Datasheet

6
LT1300
APPLICATIONS INFORMATION
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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 V
IN
respectively. It is permissible to tie SELECT to a voltage
higher than V
IN
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 V
IN
, no reduction in switch
efficiency occurs when in 3.3V mode. When V
IN
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.
I
LIM
Function
The LT1300’s current limit (I
LIM
) 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 I
LIM
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.
Figure 4. Addition of R1 and C3 Limit Input Current at Startup
500µs/DIV
REP RATE = 1Hz
LT1300 F5
V
OUT
2VDIV
I
BATTERY
500mA/DIV
V
SHDN
10V/DIV
C2
100µF

C3
0.1µF

R1
1M
5V/3.3V
OUTPUT
+
+
L1
10µH
D1
C1
100µF
SW
I
LIM
PGND GND
SENSE
LT1300
SHDN
SELECT
V
IN
Figure 5. Startup Waveforms using Soft-Start Circuitry
I
LOAD
= 100mA, V
OUT
= 5V
R
LIM
(Ω)
500
SWITCH CURRENT (mA)
700
800
1000
1100
100 10k 100k 1M
LT1300 F1B
300
1k
900
600
400
1.6V ≤ V
IN
≤ 5V
Figure 6. Peak Switch Current vs. R
LIM