MIC2545A/2549A
Micrel
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:
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.
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
, is defined by the value of
LIMIT
Input and Output
R
as explained further in this section.
SET
INisthepowersupplyconnectiontothelogiccircuitryandthe
drain of the output MOSFET. OUT is the source of the output
MOSFET. In a typical circuit, current flows from IN to OUT
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
called
LIMIT
toward the load. If V
is greater than V , current will flow
OUT
IN
I
. This further protects the load and
SHORT
from OUT to IN since the switch is bidirectional when en-
abled. The output MOSFET and driver circuitry are also
designedtoallowtheMOSFETsourcetobeexternallyforced
reduces device power dissipation. Refer to the
“Typical Characteristics: Output Current vs.
Output Voltage” graph for details.
to a higher voltage than the drain (V
> V ) when the
OUT
IN
3. When a load is increased, the output current will
proportionally increase up to the current-limit
switchisdisabled.Inthissituation,theMIC2545A/49Aavoids
undesirable current flow from OUT to IN. Both IN pins must
be connected together, and both OUT pins must be con-
nected together.
threshold, I , as shown in the “Functional
TH
Characteristics: Current-Limit Response” graph.
The device in this case will supply current
slightly higher than the current-limit set point
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.
defined by R
further the current folds back to I
. As the load is increased
SET
.
LIMIT
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:
, is set with an external
LIMIT
The MIC2549A features an internal latch which causes the
part to remain off after thermal shutdown until a reset pulse
isprovidedviatheenablepin(pin1).Whileincurrent-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.
230
RSET
=
ILIMIT
where:
76.8Ω < R
< 459Ω
SET
Enable Input
For example, to set a 1A nominal current-limit, R
calculated as:
is
SET
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.
230
= 230Ω
1A
Refer to the “Typical Characteristics: Current-Limit vs R
graph for more details.
”
Current-Limit Operation
SET
The current limit is user adjustable with an external set
resistor. Current limiting in the range of 500mA to 3A is
availablewithasetpointaccuracyofbetterthan±30%(±20%
Current through R
voltage across R
increases with output current. The
could be monitored with a high imped-
SET
SET
ance comparator to provide an indication of output current.
should be between 76.8Ω and 459Ω (1% resister
value).
for I
1 to 2.5A). The current-limit circuit prevents damage
SET
R
to the output MOSFET and external load.
SET
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
10mAloadtotypically100mVaboveground.Forapplications
with V ≤ 3.6V, it is recommended that flag current be limited
IN
to 5mA or less.
M9999-041204
12
April 2004
MICREL [ MICREL SEMICONDUCTOR ]
