Datasheet

LT3487
11
3487f
Figure 4. V
CE
vs I of Output Disconnect
Choosing a Feedback Node
The positive channel feedback resistor, R1, may be con-
nected to the V
POS
pin or to the CAP pin (see Figure 5).
Regulating the V
POS
pin eliminates the output offset result-
ing from the voltage drop across the output disconnect.
However, in the case of a short-circuit fault at the V
POS
pin, the LT3487 will switch continuously because the FBP
pin is low. While operating in this open-loop condition, the
rising voltage at the CAP pin is limited only by the current
limit of the output disconnect. Given worst-case parameters
this voltage may reach 18V in a Li-Ion application. Care
must be taken in high V
IN
applications when regulating
from the V
POS
pin. When the short-circuit is removed, the
V
POS
pin will bounce up to the voltage on the CAP pin,
potentially exceeding the programmed output voltage until
the capacitor voltages fall back into regulation. While this
is harmless to the LT3487, this should be considered in
the context of the external circuitry if short-circuit events
are expected. Regulating the CAP pin ensures that the
voltage on the V
POS
pin never exceeds the set output volt-
age after a short-circuit event. However, this setup does
not compensate for the voltage drop across the output
disconnect, resulting in an output voltage that is slightly
lower than the voltage set by the feedback resistor. This
voltage drop (V
DISC
) can be accounted for when using
the CAP pin as the feedback node by setting the output
voltage according to the following formula (using V
DISC
from Figure 4):
R
V
µA
DISC
1
123
25
=
+V
POS
–.
V
BAT
The V
BAT
pin is a new innovation in the LT3487 that allows
output disconnect operation in a wide range of applica-
tions. The V
BAT
pin allows the part to stay on until CAP is
less than 1.2V above V
BAT
. This ensures that the positive
bias doesn’t fall before the negative bias discharges. In
some applications it may be useful to power the inductors
from a different source than V
IN
. In this case, connect
V
BAT
to the source powering the inductors to allow proper
operation of the disconnect. For example, in an automotive
system there may already be a buck regulator producing
3.3V from a 12V battery. The LT3487 enables the user
to power V
IN
from the 3.3V rail, but power the V
BAT
pin
APPLICATIO S I FOR ATIO
WUU
U
Figure 5. Feedback Connection Using the V
POS
and CAP Pins
DISCONNECT CURRENT (mA)
0
0
DISCONNECT SATURATION VOLTAGE (mV)
50
100
150
200
250
300
20 40 60 80
2400 G31
100
SWN
V
IN
CAP
FBP
V
POS
V
BAT
LT3487
GND
SWP
DN
FBN
RUN/SS
3487 F05
V
POS
SWN
V
IN
CAP
FBP
V
POS
V
BAT
LT3487
GND
SWP
DN
FBN
RUN/SS
V
POS