMPERATURE (°C)
50
DUTY CYCLE (%)
55
60
65
70
80
LT1108 • TPC08
75
–50
25
75
–25 0 50
100
Minimum/Maximum Frequency Switch Saturation Voltage Switch Saturation Voltage
vs ON-Time Step-Up Mode Step-Down Mode
PI
U
FU
U
C
U
S
O
TI
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.
TEMPERATURE (˚C)
–50

38
40
42
25 75
LT1108 • TPC09
36
34
–25 0
50 100
32
30
SWITCH-ON TIME (µs)
44
TEMPERATURE (°C)
–50
V
CESAT
(V)
0.3
0.4
0.5
25
75
LT1108 • TPC11
0.2
0.1
0
–25 0 50
0.6
0.7
0.8
100
I
SW
= 650mA
TEMPERATURE (°C)
–50
0.8
V
SAT
(V)
1.0
1.1
1.2
1.3
1.4
1.5
–25
0
25 50
LT1108 • TPC12
75
1.6
1.7
1.8
0.9
100
I
SW
= 650mA
ON-TIME (µs)
25
FREQUENCY (kHz)
20
24
28
45
LT1108 • TPC10
16
12
0
30
35
40
50
18
22
26
14
10