MIC2012

Micrel

MIC2012/2072

8

September 2000

Functional Description

The MIC2012/2072 are designed to support the power distri-

bution requirements for USB wakeup from the ACPI S3 state.

It integrates two independent channels under control of input

S3#. When S3# is asserted LOW (S3 state) the MIC2012/

2072 will switch a 500m

Ω, 100mA MOSFET switch from the

AUX input to each of its two outputs. Conversely when the

S3# input is HIGH (S0 state) the MIC2012/72 will switch a

100m

Ω, 500mA MOSFET switch from the MAIN input to each

of its two outputs. The lower current limit during the ACPI S3

state helps to ensure that the standby supply maintains

regulation even during fault conditions.

Thermal Shutdown

Thermal shutdown is employed to protect the device from

damage should the die temperature exceed safe margins

due mainly to short circuit faults. Thermal shutdown shuts off

the output MOSFET and asserts the FAULT output if the die

temperature reaches 140

°C and the overheated channel is in

current limit. The other channel is not affected. If, however,

the die temperature exceeds 160

°C, both channels will be

shut off even if neither channel is in current limit.

Power Dissipation

The device’s junction temperature depends on several fac-

tors such as the load, PCB layout, ambient temperature and

package type. The power dissipated in each channel is

P

internal MOSFETs and I

Total power dissipation of the device will be the summation of

P

the following equation can be used:

TJ = P

where:

Functional Diagram

Current

Limit

10ms

Timer

Charge Pump

+

Gate Control

S3#

EN

Current

Limit

MAIN

OUT

AUX

/FAULT

AUX

Latch

To Other Channel

*

MAIN

FET

AUX

FET

*MIC2070-1/2 Only

Thermal

Sense

* 'P' options only

T

T

θ

Current Sensing and Limiting

The current-limit thresholds are preset internally for each

state. The preset level prevents damage to the device and

external load but still allows a minimum current of 100mA or

500mA to be delivered to the load depending on the state of

the device according to the S3# input. When S3# is LOW the

current-limit is set at 100mA minimum. When S3# is HIGH the

current-limit is set at 500mA minimum.

Should an over-current condition last longer than t

MIC2072 will latch the faulty output off. The output will remain

off until either the load is removed or the EN signal (MIC2072-

1, -2) is toggled. When the MIC2072 enters a latched output

condition a 1mA pull-up current source is activated. This

provides a way to automatically reset the output once the load

is removed without the need to toggle the enable input such

as in the MIC2072. Please refer to Figure 7 for timing details.

The MIC2012 will automatically reset its output when the die

temperature cools down to 120

°C. The MIC2012 output and

FAULT signal will continue to cycle on and off until the device

is disabled or the fault is removed. Figure 6 depicts typical

timing. Depending on PCB layout, package, ambient tem-

perature, etc., it may take several hundred milliseconds from

the incidence of the fault to the output MOSFET being shut

off. This time duration will be shortest in the case of a dead

short on the output.