Datasheet

LT3507A
14
3507af
the output is already in regulation, the boost capacitor
may not be fully charged. Because the boost capacitor
charges 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 current 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. Even without an output load current, in
many cases the discharged output capacitor will present
a load to the switcher that will allow it to start.
The boost current is generally small but can become sig-
nificant at high duty cycles. The required boost current is:
I
BOOST
=
V
OUT
V
IN






I
OUT
40






Converter with Backup Output Regulator
There is another situation to consider in systems where
the output will be held high when the input to the LT3507A
is absent. If the V
IN
and one of the RUN pins are allowed
to float, then the LT3507A’s internal circuitry will pull its
quiescent current through its SW pin. This is acceptable
APPLICATIONS INFORMATION
Figure 3. Generating the Boost Voltage
Figure 4. The Minimum Input Voltage Depends on Output Voltage, Load Current and Boost Circuit
V
IN
BOOST
GND
SW
V
IN
LT3507A
(3a)
D2
V
OUT
C3
V
BOOST
– V
SW
≅ V
OUT
MAX V
BOOST
≅ V
IN
+ V
OUT
V
IN
BOOST
GND
SW
V
IN
LT3507A
(3b)
D2
V
OUT
C3
V
BOOST
– V
SW
≅ V
IN
MAX V
BOOST
≅ 2V
IN
3507A F03
D2
V
IN
BOOST
GND
SW
V
IN
LT3507A
(3c)
V
OUT
V
BOOST
– V
SW
≅ V
INB
MAX V
BOOST
≅ V
INB
+ V
IN
MINIMUM VALUE FOR V
INB
=
3V
V
INB
> 3V
C3
LOAD CURRENT (A)
0.001
INPUT VOLTAGE (V)
5.5
5.0
4.0
3.0
4.5
3.5
2.5
3507 F04b
1.0000.010 0.100
T
A
= 25°C
V
OUT
= 3.3V
TO START
TO RUN
LOAD CURRENT (A)
0.001
INPUT VOLTAGE (V)
8.0
7.5
6.5
5.5
4.5
7.0
6.0
5.0
4.0
3507A F04a
1.0000.010 0.100
T
A
= 25°C
V
OUT
= 5V
TO START
TO RUN