LT1933 Datasheet, PDF (10/16 Pages) Linear Technology – 600mA, 500kHz Step-Down Switching Regulator in SOT-23

LT1933
APPLICATIO S I FOR ATIO
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
D2
BOOST
C3
LT1933
V_IN
V_IN
SW
GND
V_BOOST – V_SW ≅ V_OUT
MAX V_BOOST ≅ V_IN + V_OUT
(2a)
D2
V_OUT
1933 F02a
BOOST
C3
LT1933
V_IN
V_IN
SW
V_OUT
GND
V_BOOST – V_SW ≅ V_IN
MAX V_BOOST ≅ 2V_IN
(2b)
1933 F02b
Figure 2. Two Circuits for Generating the Boost Voltage
10
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 V_IN is ramping very slowly.
For lower start-up voltage, the boost diode can be tied to
V_IN; however, this restricts the input range to one-half of
the absolute maximum rating of the BOOST pin.
Minimum Input Voltage V_OUT = 3.3V
6.0
5.5
TO START
5.0
V_OUT = 3.3V
T_A = 25°C
L = 22µH
4.5
4.0
3.5
TO RUN
3.0
1
10
100
LOAD CURRENT (mA)
1933 F03a
Minimum Input Voltage V_OUT = 5V
8
V_OUT = 5V
T_A = 25°C
L = 33µH
7
TO START
6
TO RUN
5
4
1
10
100
LOAD CURRENT (mA)
1933 F03b
Figure 3. The Minimum Input Voltage Depends
on Output Voltage, Load Current and Boost Circuit
1933f