Datasheet

LT1941
13
1941fb
APPLICATIONS INFORMATION
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 0.18μF capacitor and fast switching diode (such
as the CMDSH-3 or MMSD914LT1) will work well. Figure 3
shows four 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
fi 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, be sure that the maximum voltage at the BOOST pin
is less than 35V and the voltage difference between the
BOOST and SW pins is less than 25V.
The boost circuit can also run directly from a DC voltage
that is higher than the input voltage by more than 2.5V + V
F
,
as in Figure 3d. The diode prevents damage to the LT1941
Figure 3. Generating the Boost Voltage
in case V
IN2
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 LT1941 may be higher. Also, if V
IN2
is absent the
LT1941 will still attempt to regulate the output, but will
do so with low effi ciency and high dissipation because
the switch will not be able to saturate, dropping 1.5V to
2V in conduction.
The minimum operating voltage of an LT1941 application
is limited by the undervoltage lockout (3.5V) 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 LT1941 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. 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.
V
IN
BOOST
GND
SW
V
IN
LT1941
(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
LT1941
(3b)
D2
V
OUT
C3
V
BOOST
– V
SW
V
IN
MAX V
BOOST
2V
IN
V
IN
BOOST
GND
SW
V
IN
LT1941
(3d)
1941 F03
V
OUT
MAX V
BOOST
– V
SW
V
IN2
MAX V
BOOST
V
IN2
MINIMUM VALUE FOR V
IN2
=
V
IN
+ 3V
V
IN2
>V
IN
+ 3V
D2
V
IN
BOOST
GND
SW
V
IN
LT1941
(3c)
V
OUT
V
BOOST
– V
SW
V
IN2
MAX V
BOOST
V
IN2
+ V
IN
MINIMUM VALUE FOR V
IN2
=
3V
D2
V
IN2
> 3V
C3