LT3507

14

3507f

maximum duty cycle. The boost circuit also limits the

minimum input voltage for proper start-up. If the input

voltage ramps slowly, or the LT3507 turns on when the

output is already in regulation, the boost capacitor may

not be fully charged. Because the boost capacitor charges

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 current generally

goes to zero once the circuit has started. Figure 4 shows

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

of input voltage. Even without an output load current, in

many cases the discharged output capacitor will present

a load to the switcher that will allow it to start.

The boost current is generally small but can become signifi-

cant at high duty cycles. The required boost current is:

40

Converter with Backup Output Regulator

There is another situation to consider in systems where

the output will be held high when the input to the LT3507

APPLICATIONS INFORMATION

Figure 3. Generating the Boost Voltage

Figure 4. The Minimum Input Voltage Depends on Output Voltage, Load Current and Boost Circuit

BOOST

GND

SW

LT3507

(3a)

D2

C3

BOOST

GND

SW

LT3507

(3b)

D2

C3

3507 F03

D2

BOOST

GND

SW

LT3507

(3c)

C3

LOAD CURRENT (A)

0.001

5.5

5.0

4.0

3.0

4.5

3.5

2.5

3507 F04b

1.000

0.010

0.100

TO START

TO RUN

LOAD CURRENT (A)

0.001

8.0

7.5

6.5

5.5

4.5

7.0

6.0

5.0

4.0

3507 F04a

1.000

0.010

0.100

TO START

TO RUN