MIC2090_11 Datasheet, PDF (16/23 Pages) Micrel Semiconductor – Current Limiting Power Distribution Switches

Micrel, Inc.
During an overcurrent or short circuit, The FAULT/ signal
asserts after a brief delay period, t_{D_FAULT/}, in order to
filter out false or transient over-current conditions.
The FAULT/ output is open-drain and must be pulled
HIGH with an external resistor. The FAULT/ signal may
be wire-OR’d with other similar outputs, sharing a single
pull-up resistor.
Power Dissipation and Thermal Shutdown
Thermal shutdown is used to protect the
MIC2090/MIC2091 switch from damage should the die
temperature exceed a safe operating temperature.
Thermal shutdown shuts off the output MOSFET and
asserts the FAULT/ output if the die temperature
reaches the over-temperature threshold, T_OVERTEMP.
The switch will automatically resume operation when the
die temperature cools down to 140°C. If resumed
operation results in reheating of the die, another
shutdown cycle will occur and the switch will continue
cycling between ON and OFF states until the reason for
the overcurrent condition has been resolved.
Depending upon the PCB layout, package type, ambient
temperature, etc., hundreds of milliseconds may elapse
from the time a fault occurs to the time the output
MOSFET will be shut off. This delay is caused because
of the time it takes for the die to heat after the fault
condition occurs.
Power dissipation depends on several factors such as
the load, PCB layout, ambient temperature, and supply
voltage. Calculation of power dissipation can be
accomplished by Equation 2:
P_D = R_DS(ON) × (I_OUT)²
Eq.2
To relate this to junction temperature, Equation 3 can be
used:
T_J = P_D × R_θ(J-A) + T_A
Where:
T_J = Junction Temperature
T_A = Ambient Temperature
Eq. 3
R_θ(J-A) is the thermal resistance of the package.
In normal operation, excessive switch heating is most
often caused by an output short circuit. If the output is
shorted, when the switch is enabled, the
MIC2090/MIC2091 switch limits the output current to the
maximum value. The heat generated by the power
dissipation of the switch continuously limiting the current
MIC2090/MIC2091
may exceed the package and PCB’s ability to cool the
device and the MIC2090/MIC2091 will shut down and
signal a fault condition. Please see the “Fault Output”
description for more details on the FAULT/ output.
After the MIC2090/MIC2091 shuts down, and cools, it
will re-start itself if the enable signal remains true.
n Figure 2, die temperature is plotted against I_OUT
assuming a constant ambient temperature of 85°C and a
worst case internal switch on-resistance (R_ON). This plot
is valid for both the MIC2090 and MIC2091.
Die Temperature vs. Output Current
(Ambient Temperature = 85°C)
90
89
88
87
86
85
84
83
0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1
I_OUT (A)
Figure 2. Die Temperature vs. I_OUT
I_LIMIT vs. I_OUT Measured (-1 version only)
When the MIC2090/MIC2091 is current limiting, it is
designed to act as a constant current source to the load.
As the load tries to pull more than the maximum current,
V_OUT drops and the input to output voltage differential
increases. When V_OUT drops below 1.8V, then the output
switch momentarily turns off to insure the internal
MOSFET switch is not damaged by a very fast short
circuit event.
When measuring I_OUT in an overcurrent condition, it is
important to remember voltage dependence, otherwise
the measurement data may appear to indicate a problem
when none really exists. This voltage dependence is
illustrated in Figures 3 and 4.
In Figure 3, output current is measured as V_OUT is pulled
below V_IN, with the test terminating when V_OUT is 2.5V
below V_IN. Observe that once I_LIMIT is reached I_OUT
remains constant throughout the remainder of the test.
Figure 4 repeats this test but simulates operation deeper
into an overcurrent condition. When V_OUT drops below
1.8V, the switch turns off for a few microseconds before
turning back on.
July 2011
16
M9999-070611-B