LT1933

11

1933fe

APPLICATIONS INFORMATION

BOOST Pin Considerations

Capacitor C3 and diode D2 are used to generate a boost

voltage that is higher than the input voltage. In most cases

a 0.1μF capacitor and fast switching diode (such as the

1N4148 or 1N914) will work well. Figure 2 shows two

ways to arrange the boost circuit. The BOOST pin must

be at least 2.3V above the SW pin for best efficiency. For

outputs of 3V and above, the standard circuit (Figure 2a)

is best. For outputs between 2.5V and 3V, use a 0.47μF

capacitor and a small Schottky diode (such as the BAT-54).

For lower output voltages the boost diode can be tied to

the input (Figure 2b). The circuit in Figure 2a is more ef-

ficient because the BOOST pin current comes from a lower

voltage source. You must also be sure that the maximum

voltage rating of the BOOST pin is not exceeded.

The minimum operating voltage of an LT1933 application

is limited by the undervoltage lockout (~3.35V) and by

the maximum duty cycle as outlined above. For proper

startup, the minimum input voltage is also limited by the

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

LT1933 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 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 3 shows a plot of minimum load

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

Figure 2. Two Circuits for Generating the Boost Voltage

Figure 3. The Minimum Input Voltage Depends on Output Voltage, Load Current and Boost Circuit

BOOST

GND

SW

LT1933

(2a)

D2

1933 F02a

C3

BOOST

GND

SW

LT1933

(2b)

D2

1933 F02b

C3

LOAD CURRENT (mA)

4.5

5.0

5.5

6.0

1933 F03a

3.0

3.5

4.0

L = 22μH

TO START

TO RUN

1

10

100

LOAD CURRENT (mA)

5

6

7

8

1933 F03b

4

L = 33μH

TO START

TO RUN

1

10

100