Datasheet

10
LT1616
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 0.01µF capacitor and fast switching diode (such as the
1N4148 or 1N914) will work well. Figure 5 shows two
ways to arrange the boost circuit. The BOOST pin must be
more than 2.5V above the SW pin for best efficiency. For
outputs of 3.3V and above, the standard circuit (Figure 5a)
is best. For outputs between 2.8V and 3.3V, use a 0.033µF
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 5b). The circuit in Figure 5a 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 LT1616 application
is limited by the undervoltage lockout (<3.6V) and by the
maximum duty cycle as outlined above. For proper start-
up, the minimum input voltage is also limited by the boost
circuit. If the input voltage is ramped slowly, or the LT1616
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 6 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.
Use a Schottky diode (such as the BAT-54) for the lowest
start-up voltage.
APPLICATIO S I FOR ATIO
WUUU
V
IN
BOOST
GND
SW
V
IN
LT1616
(5a)
D2
1616 F05a
V
OUT
C3
V
BOOST
– V
SW
≅ V
OUT
MAX V
BOOST
≅ V
IN
+ V
OUT
V
IN
BOOST
GND
SW
V
IN
LT1616
(5b)
D2
1616 F05b
V
OUT
C3
V
BOOST
– V
SW
≅ V
IN
MAX V
BOOST
≅ 2V
IN
Figure 5. Two Circuits for Generating the Boost Voltage Figure 6. The Minimum Input Voltage Depends
on Output Voltage, Load Current and Boost Circuit
Minimum Input Voltage V
OUT
= 3.3V
Minimum Input Voltage V
OUT
= 5V
LOAD CURRENT (mA)
1
INPUT VOLTAGE (V)
6
7
10 100 500
1616 F06a
5
4
3
BOOST DIODE
TIED TO OUTPUT
V
OUT
= 3.3V
D
BOOST
= BAT54
BOOST DIODE
TIED TO INPUT
V TO START
V TO RUN
LOAD CURRENT (mA)
1
INPUT VOLTAGE (V)
7
8
9
10 100 500
1616 F06b
6
5
4
BOOST DIODE
TIED TO OUTPUT
V
OUT
= 5V
D
BOOST
= BAT54
V TO START
V TO RUN
BOOST DIODE
TIED TO INPUT
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