LT1933HS6 View Datasheet(PDF) - Linear Technology

Part Name

Description

Manufacturer

LT1933HS6

600mA, 500kHz Step-Down Switching Regulator in SOT-23 and DFN Packages

Linear Technology 

LT1933

APPLICATIONS INFORMATION

cases the discharged output capacitor will present a load

to the switcher which will allow it to start. The plots show

the worst-case situation where VIN is ramping very slowly.

For lower start-up voltage, the boost diode can be tied to

VIN; however, this restricts the input range to one-half of

the absolute maximum rating of the BOOST pin.

At light loads, the inductor current becomes discontinu-

ous and the effective duty cycle can be very high. This

reduces the minimum input voltage to approximately

300mV above VOUT. At higher load currents, the inductor

current is continuous and the duty cycle is limited by the

maximum duty cycle of the LT1933, requiring a higher

input voltage to maintain regulation.

Soft-Start

The SHDN pin can be used to soft-start the LT1933, reducing

the maximum input current during start up. The SHDN pin

is driven through an external RC filter to create a voltage

ramp at this pin. Figure 4 shows the start up waveforms

with and without the soft-start circuit. By choosing a large

RC time constant, the peak start up current can be reduced

to the current that is required to regulate the output, with

no overshoot. Choose the value of the resistor so that it

can supply 60μA when the SHDN pin reaches 2.3V.

RUN SHDN

GND

1933 F04a

RUN

5V/DIV

IIN

100mA/DIV

VOUT

5V/DIV

50μs/DIV

RUN

15k

0.1μF

SHDN

GND

1933 F04b

RUN

5V/DIV

IIN

100mA/DIV

VOUT

5V/DIV

0.5ms/DIV

Figure 4. To Soft-Start the LT1933, Add a Resistor and Capacitor to

the SHDN Pin. VINI = 12V, VOUT = 3.3V, COUT = 22μF, RLOAD = 10Ω

1933fe

12

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