MIC2095_11 Datasheet, PDF (17/25 Pages) Micrel Semiconductor – Current-Limiting Power Distribution Switches

Micrel, Inc.
Figure 1. MIC2097 Kickstart Operation
Figure 1 Label Key:
A. The MIC2097 is enabled into an excessive load
(slew-rate limiting not visible at this time scale) The
initial current surge is limited by either the overall
circuit resistance and power-supply compliance, or
the secondary current limit, whichever is less.
B. R_ON of the power FET increases due to internal
heating.
C. Kickstart period.
D. Current limiting initiated. FAULT/ goes low.
E. V_OUT is non-zero (load is heavy, but not a dead short
where V_OUT = 0V. Limiting response will be the same
for dead shorts).
F. Thermal shutdown followed by thermal cycling.
G. Excessive load released, normal load remains.
MIC2097 drops out of current limiting.
H. FAULT/ delay period followed by FAULT/ going
HIGH.
Enable Input
The ENABLE pin is a logic level compatible input which
turns on or off the main MOSFET switch. There are two
versions of each device. The −1 version has an active
high (ENABLE) and the −2 version has an active low
(ENABLE/).
Fault Output
The FAULT/ is an N-channel open-drain output, which is
asserted (LOW true) when the device either begins
current limiting or enters thermal shutdown. The FAULT/
signal asserts after a brief delay period in order to filter
out very brief over current conditions. After an over-
current or over-temperature fault clears, the FAULT/ pin
remains asserted (low) for the delay period.
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
MIC2095/97/98/99
pull-up resistor. FAULT/ may be tied to a pull-up voltage
source which is less than or equal to V_IN.
Soft-Start Control
Large capacitive loads can create significant inrush
current surges when charged through the current limiting
switch. When the switch is enabled, the built-in soft-start
limits the initial inrush current by slowly turning on the
output.
Power Dissipation and Thermal Shutdown
Thermal shutdown is used to protect the current limiting
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 145°C (typical).
The switch will automatically resume operation when the
die temperature cools down to 135°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 on 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 the following equation:
( ) P_D = R_DS(ON) × I_OUT ²
Eq. 2
To relate this to junction temperature, the following
equation can be used:
T_J = P_D × R_{θ (J-A)} + T_A
Eq. 3
Where T_J = junction temperature, T_A = ambient
temperature, and 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 switch limits the
output current to the maximum value. The heat
generated by the power dissipation of the switch
continuously limiting the current may exceed the
package and PCB’s ability to cool the device and the
switch will shut down and signal a fault condition. Please
see the Fault Output description in the previous page for
more details on the FAULT/ output. After the switch
August 2011
17
M9999-080211-C