MIC2544/2548
Micrel
Block Diagram
EN
THERMAL
SHUTDOWN
1.2V
REFERENCE
OSC.
IN
CHARGE
PUMP
GATE
CONTROL
CURRENT
LIMIT
ILIM
R
SET
RESET
THERMAL
SHUTDOWN
LATCH
(MIC2548)
OUT
FLG
GND
Functional Description
The MIC2544 and MIC2548 are high-side N-channel switches
available with active-high or active-low enable inputs. 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
OUT
is greater than V
IN
, current will flow
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
OUT
> V
IN
) when the
switch is disabled. In this situation, the MIC2544/48 avoids
undesirable current flow from OUT to IN. Both OUT pins 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 MIC2548 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, and V
EN
should be less than
or equal to V
IN
.
June 2000
9
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 ILIM and GND. For a desired current-limit, the value
of the external set resistor is given by:
230V
R
SET
=
I
LIMIT
where:
154Ω < R
SET
< 2.29kΩ
For example, to set a 1A nominal current-limit, R
SET
is
calculated as:
230V
=
230
Ω
1A
Current through R
SET
increases with OUT current. The
voltage across R
SET
could be monitored with a high imped-
ance comparator to provide an indication of output current.
R
SET
should be between 154Ω
±0.5%
and 2.29kΩ
±0.5%.
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
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.
MIC2544/2548
MICREL [ MICREL SEMICONDUCTOR ]
