LT3682EDD データシートの表示（PDF） - Linear Technology
部品番号
コンポーネント説明
メーカー
LT3682EDD Datasheet PDF : 24 Pages
| |||
LT3682
APPLICATIONS INFORMATION
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 efficiency.
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 VIN reduces the maximum input voltage to
25V. The circuit in Figure 5a is more efficient 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 VOUT and BD to maintain
the BD pin voltage between 3V and 8V.
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
BD
BOOST
VOUT
VIN
VIN LT3682
C3
SW
D1
DA
GND PGND
(5a) For VOUT > 2.8V; VIN(MIN) = 4.3V if VOUT < 3V
BD
BOOST
VOUT
D2
VIN
VIN LT3682
C3
SW
D1
DA
GND PGND
(5b) For 2.5V < VOUT < 2.8V; VIN(MIN) = 4.3V
BD
BOOST
VOUT
VIN
VIN LT3682
C3
SW
D1
DA
GND PGND
(5c) For VOUT < 2.5V; VIN(MAX) = 25V
3682 F05
Figure 5. Three Circuits For Generating The Boost Voltage
3682f
18
Share Link: 