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LT3502/LT3502A

3502fc

APPLICATIONS INFORMATION

Figure 6

(6a) LT3502A Typical Minimum Input Voltage, VOUT = 3.3V

(6b) LT3502A Typical Minimum Input Voltage, VOUT = 5V

(6c) LT3502 Typical Minimum Input Voltage, VOUT = 3.3V

(6d) LT3502 Typical Minimum Input Voltage, VOUT = 5V

The minimum operating voltage of an LT3502/LT3502A

application is limited by the undervoltage lockout (3V) and

by the maximum duty cycle as outlined above. For proper

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

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

LT3502/LT3502A is turned on with its SHDN 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 the 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 plots 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 which will allow it to

start. The plots show the worst-case situation where VIN

is ramping very slowly. 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 400mV above VOUT. At higher load currents,

the inductor current is continuous and the duty cycle is

limited by the maximum duty cycle of the LT3502/LT3502A,

requiring a higher input voltage to maintain regulation.

LOAD CURRENT (A)

0.001

3502 G19

0.01

0.1

RUN

START

LOAD CURRENT (A)

0.001

0.01

0.1

3502 G20

START

RUN

LOAD CURRENT (A)

0.001

3502 G21

0.01

0.1

RUN

START

LOAD CURRENT (A)

0.001

0.01

0.1

3502 G22

START

RUN

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