Datasheet

LT3663
10
3663fc
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applicaTions inForMaTion
BOOST and BIAS Pin Considerations
Capacitor C3 and the internal boost diode (see the Block
Diagram) are used to generate a boost voltage that is higher
than the input voltage. In most cases a 0.1µF capacitor
works well. Figure 4 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 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 connect an external Schottky
diode to the input (Figure 4c), or to another supply greater
than 2.8V. Using V
IN
reduces the maximum input voltage
to 25V. The circuit in Figure 4a is more efficient because
the BOOST pin current comes from a lower voltage source.
Take care to ensure that the maximum voltage ratings of
the BOOST pin is not exceeded.
Figure 4. Three Circuits to Generate BOOST Pin Voltage
LT3663
(4c) For V
OUT
< 2.5V; V
IN(MAX)
= 25V
V
IN
V
IN
3663 F04
BOOST
SW
D3
C3
I
SENSE
V
OUT
V
OUT
LT3663
(4b) For 2.5V < V
OUT
< 2.8V
V
IN
V
IN
BOOST
SW
D3
C3
I
SENSE
V
OUT
V
OUT
LT3663
(4a) For V
OUT
> 2.8V
V
IN
V
IN
BOOST
SW
C3
I
SENSE
V
OUT
V
OUT