LT3682 LINER [Linear Technology], LT3682 Datasheet - Page 18

• Manufacturer Part Number: LT3682
• Description: 1A Micropower Step-Down
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
LT3682
BOOST and BD 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 between 3V and 8V, 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. Keep in mind that a minimum input voltage of 4.3V
is required if the voltage at the BD pin is smaller than 3V.
Tying BD to V
25V. The circuit in Figure 5a is more effi cient because the
BOOST pin current and BD pin quiescent current come
from a lower voltage source. You must also be sure that
the maximum voltage ratings of the BOOST and BD pins
are not exceeded.
As mentioned, a minimum of 2.5V across the BOOST
capacitor is required for proper operation of the internal
BOOST circuitry to provide the base current for the power
NPN switch. For BD pin voltages higher than 3V, the excess
voltage across the BOOST capacitor does not bring an
increase in performance but dissipates additional power in
the internal BOOST circuitry instead. The BOOST circuitry
tolerates reasonable amounts of power, however excessive
power dissipation on this circuitry may impair reliability. For
reliable operation, use no more than 8V on the BD pin for
the circuit in Figure 5a. For higher output voltages, make
sure that there is no more than 8V at the BD pin either by
connecting it to another available supply higher than 3V or
by using a Zener diode between V
the BD pin voltage between 3V and 8V.
18
IN
reduces the maximum input voltage to
OUT
and BD to maintain
The minimum operating voltage of an LT3682 applica-
tion is limited by the minimum input voltage and by the
maximum duty cycle as outlined previously. For proper
startup, the minimum input voltage is also limited by the
boost circuit. If the input voltage is ramped slowly, or the
LT3682 is turned on with its RUN/SS pin when the output
Figure 5. Three Circuits For Generating The Boost Voltage
(5a) For V
V
V
(5b) For 2.5V < V
V
IN
IN
IN
(5c) For V
OUT
> 2.8V; V
V
V
V
IN
IN
IN
OUT
GND
GND
GND
OUT
LT3682
LT3682
LT3682
< 2.5V; V
BD
BD
BD
IN(MIN)
PGND
PGND
PGND
< 2.8V; V
BOOST
BOOST
BOOST
SW
SW
SW
DA
DA
DA
IN(MAX)
= 4.3V if V
IN(MIN)
C3
D1
C3
D1
C3
D1
= 25V
D2
3682 F05
= 4.3V
OUT
V
OUT
V
V
< 3V
OUT
OUT
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