LTC4413 datasheet(8/12 Pages) LINER

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LTC4413

4413f

OPERATIO

The LTC4413 is described with the aid of the Block

Diagram (Figure 1). Operation begins when the power

source at VINA or VINB rises above the undervoltage

lockout (UVLO) voltage of 2.4V and either of the ENBA or

ENBB control pins is low. If only the voltage at the VINA pin

is present, the power source to the LTC4413 (VDD) will be

supplied from the VINA pin. The amplifier (A) will pull a

current proportional to the difference between VINA and

VOUTA from the gate (VGATEA) of the internal PFET (PA),

driving this gate voltage below VINA. This will turn on PA.

As VOUTA is pulled up to a forward voltage drop (VFWD) of

20mV below VINA, the LTC4413 will regulate VGATEA to

maintain the small forward voltage drop. The system is

now in forward regulation and the load at VOUTA will be

powered from the supply at VINA. As the load current

varies, VGATEA will be controlled to maintain VFWD until the

load current exceeds the transistor’s (PA) ability to deliver

the current as VGATEA approaches GND. At this point the

PFET will behave as a fixed resistor with resistance RON,

whereby the forward voltage will increase slightly with

increased load current. As the magnitude of IOUT increases

further (such that ILOAD > IOC), the LTC4413 will fix the load

current to the constant value IOC to protect the device. The

characteristics for parameters RFWD, RON, VFWD and IOC

are specified with the aid of Figure 2, illustrating the

LTC4413 forward voltage drop versus that of a Schottky

diode.

If another supply is provided at VINB, the LTC4413 will

likewise regulate the gate voltage on PB to maintain the

output voltage VOUTB just below the input voltage VINB. If

this alternate supply, VINB, exceeds the voltage at VINA, the

LTC4413 will select this input voltage as the internal

supply (VDD). This second ideal diode operates indepen-

dently of the first ideal diode function.

When an alternate power source is connected to the load

at VOUTA (or VOUTB), the LTC4413 will sense the increased

voltage at VOUTA and amplifier A will increase the voltage

VGATEA, reducing the current through PA. When VOUTA is

higher than VINA + VRTO, VGATEA will be pulled up to VDD,

which will turn off PA. The internal power source for the

LTC4413 (VDD) will then be diverted to source current

from the VOUTA pin, only if VOUTA is larger than VINB (or

VOUTB). The system is now in the reverse turn-off mode.

Power to the load is being delivered from an alternate

supply and only a small current is drawn from VINA to

sense the potential at VINA.

When the selected channel of the LTC4413 is in reverse

turn-off mode or both channels are disabled, the STAT pin

will sink 9µA of current (ISON) if connected.

Channel selection is accomplished using the two ENB

pins, ENBA and ENBB. When the ENBA input is asserted

(high), PA will have its gate voltage pulled to VDD at a

controlled rate, limiting the turn-off time to avoid voltage

spiking at the input when being driven by an inductive

source impedance. A 3µA pull-down current on the ENB

pins will ensure a low level at these inputs if left floating.

Overcurrent and Short-Circuit Protection

During an overcurrent condition, the output voltage will

droop as the load current exceeds the amount of current

that the LTC4413 can supply. At the time when an over-

current condition is first detected, the LTC4413 will take

some time to detect this condition before reducing the

current to IMAX. For short durations after the output is

shorted, the current may exceed IMAX. The magnitude of

this peak short-circuit current can be large, depending on

the load current immediately before the short circuit

Figure 2

FORWARD VOLTAGE (V)

IFWD

IOC

SLOPE

1/RON

SLOPE

1/RFWD

LTC4413

SCHOTTKY

DIODE

4413 F02

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