LM3526

SNVS054E – FEBRUARY 2000 – REVISED MARCH 2013

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FUNCTIONAL DESCRIPTION

The LM3526-H and LM3526-L are high side P-Channel switches with active-high and active-low enable inputs,

respectively. Fault conditions turn-off and inhibit turn-on of the output transistor and activate the open-drain error

flag transistor sinking current to the ground.

INPUT AND OUTPUT

IN (Input) is the power supply connection to the control circuitry and the source of the output MOSFET.

OUT (Output) is the connection to the drain of the output MOSFET. In a typical application circuit, current flows

through the switch from IN to OUT towards the load.

If VOUT is greater than VIN when the switch is enabled, current will flow from OUT to IN since the MOSFET is

bidirectional.

THERMAL SHUTDOWN

The LM3526 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 150°C,

both switches are turned off and both 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.

UNDERVOLTAGE LOCKOUT

UVLO prevents the MOSFET switch from turning on until input voltage exceeds 1.8V (typical).

If input voltage drops below 1.8V (typical), UVLO shuts off the MOSFET switch and signals the fault flag. UVLO

functions only when device is enabled.

CURRENT LIMIT

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

damage caused by excessive currents. The current limit threshold is set internally to allow a minimum of 500 mA

through the MOSFET but limits the output current to approximately 1.0A typical.

FAULT FLAG

The fault flag is an open-drain output capable of sinking 10 mA load current to typically 100 mV above ground.

A parasitic diode exists between the flag pins and VIN pin. Pulling the flag pins to voltages higher than VIN will

forward bias this diode and will cause an increase in supply current. This diode will also clamp the voltage on the

flag pins to a diode drop above VIN.

The fault flag is active (pulled low) when any of the following conditions are present: under-voltage, current limit,

or thermal shutdown.

A 1ms (typ.) delay in reporting the fault condition prevents erroneous fault flags and eliminates the need for an

external RC delay network.

Application Information

FILTERING

The USB specification indicates that “no less than 120 µF tantalum capacitors” must be used on the output of

each downstream port. This bulk capacitance provides the short-term transient current needed during a hot plug-

in. Current surges caused by the input capacitance of the down stream device could generate undesirable EMI

signals. Ferrite beads in series with all power and ground lines are recommended to eliminate or significantly

reduce EMI.

In selecting a ferrite bead, the DC resistance of the wire used must be kept to a minimum to reduce the voltage

drop.

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