LM3544

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SNVS062C – AUGUST 2000 – REVISED MAY 2013

FUNCTIONAL DESCRIPTION

POWER SWITCHES

The power switches that comprise the four ports of the LM3544 are N-Channel MOSFETs. They have a typical

onstate drain-to-source resistance of 90 m

Ω when the input voltage is 5 V. When enabled, each switch will

supply a 500 mA minimum current to its load. In the unlikely event that a switch is enabled and the output

voltage of that switch is pulled above the input voltage, the bi-directional nature of the switch results in current to

flow from the output to the input. When a switch is disabled, current flow through the switch is prevented in both

directions.

CHARGE PUMP AND DRIVER

The gate voltages of the high-side NFET power switches are supplied by an internal charge-pump and driver

circuit combination. The charge pump is a low-current, switched capacitor circuit that efficiently generates

voltages above the LM3544 input supply. The charge pump output is used to supply a transconductance

amplifier driver circuit that controls the gate voltages of the power switches. Rise and fall times on the gates are

typically kept between 2 ms and 4 ms to limit large current surges and associated electromagnetic interference

(EMI).

ENABLE (ENX OR ENX)

The LM3544 comes in two versions: an active-high enable version, LM3544-H, and an active-low enable version,

LM3544-L. In the LM3544-H, the ENX pins are active-high logic inputs that, when asserted, turn on the

associated power supply switch(es). Power supply switches are controlled by the ENX active-low logic inputs in

the LM3544-L. With all four ports disabled on either version of the LM3544, less than 5

μA of supply current is

consumed. Both types of enable inputs, active-high and active-low, are TTL and CMOS logic compatible.

INPUT AND OUTPUT

The power supply to the control circuitry and the drains of the power-switch MOSFETs are connected to the two

input pins, IN1 and IN2. These two pins are connected externally in most standard applications. The two ground

nodes GND1 and GND2 must be connected externally in all applications. Pins OUT1, OUT2, OUT3, and OUT4

are connections to the source nodes of the power-switch MOSFETs. In a typical application circuit, current flows

through the switches from IN1 and IN2 to OUTX toward the load.

UNDERVOLTAGE LOCKOUT (UVLO)

Undervoltage Lockout (UVLO) prevents the MOSFET switches from turning on until the input voltage exceeds a

typical value of 1.8V.

If the input voltage drops below the UVLO threshold, the MOSFET switches are opened and fault flags are

activated. UVLO flags function only when one or more of the ports is enabled. If a port is enabled in a UVLO

condition, flags corresponding to the enabled port and its dual (port 1 is paired with port 2, port 3 is paired with

port 4) are asserted.

CURRENT LIMIT AND FOLD-BACK

The current limit circuit is designed to protect the system supply, the LM3544 switches, and the load from

potential damage resulting from excessive currents. If a direct short occurs on an output of the LM3544, the input

capacitor(s) rapidly discharge through the part, activating current limit circuitry. The threshold for activating

current limiting is 2.0A (typ.). Protection is achieved by momentarily opening the MOSFET switch and then

gradually turning it on. Turn-on is halted when the current through the switch reaches the current-limit level of

1.0A (typ.) The current is held at this level until either the excessive load/short is removed or the part overheats

and thermal shutdown occurs (see THERMAL SHUTDOWN). The fault flag of a switch is asserted whenever the

switch is current limiting.

If a port on the LM3544 is enabled into a short condition, the output current of that port will rise to the current-

limit level and hold there.

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