LT1300CS8 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LT1300CS8' PDF : 8 Pages View PDF
LT1300
APPLICATIONS INFORMATION
Output Voltage Selection
The LT1300 can be selected to 3.3V or 5V under logic
control or fixed at either by tying SELECT to ground or VIN
respectively. It is permissible to tie SELECT to a voltage
higher than VIN as long as it does not exceed 10V.
Efficiency in 3.3V mode will be slightly less that in 5V mode
due to the fact that the diode drop is a greater percentage
of 3.3V than 5V. Since the bipolar switch in the LT1300
gets its base drive from VIN, no reduction in switch
efficiency occurs when in 3.3V mode. When VIN exceeds
the programmed output voltage the output will follow the
input. This is characteristic of the simple step-up or
“boost” converter topology. A circuit example that pro-
vides a regulated output with an input voltage above or
below the output (called a buck-boost or SEPIC) is shown
in the Typical Applications section.
Shutdown
The converter can be turned off by pulling SHDN (pin 3)
high. Quiescent current drops to 10µA in this condition.
Bias current of 3µA to 5µA flows into the pin (at 2.5V input).
It is recommended that SHDN not be left floating. Tie the
pin to ground if the feature is not used.
ILIM Function
The LT1300’s current limit (ILIM) pin can be used for soft
start. Upon start-up, switching regulators require maxi-
mum current from the supply. The high currents flowing
can create IR drops along supply and ground lines and
are especially demanding on alkaline batteries. By in-
stalling an R1 and C3 as shown in Figure 4, the switch
current in the LT1300 is limited to 400mA until the 15µA
flowing out of the ILIM pin charges up the 0.1µF capaci-
tor. Input current is held to under 500mA while the
output voltage ramps up to 5V as shown in Figure 5. The
1Meg resistor provides a discharge path for the capacitor
without appreciably decreasing peak switch current. When
the full capability of the LT1300 is not required, peak
current can be reduced by changing the value of R3 as
shown in Figure 6. With R3 = 0, switch current is limited
to approximately 400mA.
L1
10µH
D1
VIN
SW
+ C1
SELECT
SENSE
100µF
LT1300
SHDN
PGND
ILIM
GND
R1
1M
5V/3.3V
OUTPUT
+
C3
0.1µF
C2
100µF
Figure 4. Addition of R1 and C3 Limit Input Current at Startup
VOUT
2VDIV
IBATTERY
500mA/DIV
VSHDN
10V/DIV
500µs/DIV
REP RATE = 1Hz
LT1300 F5
Figure 5. Startup Waveforms using Soft-Start Circuitry
ILOAD = 100mA, VOUT = 5V
1100
1.6V ≤ VIN ≤ 5V
1000
900
800
700
600
500
400
300
100
1k
10k
RLIM (Ω)
100k
1M
LT1300 F1B
Figure 6. Peak Switch Current vs. RLIM
6
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