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LT3684

3684f

proper start-up, the minimum input voltage is also limited

by the boost circuit. If the input voltage is ramped slowly,

or the LT3684 is turned on with its RUN/SS pin when the

output 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 5 shows a plot

of minimum load to start and to run as a function of input

voltage. In many cases the discharged output capacitor

will present a load to the switcher and the minimum input

to start will be the same as the minimum input to run.

This occurs, for example, if RUN/SS is asserted after VIN

is applied. 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 LT3684, requiring a higher

input voltage to maintain regulation.

Figure 4. Three Circuits For Generating The Boost Voltage

Figure 5. The Minimum Input Voltage Depends on

Output Voltage, Load Current and Boost Circuit

APPLICATIONS INFORMATION

VIN

BOOST

VIN

VOUT

4.7

µF

GND

LT3684

VIN

BOOST

VIN

VOUT

4.7

µF

GND

LT3684

VIN

BOOST

VIN

VOUT

4.7

µF

GND

LT3684

3684 FO4

(4a) For VOUT > 2.8V

(4b) For 2.5V < VOUT < 2.8V

(4c) For VOUT < 2.5V

3684 F05

LOAD CURRENT (A)

0.001

4.0

4.5

5.0

3.5

3.0

2.0

0.01

0.1

2.5

6.0

5.5

TO START

TO RUN

VOUT = 3.3V

TA = 25°C

L = 4.7

µH

f = 800kHz

LOAD CURRENT (A)

0.001

5.0

6.0

7.0

4.0

2.0

0.01

0.1

3.0

8.0

TO START

TO RUN

VOUT = 5V

TA = 25°C

L = 4.7

µH

f = 800kHz

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