Datasheet

LT3506/LT3506A
12
3506afc
BOOST Pin Considerations
The capacitor and diode tied to the BOOST pin generate
a voltage that is higher than the input voltage. In most
cases a 0.1F capacitor and fast switching diode (such
as the CMDSH-3 or FMMD914) will work 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
effi ciency. 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 BAT-54).
For lower output voltages the boost diode can be tied to
the input (Figure 3b). The circuit in Figure 3a is more ef-
cient 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 any
case, you must also be sure that the maximum voltage at
the BOOST pin is less than the maximum specifi ed in the
Absolute Maximum Ratings section.
The boost circuit can also run directly from a DC voltage
that is higher than the input voltage by more than 3V,
as in Figure 3d. The diode is used to prevent damage to
the LT3506 in case V
INB
is held low while V
IN
is present.
The circuit saves several components (both BOOST pins
can be tied to D2). However, effi ciency may be lower and
dissipation in the LT3506 may be higher. Also, if V
INB
is
absent, the LT3506 will still attempt to regulate the output,
but will do so with very low effi ciency and high dissipation
because the switch will not be able to saturate, dropping
1.5V to 2V in conduction.
V
IN
BOOST
GND
SW
V
IN
LT3506
(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
LT3506
(3b)
D2
V
OUT
C3
V
BOOST
– V
SW
V
IN
MAX V
BOOST
2V
IN
V
IN
BOOST
GND
SW
V
IN
LT3506
(3d)
3506 F03
V
OUT
MAX V
BOOST
– V
SW
V
INB
MAX V
BOOST
V
INB
MINIMUM VALUE FOR V
INB
=
V
IN
+ 3V
V
INB
>V
IN
+ 3V
D2
V
IN
BOOST
GND
SW
V
IN
LT3506
(3c)
V
OUT
V
BOOST
– V
SW
V
INB
MAX V
BOOST
V
INB
+ V
IN
MINIMUM VALUE FOR V
INB
=
3V
D2
V
INB
> 3V
C3
Figure 3. Generating the Boost Voltage
APPLICATIONS INFORMATION