lt3663 Linear Technology Corporation, lt3663 Datasheet - Page 8

• Manufacturer Part Number: lt3663
• Description: 1.2a, 1.5mhz Step-down Switching Regulator With Output Current Limit
• Manufacturer: Linear Technology Corporation

LT3663
nected from I
Table 1 details the I
current limits.
Table 1. Output Current Limit vs R
There may be a reason in some applications to move the
output current limit. The following simple circuit demon-
strates how the output current limit can be adjusted:
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 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 connect an external Schottky
APPLICATIONS INFORMATION
8
Figure 3. Circuit for Multiple Values of Output Current Limit
OUTPUT CURRENT LIMIT (A)
OFF , R
ON, R
ILIM
= RA + RB
ILIM
0.4
0.6
0.8
1.0
1.2
LIM
= RA
pin to GND programs the output current.
LIM
resistor values for specifi c output
RA
RB
LIM
Value
I
LIM
R
ILIM
LT3663
VALUE (kΩ)
48.7
36.5
28.7
140
75
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diode to the input (Figure 4c), or to another supply greater
than 2.8V. Using V
to 25V. The circuit in Figure 4a is more effi cient 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
(4c) For V
V
V
V
IN
IN
IN
(4b) For 2.5V < V
(4a) For V
OUT
V
V
V
IN
IN
IN
IN
reduces the maximum input voltage
< 2.5V; V
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D3
OUT
BOOST
BOOST
BOOST
I
I
I
SENSE
SENSE
OUT
SENSE
V
V
V
> 2.8V
OUT
OUT
OUT
SW
SW
SW
IN(MAX)
< 2.8V
= 25V
3663 F04
C3
C3
C3
V
V
OUT
OUT
D3
V
OUT
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