June 2000

11

MIC2545A/2549A

MIC2545A/2549A

Micrel

Functional Description

The MIC2545A and MIC2549A are high-side N-channel

switches available with active-high or active-low enable in-

puts. Fault conditions turn off or inhibit turn-on of the output

transistor and activate the open-drain error flag transistor

making it sink current to ground.

Input and Output

IN is the power supply connection to the logic circuitry and the

drain of the output MOSFET. OUT is the source of the output

MOSFET. In a typical circuit, current flows from IN to OUT

toward the load. If V

from OUT to IN since the switch is bidirectional when en-

abled. The output MOSFET and driver circuitry are also

designed to allow the MOSFET source to be externally forced

to a higher voltage than the drain (V

switch is disabled. In this situation, the MIC2545A/49A avoids

undesirable current flow from OUT to IN. Both IN pins must

be connected together, and both OUT pins must be con-

nected together.

Thermal Shutdown

Thermal shutdown shuts off the output MOSFET and signals

the fault flag if the die temperature exceeds 135

°C. 10°C of

hysteresis prevents the switch from turning on until the die

temperature drops to 125

°C. Overtemperature detection

functions only when the switch is enabled.

The MIC2549A features an internal latch which causes the

part to remain off after thermal shutdown until a reset pulse

is provided via the enable pin (pin 1). While in current-limit, the

thermal shutdown latch prevents on/off cycling of the output.

Refer to Figure 2 for timing diagram. The flag remains low

until reset.

Enable Input

EN must be driven logic high or logic low, or be pulled high or

low for a clearly defined input. Floating the input may cause

unpredictable operation. EN should not be allowed to go

negative with respect to GND.

Current-Limit Operation

The current limit is user adjustable with an external set

resistor. Current limiting in the range of 500mA to 3A is

available with a set point accuracy of better than

±30% (±20%

for I

to the output MOSFET and external load.

The current-limit response of the MIC2545A/49A is based on

the type of load that is applied to the output and is defined in

three parts:

1. The first mode of operation is where the device

enters a constant-current mode preventing

further increases in output current. The value of

this current, I

R

2. When a short circuit is applied to the output of

an enabled device the output current immedi-

ately folds back to a value less than I

I

reduces device power dissipation. Refer to the

“Typical Characteristics: Output Current vs.

Output Voltage” graph for details.

3. When a load is increased, the output current will

proportionally increase up to the current-limit

threshold, I

Characteristics: Current-Limit Response” graph.

The device in this case will supply current

slightly higher than the current-limit set point

defined by R

further the current folds back to I

The nominal current-limit value, I

resistor between ILIM (pin 4) and GND (pin 3). For a desired

current-limit, the value of the external set resistor is given by:

R

=

I

SET

LIMIT

230









where:

76.8

Ω < R

For example, to set a 1A nominal current-limit, R

calculated as:

230

1A









=Ω

230

Refer to the “Typical Characteristics: Current-Limit vs R

graph for more details.

Current through R

voltage across R

ance comparator to provide an indication of output current.

R

value).

Fault Flag

FLG is an N-channel, open-drain MOSFET output. The fault-

flag is active (low) for current-limit or thermal shutdown

conditions. The flag output MOSFET is capable of sinking a

10mA load to typically 100mV above ground. For applications

with V

to 5mA or less.