LT1301CS8 datasheet(6/12 Pages) LINER

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LT1301

APPLICATIONS INFORMATION

IIN

500mA/DIV

VOUT

5V/DIV

VSHDN

10V/DIV

200

µs/DIV

VIN = 5V, VOUT = 12V

LT1300 F5

VOUT

100mV/DIV

AC COUPLED

VSW

10V/DIV

500mA/DIV

µs/DIV

VIN = 5V, VOUT = 12V, L = 33µH

COUT = 33µF, ILOAD = 90mA

LT1300 F4

Figure 4. Burst Mode Operation in Action

Figure 5. Start-Up Response

Output Voltage Selection

The LT1301 can be selected to 5V or 12V 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 5V mode will be slightly less that in 12V mode

due to the fact that the diode drop is a greater percentage

of 5V than 12V. Since the bipolar switch in the LT1301 gets

its base drive from VIN, no reduction in switch efficiency

occurs when in 5V mode. When VIN exceeds the pro-

grammed output voltage the output will follow the input.

This is characteristic of the simple step-up or “boost”

converter topology. A circuit example that provides a

regulated output with an input voltage above or below the

output (known as 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 8

µA to 10µA flows into the pin (at 5V input).

It is recommended that SHDN not be left floating. Tie the

pin to ground if the feature is not used. SHDN can be driven

high even if VIN is floating.

ILIM Function

The LT1301’s current limit (ILIM) pin can be used for soft

start. Upon start-up, the LT1301 will draw maximum

current from the supply (about 1A) from the supply to

charge the output capacitor. Figure 5 shows VOUT and IIN

waveforms as the device is turned on. The high current

flow can create IR drops along supply and ground lines

or cause the input supply to drop out momentarily. By

adding R1 and C3 as shown in Figure 6, the switch

current in the LT1301 is initially limited to 400mA until

the 15

µA flowing out of the ILIM pin charges up C3. Input

current is held to under 500mA while the output voltage

ramps up to 12V as shown in Figure 7. R1 provides a

discharge path for the capacitor without appreciably de-

creasing peak switch current. When using the ILIM pin soft-

start mode a minimum load of a few hundred microam-

peres is recommended to prevent C3 from discharging, as

no current flows out of ILIM when the LT1301 is not

IIN

500mA/DIV

200

µs/DIV

VOUT

5VDIV

VSHDN

10V/DIV

LT1300 F5

VIN = 5V, VOUT = 12V

Figure 7. Startup Response Soft-Start Circuitry Added

Figure 6.

33µF

0.1µF

33µH

1N5817

47µF

ILIM

PGND

GND

SENSE

LT1301

SELECT

LT1301 F6

SHDN

SHUTDOWN

12V

VIN

3.3V OR 5V

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