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LT3682

3682f

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 discontinuous 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 LT3682, requiring a higher input voltage

to maintain regulation.

Soft-Start

The RUN/SS pin can be used to soft-start the LT3682,

reducing the maximum input current during start-up. The

RUN/SS pin is driven through an external RC network to

create a voltage ramp at this pin. Figure 7 shows the startup

and shut-down waveforms with 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 20μA when the RUN/SS

pin reaches 2.5V.

5ms/DIV

VRUN/SS

5V/DIV

VOUT

5V/DIV

1A/DIV

VRUN

5V/DIV

3682 F07

RUN/SS

RUN

GND

15k

0.22μF

Figure 7. To Soft-Start the LT3682, Add a

Resistor and Capacitor to the RUN/SS Pin

LOAD CURRENT (mA)

4.5

5.5

1000

3.5

2.5

100

LOAD CURRENT(mA)

3682 F06

6.5

7.5

1000

4.5

5.5

3.5

2.5

TO RUN

TO START

(WORST CASE)

VOUT = 3.3V

TA = 25˚C

L = 10μH

f = 800kHz

TO START

(WORST CASE)

TO RUN

VOUT = 5V

TA = 25˚C

L = 10μH

f = 800kHz

Figure 6. The Minimum Input Voltage Depends on

Output Voltage, Load Current and Boost Circuit

is already in regulation, then the boost capacitor may not

be fully charged. Because the boost capacitor is charged

with the energy stored in the inductor, the circuit will rely

on some minimum load current to get the boost circuit

running properly. This minimum load will depend on input

and output voltages, and on the arrangement of the boost

circuit. The minimum load generally goes to zero once the

circuit has started. Figure 6 shows a plot of minimum load

to start and to run as a function of input voltage. In many

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