MIC2044/2045

Micrel

MIC2044/2045

12

October 2003

/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

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:

VV

1

R2

R3

OUT(GOOD)

TH

=× +









(3)

In substituting the resistor values of the circuit and the typical

PGREF threshold, the resulting V

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-