LT1308B Linear Technology, LT1308B Datasheet - Page 6

• Manufacturer Part Number: LT1308B
• Description: High Current/ Micropower Single Cell/ 600kHz DC/DC Converters
• Manufacturer: Linear Technology

LT1308
APPLICATIONS
distance from the circuit, an additional input capacitor may
be required. A 220 F aluminum electrolytic unit works well
in these cases. This capacitor need not have low ESR.
OPERATION FROM A LABORATORY POWER SUPPLY
If a lab supply is used, the leads used to connect the circuit
to the supply can have significant inductance at the
LT1308’s switching frequency. As in the previous situa-
tion, an electrolytic capacitor may be required at the circuit
in order to reduce the AC impedance of the input suffi-
ciently. An alternative solution is to attach the circuit
directly to the power supply at the supply terminals,
without the use of leads. The power supply’s output
capacitance will then provide the bulk capacitance the
LT1308 circuit requires.
SHUTDOWN PIN
The LT1308 has a shutdown pin (SHDN) that must be
grounded to shut the device down or tied to a voltage equal
or greater than V
shown in Figure 3.
Note that allowing SHDN to float turns on both the start-
up current (Q2) and the shutdown current (Q3) for V
2V
and behaves erratically. SHDN voltage above V
lowed. This merely reverse-biases Q3’s base emitter junc-
tion, a benign condition.
6
LOW-BATTERY DETECTOR
The LT1308’s low-battery detector is a simple PNP input
gain stage with an open collector NPN output. The nega-
BE
. The LT1308 doesn’t know what to do in this situation
SHDN
Figure 3. Shutdown Circuit
IN
U
to operate. The shutdown circuit is
INFORMATION
Q1
R2
400k
400k
U
V
IN
Q3
Q2
SHUTDOWN
CURRENT
START-UP
CURRENT
1308 F03
W
U
IN
is al-
IN
>
GSM PHONES
The LT1308 is suitable for converting a single Li-Ion cell
to 5V for powering GSM RF power stages. Figure 6 details
a Li-Ion to 5V converter circuit using frequency compen-
sation optimized for a typical GSM pulsed load. Figure 7
details transient response of Figure 6’s circuit with a
100mA to 1A pulsed load. A slower time sweep is used to
show several transmit pulses in Figure 8. At a V
additional output capacitance is recommended to help
minimize V
voltage of 2.7V. Figure 10 expands the horizontal sweep
speed to 500 s/division to show detail of one transmit
pulse.
tive input of the gain stage is tied internally to a 200mV
reference. The positive input is the LBI pin. Arrangement
as a low-battery detector is straightforward. Figure 4
details hookup. R1 and R2 need only be low enough in
value so that the bias current of the LBI pin doesn’t cause
large errors. For R2, 100k is adequate. The 200mV refer-
ence can also be accessed as shown in Figure 5.
V
BAT
V
BAT
Figure 4. Setting Low-Battery Detector Trip Point
R1
R2
100k
OUT
Figure 5. Accessing 200mV Reference
200mV
V
REF
2N3906
droop. Figure 9 shows V
LBI
GND
200mV
INTERNAL
REFERENCE
10k
+
–
V
+
IN
200k
10 F
LT1308
LBO
1308 F04
LBO
LBI
LT1308
3.3V
GND
V
OUT
IN
1M
1308 F05
R1 =
with an input
V
TO PROCESSOR
LB
IN
– 200mV
2 A
of 2.7V,