Functional Description

Power Switches

The power switches that comprise the three ports of the

LM3543 are N-Channel MOSFETs. They have a typical

on-state 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 LM3543 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 (EN

The LM3543 comes in two versions: an active-high enable

version, LM3543-H, and an active-low enable version,

LM3543-L. In the LM3543-H, the EN

logic inputs that, when asserted, turn on the associated

power supply switch(es). Power supply switches are

controlled by the EN

x

active-low logic inputs in the

LM3543-L. With all three ports disabled on either version of

the LM3543, 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, and OUT3 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 OUT

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. Due to the paired nature of the design, both FLAG1

and FLAG2 will assert if either port1 or port2 is enabled in a

UVLO condition.

Current Limit and Foldback

The current limit circuit is designed to protect the system

supply, the LM3543 switches, and the load from potential

damage resulting from excessive currents. If a direct short

occurs on an output of the LM3543, 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 section, below). The fault flag of a switch is

asserted whenever the switch is current limiting.

If a port on the LM3543 is enabled into a short condition, the

output current of that port will rise to the current-limit level

and hold there.

When a port is in a current-limit condition, the LM3543

senses the output voltage on that port and, if it is less than

1.0V (typ.), will reduce the output current through that port.

This operation is shown in

Figure 2, below. The current

reduction, or foldback, reduces power dissipation through

the overloaded MOSFET switch. An additional advantage of

the foldback feature is the reduction of power required from

the source supply when one or more output ports are

shorted.

Thermal Shutdown

The LM3543 is internally protected against excessive power

dissipation by a two-stage thermal protection circuit. If the

device temperature rises to approximately 145˚C, the

thermal shutdown circuitry turns off any switch that is current

limited. Non-overloaded switches continue to function

normally. If the die temperature rises above 160˚C, all

switches are turned off and all three fault flag outputs are

activated. Hysteresis ensures that a switch turned off by

thermal shutdown will not be turned on again until the die

temperature is reduced to 135˚C. Shorted switches will

continue to cycle off and on, due to the rising and falling die

temperature, until the short is removed.

The thermal shutdown function is shown graphically in

Figure 3 and Figure 4.

10125817

FIGURE 2. Short-Circuit Output Current (with

Foldback) vs. Output Voltage

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