MIC2546 Datasheet, PDF (10/12 Pages) Micrel Semiconductor – Dual Programmable Current Limit Switch Preliminary Information

MIC2546/2547
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_OUT is greater than V_IN, current will
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_OUT > V_IN) when the
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_ENX should be less than
or equal to V_INX.
Micrel
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_SETX
=
 _230V
 ^I_LIMITX


where:
154Ω < R_SET < 2.29kΩ
For example, to set a 1A nominal current-limit, R_SET is
calculated as:

230V
1A

=
230Ω
Current through R_SETX increases with OUT current. The
voltage across R_SETX could be monitored with a high imped-
ance comparator to provide an indication of output current.
R_SETX should be between 154Ω and 2.29kΩ (0.5% resistor
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.
MIC2546/2547
10
June 2000