11

LT1933

1933f

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

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.

Shorted and Reversed Input Protection

If the inductor is chosen so that it won’t saturate exces-

sively, an LT1933 buck regulator will tolerate a shorted

output. There is another situation to consider in systems

where the output will be held high when the input to the

LT1933 is absent. This may occur in battery charging

applications or in battery backup systems where a battery

or some other supply is diode OR-ed with the LT1933’s

is held high (either by a logic signal or because it is tied to

quiescent current through its SW pin. This is fine if your

system can tolerate a few mA in this state. If you ground

the SHDN pin, the SW pin current will drop to essentially

is held high, then parasitic diodes inside the LT1933 can

APPLICATIO S I FOR ATIO

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

RUN

5V/DIV

100mA/DIV

5V/DIV

RUN

SHDN

GND

1933 F04a

RUN

5V/DIV

100mA/DIV

5V/DIV

RUN

15k

0.1µF

SHDN

GND

1933 F04b

0.5ms/DIV

50µs/DIV