MIC2546/2547

Micrel

MIC2546/2547

10

June 2000

Functional Description

The MIC2546 and MIC2547 are dual 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

INX is the power supply connection to the logic circuitry and

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

output MOSFET. In a typical circuit, current flows from INX to

OUTX toward the load. If V

flow from OUTX to INX since the switch is bidirectional when

enabled. 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 MIC2546/47 avoids

undesirable current flow from OUTX to INX. Both OUT pins

for a given channel must be connected together.

Thermal Shutdown

Thermal shutdown shuts off the output MOSFET and signals

the fault flag if the die temperature exceeds 140

°C. 10°C of

hysteresis prevents the switch from turning on until the die

temperature drops to 130

°C. Overtemperature detection

functions only when the switch is enabled.

The MIC2547 features an internal latch which causes the part

to remain off after thermal shutdown until a reset pulse is

provided via the enable pin. 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

ENX 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. ENX should not be allowed to go

negative with respect to GND, and V

or equal to V

Adjustable Current-Limit

The short-circuit current-limit is user-adjustable with an exter-

nal set resistor. Current-limit in the range of 100mA to 1.5A is

available with a set point accuracy of better than

±20%. The

current-limit circuit prevents damage to the output MOSFET

and external load.

The nominal current-limit value is set with an external resistor

between ILIMX and GND. For a desired current-limit, the

value of the external set resistor is given by:

R

230V

I

SETX

LIMITX

=









where:

154

Ω < R

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

calculated as:

230V

1A

230







Ω

Current through R

voltage across R

ance comparator to provide an indication of output current.

R

value).

Short-Circuit Protection

In the event of a short-circuit, the output current will fold back

to approximately 80% of the short-circuit current-limit.

Fault Flag

FLGX 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.