MIC2044/2045
Micrel
MIC2044/2045
12
January 2005
/FAULT
The /FAULT signal is an N-Channel, open-drain MOSFET
output. An external pull-up resistor tied to a maximum 6V rail
is required for the /FAULT pin. The /FAULT pin is asserted
(active-low) when either an overcurrent or thermal shutdown
condition occurs. During a hot insert of a PCB or when turning
on into a highly capacitive load, the resulting high transient
inrush current may exceed the current limit threshold of the
MIC2044/45. In the case where an overcurrent condition
occurs, /FAULT will assert only after the flag delay time has
elapsed, typically 32ms. This ensures that /FAULT is as-
serted only upon valid overcurrent conditions and that nui-
sance error reporting is prevented.
Thermal Shutdown
For the MIC2044, thermal shutdown is employed to protect
the device from damage should the die temperature exceed
safe margins due to a short circuit or an excessive load.
Thermal shutdown shuts off the output MOSFET and asserts
the /FAULT output if the die temperature exceeds 140
°
C.
The MIC2044 automatically resets its output and resumes
supplying current to the load when the die temperature drops
to 120
°
C. If the fault is still present, the MIC2044 will quickly
re-heat and shut down again. This process of turning
ON-OFF-ON is called thermal cycling and will continue as
long as the power switch is enabled while the fault or
excessive load is present.
Depending on PCB layout (including thermal considerations
such as heat sinking), package, and ambient temperature, it
may take several hundred milliseconds from the incidence of
the fault to the output MOSFET being shut off.
Circuit Breaker Function (MIC2045)
The MIC2045 is designed to shut off all power to the load
when a fault condition occurs, just as a circuit breaker would
do. In this case, a fault condition is deemed to be anytime the
output current exceeds the current limit for more than the flag
delay period, nominally 32ms. Once the output shuts off, it
remains off until either the fault load is removed from VOUT
or the EN input is cycled ON-OFF-ON. If the fault is still
present after EN has been cycled, the MIC2045 will again
shut off all power to the load after 32ms. Once the fault has
been removed, then normal operation will resume.
Open Load Detection
The MIC2045 will automatically reset its output when the fault
load is cleared. This is accomplished by applying a small
current to VOUT and watching for the voltage at VOUT to rise
to within 200mV of VIN. This current is supplied by an internal
resistor connected to VIN and is connected to VOUT when
MIC2045 latches off.
Power-Good Detection
The MIC2044/45 can detect when the output voltage is above
or below a preset threshold that is monitored by a comparator
at the PGREF input. The PWRGD signal is an N-Channel
open-drain MOSFET output and an external pull-up resistor
up to a 6V maximum rail is required for the PWRGD pin.
Whenever the voltage at the PGREF pin exceeds its thresh-
old (V
TH
), typically 230mV, the PWRGD output is asserted.
Using the typical applications circuit from page 1 that switches
3.3V as an example, the output voltage threshold determin-
ing “power is good” is calculated by the following equation:
V
V
1
R2
R3
OUT(GOOD)
TH
=
×
+
(3)
In substituting the resistor values of the circuit and the typical
PGREF threshold, the resulting V
OUT(GOOD)
is calculated as
3.0V for this 3.3V switching application.
SLEW
The MIC2044/45’s output rise time is controlled at turn-on to
a minimum of 1.5ms and is controlled by an internal slew-rate
limiting circuit. A slew-rate adjustment control pin is available
for applications requiring slower rise times. By placing a
capacitor between SLEW and ground, longer rise times can
be achieved. For further detail, see the
“Applications Informa-
tion”
section.
