LT3684
APPLICATIONS INFORMATION
6.0
5.5
5.0
4.5
4.0
3.5
3.0
V
OUT
TO START
BD
BOOST
V
V
IN
IN
LT3684
GND
C3
SW
4.7µF
TO RUN
V
A
= 3.3V
OUT
T
= 25°C
(4a) For V
> 2.8V
OUT
2.5
2.0
L = 4.7µH
f = 800kHz
0.1
0.01
LOAD CURRENT (A)
0.001
1
10
V
OUT
D2
BD
BOOST
8.0
7.0
6.0
5.0
4.0
3.0
2.0
TO START
V
V
IN
LT3684
IN
C3
SW
GND
4.7µF
TO RUN
(4b) For 2.5V < V
< 2.8V
OUT
V
T
= 5V
OUT
A
V
OUT
= 25°C
L = 4.7µH
f = 800kHz
BD
BOOST
V
0.1
LOAD CURRENT (A)
0.001
0.01
1
10
V
IN
LT3684
IN
C3
3684 F05
SW
GND
4.7µF
Figure 5. The Minimum Input Voltage Depends on
Output Voltage, Load Current and Boost Circuit
3684 FO4
voltage. In many cases the discharged output capacitor
will present a load to the switcher and the minimum input
to start will be the same as the minimum input to run.
(4c) For V
< 2.5V
OUT
Figure 4. Three Circuits For Generating The Boost Voltage
This occurs, for example, if RUN/SS is asserted after V
IN
is applied. The plots show the worst-case situation where
proper start-up, the minimum input voltage is also limited
by the boost circuit. If the input voltage is ramped slowly,
or the LT3684 is turned on with its RUN/SS 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 5 shows a plot
of minimum load to start and to run as a function of input
V is ramping very slowly. For lower start-up voltage, the
IN
boost diode can be tied to V ; however, this restricts the
IN
input range to one-half of the absolute maximum rating
of the BOOST pin.
At light loads, the inductor current becomes discontinu-
ous and the effective duty cycle can be very high. This
reduces the minimum input voltage to approximately
300mV above V . At higher load currents, the inductor
OUT
current is continuous and the duty cycle is limited by the
maximum duty cycle of the LT3684, requiring a higher
input voltage to maintain regulation.
3684f
15
Linear [ Linear ]
