Datasheet

LTC4412
7
4412fb
For more information www.linear.com/LTC4412
operaTion
Operation can best be understood by referring to the Block
Diagram, which illustrates the internal circuit blocks along
with the few external components, and the graph that
accompanies Figure 1. The terms primary and auxiliary
are arbitrary and may be changed to suit the application.
Operation begins when either or both power sources are
applied and the CTL control pin is below the input low
voltage of 0.35V (V
IL
). If only the primary supply is pres-
ent, the Power Source Selector will power the LTC4412
from the V
IN
pin. Amplifier A1 will deliver a current to
the Analog Controller block that is proportional to the
voltage difference in the V
IN
and SENSE pins. While the
voltage on SENSE is lower than V
IN
– 20mV (V
FR
), the
Analog Controller will instruct the Linear Gate Driver and
Voltage Clamp block to pull down the GATE pin voltage
and turn on the external P-channel MOSFET. The dynamic
pull-down current of 50µA (I
G(SNK)
) stops when the GATE
voltage reaches ground or the gate clamp voltage. The
gate clamp voltage is 7V (V
G(ON)
) below the higher of V
IN
or V
SENSE
. As the SENSE voltage pulls up to V
IN
– 20mV,
the LTC4412 will regulate the GATE voltage to maintain
a 20mV difference between V
IN
and V
SENSE
which is also
the V
DS
of the MOSFET. The system is now in the forward
regulation mode and the load will be powered from the
primary supply. As the load current varies, the GATE volt
-
age will be controlled to maintain the 20mV difference. If
the load current exceeds the P-channel MOSFET’s ability
to deliver the current with a 20mV V
DS
the GATE voltage
will clamp, the MOSFET will behave as a fixed resistor
and the forward voltage will increase slightly. While the
MOSFET is on the STAT pin is an open circuit.
When an auxiliary supply is applied, the SENSE pin will be
pulled higher than the V
IN
pin through the external diode.
The Power Source Selector will power the LTC4412 from
the SENSE pin.
As the SENSE voltage pulls above V
IN
–
20mV, the Analog Controller will instruct the Linear Gate
Driver and V
oltage Clamp block to pull the GATE voltage
up to turn off the P-channel MOSFET. When the voltage
on SENSE is higher than V
IN
+ 20mV (V
RTO
), the Analog
Controller will instruct the Linear Gate Driver and Voltage
Clamp block to rapidly pull the GATE pin voltage to the
SENSE pin voltage. This action will quickly finish turning
off the external P-channel MOSFET if it hasn’t already
turned completely off. For a clean transition, the reverse
turn-off threshold has hysteresis to prevent uncertainty.
The system is now in the reverse turn-off mode. Power to
the load is being delivered through the external diode and
no current is drawn from the primary supply. The external
diode provides protection in case the auxiliary supply is
below the primary supply, sinks current to ground or is
connected reverse polarity. During the reverse turn-off
mode of operation the STAT pin will sink 10µA of current
(I
S(SNK)
) if connected. Note that the external MOSFET is
wired so that the drain to source diode will momentarily
forward bias when power is first applied to V
IN
and will
become reverse biased when an auxiliary supply is applied.
When the CTL (control) input is asserted high, the external
MOSFET will have its gate to source voltage forced to a
small voltage V
G(OFF)
and the STAT pin will sink 10µA of
current if connected. This feature is useful to allow control
input switching of the load between two power sources
as shown in Figure 4 or as a switchable high side driver
as shown in Figure 7. A 3.5µA internal pull-down current
(I
CTL
) on the CTL pin will insure a low level input if the pin
should become open.
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