MIC2545A Datasheet, PDF (11/16 Pages) Micrel Semiconductor – Programmable Current Limit High-Side Switch

MIC2545A/2549A
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.
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 MIC2545A/49A avoids
undesirable current flow from OUT to IN. Both IN pins must
be connected together, and both OUT pins must be con-
nected together.
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.
The MIC2549A 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.
Current-Limit Operation
The current limit is user adjustable with an external set
resistor. Current limiting in the range of 500mA to 3A is
available with a set point accuracy of better than ±30% (±20%
for I_SET 1 to 2.5A). The current-limit circuit prevents damage
to the output MOSFET and external load.
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:
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_LIMIT, is defined by the value of
R_SET as explained further in this section.
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_LIMIT called
I_SHORT. This further protects the load and
reduces device power dissipation. Refer to the
“Typical Characteristics: Output Current vs.
Output Voltage” graph for details.
3. When a load is increased, the output current will
proportionally increase up to the current-limit
threshold, I_TH, as shown in the “Functional
Characteristics: Current-Limit Response” graph.
The device in this case will supply current
slightly higher than the current-limit set point
defined by R_SET. As the load is increased
further the current folds back to I_LIMIT.
The nominal current-limit value, I_LIMIT, is set with an external
resistor between ILIM (pin 4) and GND (pin 3). For a desired
current-limit, the value of the external set resistor is given by:
R _SET
=
 ₂₃₀ 
 ^I_LIMIT 
where:
76.8Ω < R_SET < 459Ω
For example, to set a 1A nominal current-limit, R_SET is
calculated as:
 213A0 _{= 230Ω}
Refer to the “Typical Characteristics: Current-Limit vs R_SET”
graph for more details.
Current through R_SET increases with output 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 76.8Ω and 459Ω (1% resister
value).
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. For applications
with V_IN ≤ 3.6V, it is recommended that flag current be limited
to 5mA or less.
June 2000
11
MIC2545A/2549A