LTC4414 datasheet(6/12 Pages) LINER

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LTC4414

4414fc

OPERATIO

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 the Typical Application drawing on the front

page of the data sheet. 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 (VIL). If only the primary supply is

present, the Power Source Selector will power the LTC4414

from the VIN pin. Amplifier A1 will deliver a current to the

Analog Controller block that is proportional to the voltage

difference in the VIN and SENSE pins. While the voltage on

SENSE is lower than VIN – 20mV (VFR), 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 300

µA (IG(SNK)) stops when the GATE voltage

reaches ground or the gate clamp voltage. The gate clamp

voltage is 8.5V (VG(ON)) below the higher of VIN or VSENSE.

As the SENSE voltage pulls up to VIN – 20mV, the LTC4414

will regulate the GATE voltage to maintain a 20mV differ-

ence between VIN and VSENSE which is also the VDS 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 voltage 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 VDS 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 VIN pin through the external diode.

The Power Source Selector will power the LTC4414

from the SENSE pin. As the SENSE voltage pulls above

VIN – 20mV, the Analog Controller will instruct the Linear

Gate Driver and Voltage Clamp block to pull the GATE

voltage up to turn off the P-channel MOSFET. When the

voltage on SENSE is higher than VIN + 20mV (VRTO), 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 a current (IS(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 VIN 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 VG(OFF) and the STAT pin will sink a mini-

mum of 50

µAofcurrentifconnected.Thisfeatureisuseful

to allow control input switching of the load between two

power sources as shown in Figure 3 or as a switchable

high side driver as shown in Figure 7. A 3.5

µAinternalpull-

down current (ICTL) on the CTL pin will insure a low level

input if the pin should become open.

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