MIC2546/2547
Micrel
Adjustable Current-Limit
Functional Description
Theshort-circuitcurrent-limitisuser-adjustablewithanexter-
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 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.
Thenominalcurrent-limitvalueissetwithanexternalresistor
between ILIMX and GND. For a desired current-limit, the
value of the external set resistor is given by:
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
230V
R
=
OUTXtowardtheload. IfV
isgreaterthanV , currentwill
SETX
OUT
IN
I
LIMITX
flow from OUTX to INX since the switch is bidirectional when
enabled. The output MOSFET and driver circuitry are also
designedtoallowtheMOSFETsourcetobeexternallyforced
where:
154Ω < R
< 2.29kΩ
SET
to a higher voltage than the drain (V
> V ) when the
OUT
IN
For example, to set a 1A nominal current-limit, R
calculated as:
is
SET
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.
230V
=
230Ω
Thermal Shutdown
1A
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.
Current through R
voltage across R
increases with OUT current. The
could be monitored with a high imped-
SETX
SETX
ance comparator to provide an indication of output current.
should be between 154Ω and 2.29kΩ (0.5% resistor
R
SETX
value).
TheMIC2547featuresaninternallatchwhichcausesthepart
to remain off after thermal shutdown until a reset pulse is
providedviatheenablepin. Whileincurrent-limit, thethermal
shutdown latch prevents on/off cycling of the output. Refer to
Figure 2 for timing diagram. The flag remains low until reset.
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.
Enable Input
ENX must be driven logic high or logic low, or be pulled high
orlowforaclearlydefinedinput. Floatingtheinputmaycause
unpredictable operation. ENX should not be allowed to go
negative with respect to GND, and V
should be less than
ENX
or equal to V
.
INX
MIC2546/2547
10
June 2000
MICREL [ MICREL SEMICONDUCTOR ]
