lt1941 Linear Technology Corporation, lt1941 Datasheet - Page 13

• Manufacturer Part Number: lt1941
• Description: Triple Monolithic Switching Regulator
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
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. Fig-
ure 3 shows four 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 stan-
dard 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 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 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
V
IN2
> 3V
V
V
IN
IN
V
MAX V
V
MAX V
MINIMUM VALUE FOR V
BOOST
BOOST
U
V
BOOST
V
BOOST
IN
– V
D2
IN
– V
SW
SW
LT1941
BOOST
LT1941
BOOST
U
≅ V
GND
≅ V
GND
≅ V
≅ V
IN
IN2
OUT
IN2
+ V
+ V
SW
SW
OUT
(3a)
IN
(3c)
IN2 =
W
3V
C3
C3
Figure 3. Generating the Boost Voltage
D2
U
V
OUT
V
OUT
LT1941 in case V
circuit saves several components (both BOOST pins can
be tied to D2). However, efficiency may be lower and
dissipation in the LT1941 may be higher. Also, if V
absent the LT1941 will still attempt to regulate the output,
but will do so with low efficiency and high dissipation
because the switch will not be able to saturate, dropping
1.5 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
+ 3V
V
V
V
IN2
IN
IN
V
MAX V
MAX V
MAX V
MINIMUM VALUE FOR V
BOOST
V
BOOST
V
BOOST
BOOST
D2
IN2
D2
– V
IN
IN
SW
LT1941
LT1941
BOOST
BOOST
is held low while V
≅ 2V
– V
≅ V
GND
GND
≅ V
SW
IN2
IN
IN
≅ V
SW
SW
IN2
(3b)
(3d)
IN2 =
V
IN
+ 3V
C3
IN
is present. The
1941 F03
LT1941
V
V
OUT
OUT
13
IN2
1941fa
is