LT3506/LT3506A
U U
W
APPLICATIO S I FOR ATIO
The minimum input voltage of an LT3506 application is
limited by the minimum operating voltage (<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 LT3506 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. The plots below show
the minimum load current to start and to run as a function
of input voltage for 3.3V and 5V outputs. 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 is ramping very slowly.
IN
Use a Schottky diode (such as the BAT-54) for the lowest
start-up voltage.
Minimum Input Voltage,
VOUT = 3.3V (LT3506A)
Minimum Input Voltage,
VOUT = 5V (LT3506A)
5.5
7.0
T
= 25°C
BOOST
T = 25°C
A
V
TO START
A
IN
D
= 1N5817
D
= 1N5817
BOOST
V
TO START
IN
6.5
6.0
5.5
5.0
4.5
5.0
4.5
4.0
3.5
3.0
BOOST DIODE
BOOST DIODE
TIED TO OUTPUT
TIED TO OUTPUT
V
TO RUN
IN
V
TO RUN
IN
BOOST DIODE
TIED TO INPUT
BOOST DIODE
TIED TO INPUT
0.001
0.01
0.1
1
0.001
0.01
0.1
1
I
(A)
I
(A)
LOAD
LOAD
3506 G14
3506 G15
3506afb
ꢀꢂ
Linear [ Linear ]
