lt3681 Linear Technology Corporation, lt3681 Datasheet - Page 14

• Manufacturer Part Number: lt3681
• Description: 36v, 2a, 2.8mhz Step-down Switching Regulator With Integrated Power Schottky Diode
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
LT3681
BOOST and BIAS Pin Considerations
Capacitor C3 and the internal boost Schottky diode (see
the Block Diagram) are used to generate a boost volt-
age that is higher than the input voltage. In most cases
a 0.22µF capacitor will work well. Figure 5 shows three
ways to arrange the boost circuit. The BOOST pin must be
more than 2.3V above the SW pin for best effi ciency. For
outputs of 2.8V and above, the standard circuit (Figure 5a)
is best. For outputs between 2.8V and 3V, use a 1µF boost
capacitor. A 2.5V output presents a special case because it
is marginally adequate to support the boosted drive stage
while using the internal boost diode. For reliable BOOST pin
operation with 2.5V outputs use a good external Schottky
diode (such as the ON Semi MBR0540), and a 1µF boost
capacitor (see Figure 5b). For lower output voltages the
boost diode can be tied to the input (Figure 5c), or to
another supply greater than 2.8V. The circuit in Figure 5a
is more effi cient because the BOOST pin current and BIAS
pin quiescent current comes from a lower voltage source.
You must also be sure that the maximum voltage ratings
of the BOOST and BIAS pins are not exceeded.
The minimum operating voltage of an LT3681 application
is limited by the minimum input voltage (3.6V) and by the
maximum duty cycle as outlined in a previous section. For
proper startup, the minimum input voltage is also limited
by the boost circuit. If the input voltage is ramped slowly,
or the LT3681 is turned on with its RUN/SS pin when the
output is already in regulation, then the boost capacitor
may not be fully charged. Because the boost capacitor is
charged 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 generally goes to zero
once the circuit has started. If, however, the LT3681 is
started by the RUN/SS pin and the output is discharged, the
discharged output capacitance will often present enough
of a load to allow the circuit to start. Figure 6 gives plots
of the input voltage required for three different situations:
the worst case situation where RUN/SS is tied to V
V
at which the circuit will regulate when start-up is controlled
by RUN/SS, and the minimum input voltage required to
maintain output regulation. For lower start-up voltage, the
14
IN
is ramped up very slowly, the minimum input voltage
IN
and
boost diode can be tied to V
input range to one-half of the absolute maximum rating
of the BOOST pin.
At light loads, the inductor current becomes discontinuous
and the effective duty cycle at the BOOST pin (not the SW
pin) can be very high. This reduces the minimum input
voltage to approximately 300mV above V
currents, the inductor current is continuous and the duty
cycle is limited by the maximum duty cycle of the LT3681,
requiring a higher input voltage to maintain regulation.
Figure 5. Three Circuits For Generating The Boost Voltage
4.7 F
4.7 F
4.7 F
V
V
V
IN
IN
IN
V
V
V
(5b) For 2.5V < V
IN
IN
IN
GND
GND
GND
(5a) For V
(5c) For V
LT3681
LT3681
LT3681
BD
BD
BD
DA
DA
DA
BOOST
BOOST
BOOST
SW
SW
SW
DC
DC
DC
IN
OUT
OUT
; however, this restricts the
OUT
< 2.5V
> 2.8V
< 2.8V
C3
C3
C3
OUT
D2
. At higher load
3681 FO5
V
OUT
V
C
V
C
C
OUT
OUT
OUT
OUT
OUT
3681f