lt3685emse-trpbf Linear Technology Corporation, lt3685emse-trpbf Datasheet - Page 14

• Manufacturer Part Number: lt3685emse-trpbf
• Description: 38v, 2a, 2.4mhz Step-down Switching Regulator
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
LT3685
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 2 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 3V and above, the standard circuit (Figure 4a)
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 4b). For lower output voltages the
boost diode can be tied to the input (Figure 4c), or to
another supply greater than 2.8V. Tying BD to V
the maximum input voltage to 30V. The circuit in Figure 4a
is more effi cient because the BOOST pin current and BD
pin quiescent current comes from a lower voltage source.
You must also be sure that the maximum voltage ratings
of the BOOST and BD pins are not exceeded.
The minimum operating voltage of an LT3685 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 LT3685 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. Figure 5 shows a plot
of minimum load to start and to run as a function of input
14
IN
reduces
voltage. In many cases the discharged output capacitor
will present a load to the switcher, which will allow it to
start. The plots show the worst-case situation where V
is ramping very slowly. For lower start-up voltage, the
boost diode can be tied to V
input range to one-half of the absolute maximum rating
of the BOOST pin.
Figure 4. Three Circuits For Generating The Boost Voltage
4.7μF
4.7μF
4.7μF
V
V
V
IN
IN
IN
(4c) For V
(4b) For 2.5V < V
V
V
V
IN
IN
IN
(4a) For V
LT3685
LT3685
LT3685
GND
GND
GND
OUT
BD
BD
BD
BOOST
BOOST
BOOST
< 2.5V; V
SW
SW
SW
IN
OUT
; however, this restricts the
OUT
> 2.8V
IN(MAX)
C3
C3
C3
< 2.8V
D2
= 30V
3685 FO4
V
OUT
V
V
OUT
OUT
3685f
IN