LT1300CS8 datasheet(6/8 Pages) LINER

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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 10

µA in this condition.

Bias current of 3

µA 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 15

µA

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.

Figure 4. Addition of R1 and C3 Limit Input Current at Startup

500

µs/DIV

REP RATE = 1Hz

LT1300 F5

VOUT

2VDIV

IBATTERY

500mA/DIV

VSHDN

10V/DIV

100µF

0.1µF

5V/3.3V

OUTPUT

10µH

100µF

ILIM

PGND

GND

SENSE

LT1300

SHDN

SELECT

VIN

Figure 5. Startup Waveforms using Soft-Start Circuitry

ILOAD = 100mA, VOUT = 5V

RLIM (Ω)

500

700

800

1000

1100

100

10k

100k

LT1300 F1B

300

900

600

400

1.6V

≤ VIN ≤ 5V

Figure 6. Peak Switch Current vs. RLIM

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