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LT1933

1933f

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

efficient 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

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.

APPLICATIO S I FOR ATIO

VIN

BOOST

GND

VIN

LT1933

(2a)

1933 F02a

VOUT

VBOOST – VSW ≅ VOUT

MAX VBOOST ≅ VIN + VOUT

VIN

BOOST

GND

VIN

LT1933

(2b)

1933 F02b

VOUT

VBOOST – VSW ≅ VIN

MAX VBOOST ≅ 2VIN

Figure 2. Two Circuits for Generating the Boost Voltage

Figure 3. The Minimum Input Voltage Depends

on Output Voltage, Load Current and Boost Circuit

Minimum Input Voltage VOUT = 3.3V

Minimum Input Voltage VOUT = 5V

LOAD CURRENT (mA)

4.5

5.0

5.5

6.0

1933 F03a

3.0

3.5

4.0

VOUT = 3.3V

TA = 25°C

L = 22µH

TO START

TO RUN

100

LOAD CURRENT (mA)

1933 F03b

VOUT = 5V

TA = 25°C

L = 33µH

TO START

TO RUN

100

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