Datasheet

LTC4412
9
4412fb
For more information www.linear.com/LTC4412
applicaTions inForMaTion
V
IN
and SENSE Pin Usage
Since the analog controller’s thresholds are small (±20mV),
the V
IN
and SENSE pin connections should be made in a
way to avoid unwanted I • R drops in the power path. Both
pins are protected from negative voltages.
GATE Pin Usage
The GATE pin controls the external P-channel MOSFET
connected between the V
IN
and SENSE pins when the
load current is supplied by the power source at V
IN
. In
this mode of operation, the internal current source, which
is responsible for pulling the GATE pin up, is limited to
a few microamps (I
G(SRC)
). If external opposing leakage
currents exceed this, the GATE pin voltage will reach the
clamp voltage (V
GON
) and V
DS
will be smaller. The internal
current sink, which is responsible for pulling the GATE pin
down, has a higher current capability (I
G(SNK)
). With an
auxiliary supply input pulling up on the SENSE pin and
exceeding the V
IN
pin voltage by 20mV (V
RTO
), the device
enters the reverse turn-off mode and a much stronger
current source is available to oppose external leakage
currents and turn off the MOSFET (V
GOFF
).
While in forward regulation, if the on resistance of the
MOSFET is too
high to maintain forward regulation, the
GATE pin will maximize the MOSFET’s V
GS
to that of the
clamp voltage (V
GON
). The clamping action takes place
between the higher of V
IN
or V
SENSE
and the GATE pin.
Status Pin Usage
During normal operation, the open-drain STAT pin can be
biased at any voltage between ground and 28V regard
-
less of the supply voltage to the LTC4412. It is usually
connected to a
resistor whose other end connects to a
voltage source. In the forward regulation mode, the STAT
pin will be open (I
S(OFF)
). When a wall adaptor input or
other auxiliary supply is connected to that input, and the
voltage on SENSE is higher than V
IN
+ 20mV (V
RTO
), the
system is in the reverse turn-off mode. During this mode of
operation the STAT pin will sink 10µA of current (I
S(SNK)
).
This will result in a voltage change across the resistor,
depending on the resistance, which is useful to turn on an
auxiliary P-channel MOSFET or signal to a microcontroller
that an auxiliary power source is connected. External
leakage currents, if
significant, should be accounted for
when determining the voltage across the resistor when
the STAT pin is either on or off.
Control Pin Usage
This is a digital control input pin with low threshold voltages
(V
IL,
V
IH
) for use with logic powered from as little as 1V.
During normal operation, the CTL pin can be biased at any
voltage between ground and 28V, regardless of the supply
voltage to the LTC4412. A logical high input on this pin
forces the gate to source voltage of the primary P-channel
MOSFET power switch to a small voltage (V
GOFF
). This
will turn the MOSFET off and no current will flow from the
primary power input at V
IN
if the MOSFET is configured
so that the drain to source diode is not forward biased.
The high input also forces the STAT pin to sink 10µA of
current (I
S(SNK)
). See the Typical Applications for various
examples on using the STAT pin. A 3.5µA internal pull-
down current (I
CTL
) on the CTL pin will insure a logical
low level input if the pin should be open.
Protection
Most of the application circuits shown provide some
protection against supply faults such
as shorted, low or
rev
ersed supply inputs. The fault protection does not protect
shorted supplies but can isolate other supplies and the load
from faults. A necessary condition of this protection is for
all components to have sufficient breakdown voltages. In
some cases, if protection of the auxiliary input (sometimes
referred to as the wall adapter input) is not required, then
the series diode or MOSFET may be eliminated.
Internal protection for the LTC4412 is provided to prevent
damaging pin currents and excessive internal self heating
during a fault condition. These fault conditions can be
a result of any LTC4412 pins shorted to ground or to a
power source that is within the pin’s absolute maximum
voltage limits. Both the V
IN
and SENSE pins are capable
of being taken significantly below ground without current
drain or damage to the IC (see Absolute Maximum Voltage
Limits). This feature allows for reverse-battery condition
without current drain or damage. This internal protection
is not designed to prevent overcurrent or overheating of
external components.
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