Datasheet

LT3991/LT3991-3.3/LT3991-5
15
3991fa
applicaTions inForMaTion
while using the internal boost diode. For reliable BOOST pin
operation with 2.5V outputs use a good external Schottky
diode (such as the ON Semi MBR0540), and a 1μF boost
capacitor (Figure 3b). For output voltages below 2.5V,
the boost diode can be tied to the input (Figure 3c), or to
another external supply greater than 2.8V. However, the
circuit in Figure 3a is more efficient because the BOOST pin
current comes from a lower voltage source. You must also
be sure that the maximum voltage ratings of the BOOST
and BD pins are not exceeded.
The minimum operating voltage of an LT3991 application
is limited by the minimum input voltage (4.3V) and by
the maximum duty cycle as outlined in the Input Voltage
Range section. For proper start-up, the minimum input
voltage is also limited by the boost circuit. If the input
voltage is ramped slowly, 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 4 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.
V
IN
BOOST
SW
BD
V
IN
V
OUT
4.7µF
C3
GND
LT3991
V
IN
BOOST
SW
BD
V
IN
V
OUT
4.7µF
C3
D2
GND
LT3991
V
IN
BOOST
SW
BD
V
IN
V
OUT
4.7µF
C3
GND
LT3991
3991 FO3
(3a) For V
OUT
> 2.8V
(3b) For 2.5V < V
OUT
< 2.8V
(3c) For V
OUT
< 2.5V; V
IN(MAX)
= 27V
Figure 4. The Minimum Input Voltage Depends on
Output Voltage, Load Current and Boost Circuit
3991 F04
LOAD CURRENT (mA)
10
INPUT VOLTAGE (V)
4.0
4.4
4.2
4.6
1000
3.6
3.8
3.4
3.0
100
10
1000
100
3.2
5.0
4.8
LOAD CURRENT (mA)
INPUT VOLTAGE (V)
5.8
6.0
6.2
5.6
5.0
5.4
5.2
6.4
TO RUN
V
OUT
= 3.3V
T
A
= 25°C
L = 10µH
f = 400kHz
V
OUT
= 5V
T
A
= 25°C
L = 10µH
f = 400kHz
TO START
TO RUN
TO START
Figure 3. Three Circuits for Generating the Boost Voltage