LT1308A/LT1308B
PIN FUNCTIONS
V
C
(Pin 1):
Compensation Pin for Error Amplifier. Con-
nect a series RC from this pin to ground. Typical values
are 47kΩ and 100pF. Minimize trace area at V
C
.
FB (Pin 2):
Feedback Pin. Reference voltage is 1.22V.
Connect resistive divider tap here. Minimize trace area at
FB. Set V
OUT
according to: V
OUT
= 1.22V(1 + R1/R2).
SHDN (Pin 3):
Shutdown. Ground this pin to turn off
switcher. To enable, tie to 1V or more. SHDN does not
need to be at V
IN
to enable the device.
GND (Pin 4):
Ground. Connect directly to local ground
plane. Ground plane should enclose all components
associated with the LT1308. PCB copper connected to
Pin 4 also functions as a heat sink. Maximize this area to
keep chip heating to a minimum.
SW (Pin 5):
Switch Pin. Connect inductor/diode here.
Minimize trace area at this pin to keep EMI down.
V
IN
(Pin 6):
Supply Pin. Must have local bypass capacitor
right at the pin, connected directly to ground.
LBI (Pin 7):
Low-Battery Detector Input. 200mV refer-
ence. Voltage on LBI must stay between –100mV and 1V.
Low-battery detector does not function with SHDN pin
grounded. If not used, float LBI pin.
LBO (Pin 8):
Low-Battery Detector Output. Open collec-
tor, can sink 50µA. A 1MΩ pull-up is recommended. LBO
is high impedance when SHDN is grounded.
BLOCK DIAGRAM
V
IN
6
R5
40k
V
OUT
R1
(EXTERNAL)
FB
R2
(EXTERNAL)
FB
2
R4
140k
RAMP
GENERATOR
600kHz
OSCILLATOR
*HYSTERESIS IN LT1308A ONLY
Figure 2. LT1308A/LT1308B Block Diagram
+
+
Σ
+
–
W
U
U
U
V
IN
Q4
2V
BE
SHDN
V
IN
R6
40k
+
g
m
V
C
1
LBI
ERROR
AMPLIFIER
BIAS
SHUTDOWN
3
–
Q1
Q2
×10
R3
30k
+
*
ENABLE
200mV
7
+
–
A4
LBO
8
–
A1
COMPARATOR
SW
FF
R
Q
S
DRIVER
5
Q3
A2
+
A=3
0.03Ω
–
4
GND
1308 BD
5
LINER [ LINEAR TECHNOLOGY ]
