Datasheet

LT3507
14
3507fb
For more information www.linear.com/LT3507
peak current, duty cycle and boost voltage; in general a
value of (0.1µF 1MHz/f
SW
) works well. Figure 3 shows
three ways to arrange the boost circuit. The BOOST pin
must be more than 2.5V above the SW pin for full ef
-
ficiency. For outputs of 3.3V and higher, the standard
circuit (Figure 3a) is best. For outputs between 2.8V and
3.3V, use a small Schottky diode (such as the BAT54).
For lower output voltages, the boost diode can be tied
to the input (Figure 3b). The circuit in Figure 3a is more
efficient because the BOOST pin current comes from a
lower voltage source. Finally, as shown in Figure 3c, the
anode of the boost diode can be tied to another source
that is at least 3V. For example, if you are generating 3.3V
and 1.8V and the 3.3V is on whenever the 1.8V is on, the
1.8V boost diode can be connected to the 3.3V output. In
this case, the 3.3V output cannot be set to track the 1.8V
output (see Output Voltage Tracking).
In any case, be sure that the maximum voltage at the
BOOST pin is less than 55V and the voltage difference
between the BOOST and SW pins is less than 25V.
The minimum operating voltage of an LT3507 applica
-
tion is limited by the internal undervoltage lockout (4V
for Channel 1, 3V
for Channels 2 and 3) and by the
maximum duty cycle. The boost circuit also limits the
minimum input voltage for proper start-up. If the input
voltage ramps slowly, or the LT3507 turns on when 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
applications inForMation
Figure 3. Generating the Boost Voltage
V
IN
BOOST
GND
SW
V
IN
LT3507
(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
LT3507
(3b)
D2
V
OUT
C3
V
BOOST
– V
SW
V
IN
MAX V
BOOST
2V
IN
3507 F03
D2
V
IN
BOOST
GND
SW
V
IN
LT3507
(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
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