LT3506/LT3506A
U U
W
APPLICATIO S I FOR ATIO
shows three ways to arrange the boost circuit. The BOOST
pin must be more than 2.5V above the SW pin for full
efficiency. 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-
ficientbecausetheBOOSTpincurrentcomesfromalower
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 specified 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 is held low while V is present.
INB
IN
The circuit saves several components (both BOOST pins
can be tied to D2). However, efficiency may be lower and
dissipation in the LT3506 may be higher. Also, if V is
INB
absent, the LT3506 will still attempt to regulate the output,
but will do so with very low efficiency and high dissipation
because the switch will not be able to saturate, dropping
1.5V to 2V in conduction.
D2
D2
C3
C3
BOOST
LT3506
BOOST
LT3506
V
V
V
V
OUT
V
SW
V
SW
IN
OUT
IN
IN
IN
GND
GND
V
– V ≅ V
V
– V ≅ V
BOOST
BOOST
SW
OUT
BOOST
SW
IN
IN
MAX V
≅ V + V
MAX V
≅ 2V
BOOST
IN
OUT
(3a)
(3b)
D2
D2
V
INB
V
> 3V
INB
>V + 3V
IN
BOOST
LT3506
BOOST
LT3506
C3
V
V
V
V
OUT
V
SW
V
SW
IN
OUT
IN
IN
IN
GND
GND
V
– V ≅ V
MAX V
MAX V
– V ≅ V
BOOST SW INB
≅ V
BOOST INB
BOOST
SW
INB
3506 F03
MAX V
≅ V + V
BOOST
INB
IN
MINIMUM VALUE FOR V = 3V
MINIMUM VALUE FOR V = V + 3V
INB
INB IN
(3c)
(3d)
Figure 3. Generating the Boost Voltage
3506afb
ꢀꢁ
Linear [ Linear ]
