MIC2545A_11 Datasheet, PDF (13/18 Pages) Micrel Semiconductor – Programmable Current-Limit High-Side Switch

Micrel, Inc.
Functional Description
The MIC2545A and MIC2549A 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 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 MIC2545A/49A avoids undesirable current
flow from OUT to IN. Both IN pins must be connected
together, and 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
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 (pin1).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.
MIC2545A/2549A
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 immediately 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 I_LIM (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
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-
impedance 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 2011
13
M9999-062711-A