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 effi ciency. 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-
fi cient 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
V
IN
BOOST
GND
SW
V
IN
LT1933
(2a)
D2
1933 F02a
V
OUT
C3
V
BOOST
– V
SW
≅ V
OUT
MAX V
BOOST
≅ V
IN
+ V
OUT
V
IN
BOOST
GND
SW
V
IN
LT1933
(2b)
D2
1933 F02b
V
OUT
C3
V
BOOST
– V
SW
≅ V
IN
MAX V
BOOST
≅ 2V
IN
LOAD CURRENT (mA)
4.5
INPUT VOLTAGE (V)
5.0
5.5
6.0
1933 F03a
3.0
3.5
4.0
V
OUT
= 3.3V
T
A
= 25°C
L = 22µH
TO START
TO RUN
1 10 100
LOAD CURRENT (mA)
5
INPUT VOLTAGE (V)
6
7
8
1933 F03b
4
V
OUT
= 5V
T
A
= 25°C
L = 33
µH
TO START
TO RUN
1 10 100
Minimum Input Voltage V
OUT
= 3.3V Minimum Input Voltage V
OUT
= 5V
