Datasheet

LT3464
10
3464f
APPLICATIO S I FOR ATIO
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Output Disconnect Considerations
The LT3464 is equipped with an output disconnect that
isolates the load from the input during shutdown. See the
Operation section for a functional diagram. The output
disconnect uses a pass PNP coupled with circuitry that
varies the base current such that the transistor is consis-
tently at the edge of saturation, thus yielding the best
compromise between V
CE(SAT)
and low quiescent current.
To remain stable, this circuit requires a bypass capacitor
connected between the OUT pin and the CAP pin or
between the OUT pin and ground. A ceramic capacitor with
a value of at least 0.1µF is a good choice.
The PNP V
CE(SAT)
varies with load current as shown in
Figure 5. This voltage drop (V
DROP
) can be accounted for
when using the CAP pin as the feedback node by setting
the output voltage according to the following formula:
RR
VV
V
OUT DROP
REF
21 1=
+






–
In addition, the disconnect circuit has a built in current
limit of 25mA (minimum) to protect the chip during short-
circuit. This feature allows the LT3464 to tolerate an
Figure 5: Output Disconnect Voltage Drop (V
DROP
) vs CurrentFigure 4. Feedback Connection Using the CAP Pin and the OUT Pin
CTRL
SHDN
V
IN
V
OUT
SW
GND
OUT
CAP
FB
LT3464
R2
R1
1
8
7
6
3
5
2
4
CTRL
SHDN
V
IN
V
OUT
SW
GND
OUT
CAP
FB
LT3464
R2
R1
1
8
7
6
3
5
2
4
3464 F01
indefinite short, but care must be taken to avoid exceeding
the maximum junction temperature.
Inrush Current
When V
IN
is stepped from ground to operating voltage
while the output capacitor is discharged, an inrush current
will flow through the inductor and integrated Schottky
diode into the output capacitor. Conditions that increase
inrush current include a larger more abrupt voltage step at
V
IN
, a larger output capacitor tied to the CAP pin, and an
inductor with a low saturation current.
While the internal diode is designed to handle such events,
the inrush current should not be allowed to exceed 1 amp.
For circuits that use output capacitor values within the
recommended range and have input voltages of less than
5V, inrush current remains low, posing no hazard to
the device. In cases where there are large steps at V
IN
(more than 5V) and/or a large capacitor is used at the CAP
pin, inrush current should be measured to ensure safe
operation.
COLLECTOR CURRENT (mA)
0
V
CAP-OUT
(SAT)
300
250
200
150
100
50
0
5101520
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