LT1308 データシートの表示（PDF） - Linear Technology
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LT1308 Datasheet PDF : 8 Pages
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LT1308
APPLICATIONS INFORMATION
distance from the circuit, an additional input capacitor may
be required. A 220µF 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 VIN to operate. The shutdown circuit is
shown in Figure 3.
Note that allowing SHDN to float turns on both the start-
up current (Q2) and the shutdown current (Q3) for VIN >
2VBE. The LT1308 doesn’t know what to do in this situation
and behaves erratically. SHDN voltage above VIN is al-
lowed. This merely reverse-biases Q3’s base emitter junc-
tion, a benign condition.
VIN
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.
VBAT
R1
LBI
R2
100k
VIN
+
–
LT1308
LBO
3.3V
1M
TO PROCESSOR
200mV
INTERNAL
REFERENCE
GND
1308 F04
R1 = VLB – 200mV
2µA
Figure 4. Setting Low-Battery Detector Trip Point
VBAT
200k
2N3906
VREF
200mV
+
10k
10µF
VIN
LBO
LT1308
LBI
GND
1308 F05
Figure 5. Accessing 200mV Reference
SHDN
R2
400k
400k
Q1
Q3
SHUTDOWN
CURRENT
START-UP
CURRENT
Q2
1308 F03
Figure 3. Shutdown Circuit
LOW-BATTERY DETECTOR
The LT1308’s low-battery detector is a simple PNP input
gain stage with an open collector NPN output. The nega-
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 VIN of 2.7V,
additional output capacitance is recommended to help
minimize VOUT droop. Figure 9 shows VOUT with an input
voltage of 2.7V. Figure 10 expands the horizontal sweep
speed to 500µs/division to show detail of one transmit
pulse.
6
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