MIC2524/2527

Micrel

MIC2524/2527

8

June 1999

Functional Description

The MIC2524/7-1 and MIC2524/7-2 are quad high-side

switches with active-high and active-low enable inputs, re-

spectively. Fault conditions turn off or inhibit turn-on of one or

more of the output transistors, depending upon the type of

fault, and activate the open-drain error flag transistors mak-

ing them sink current to ground.

Input and Output

IN (input) is the power supply connection to the logic circuitry

and the drain of the output MOSFET. OUTx (output) is the

source of its respective MOSFET. In a typical circuit, current

flows through the switch from IN to OUT toward the load. If

V

since the MOSFET is bidirectional when on.

The output MOSFET and driver circuitry are also designed to

allow the MOSFET source to be externally forced to a higher

voltage than the drain (V

this situation, the MIC2524/7 prevents reverse current flow.

If V

Thermal Shutdown

Thermal shutdown shuts off the affected output MOSFETs

and signals all fault flags if the die temperature exceeds

135

°C. 10°C of hysteresis prevents the switch from turning on

until the die temperature drops to 125

°C. Overtemperature

detection functions only when at least one switch is enabled.

Current Limit Induced Thermal Shutdown

Internal circuitry increases the output MOSFET on-resis-

tance until the series combination of the MOSFET on-resis-

tance and the load impedance limit current to typically 850mA.

The increase in power dissipation, in most cases, will cause

the MIC2524/7 to go into thermal shutdown, disabling af-

fected channels. When this is undesirable, thermal shutdown

can be avoided by externally responding to the fault and

disabling the current limited channel before the shutdown

temperature is reached. The delay between the flag indica-

tion of a current limit fault and thermal shutdown will vary with

ambient temperature, board layout, and load impedance, but

is typically several hundred milliseconds. The USB controller

must therefore recognize a fault and disable the appropriate

channel within this time. If the fault is not removed or the

switch is not disabled within this time, then the device will

enter into a thermal oscillation of about 2Hz. This does not

cause any damage to the device. Refer to “Functional Char-

acteristics: Thermal Shutdown Response.”

Undervoltage Lockout

UVLO (undervoltage lockout) prevents the output MOSFET

from turning on until V

undervoltage state, the FLAG will be low. After the switch

turns on, if the voltage drops below approximately 2.3V,

UVLO shuts off the output MOSFET and signals fault flag.

Undervoltage detection functions only when at least one

switch is enabled.

Current Sensing and Limiting

The current-limit threshold is preset internally. The preset

level prevents damage to the output MOSFET and external

load but allows a minimum current of 0.5A through the output

MOSFET of each channel.

The current-limit circuit senses a portion of the output FET

switch current. The current sense resistor shown in the block

diagram is virtual and has no voltage drop. The reaction to an

overcurrent condition varies with three scenarios:

Switch Enabled into Short Circuit

If a switch is powered on or enabled into a heavy load or short-

circuit, the switch immediately goes into a constant-current

mode, reducing the output voltage. The fault flag goes low

until the load is reduced. See the “Functional Characteristics:

Short Circuit Response, Enabled into Short Circuit” photo.

Short Circuit Applied to Output

When a heavy load is applied, a large transient current may

flow until the current limit circuitry will respond. Once this

occurs, the device limits current to less than the short-circuit

current limit specification. See the “Short Circuit Transient

Response, Short Applied to Output” graph.

Current-Limit Response

The MIC2524/7 current-limit profile exhibits a small foldback

effect of approximately 500mA. Once this current-limit thresh-

old is exceeded the device enters constant-current mode.

This constant current is specified as the short circuit current

limit in the “Electrical Characteristics” table. It is important to

note that the MIC2524/7 will deliver load current up to the

current-limit threshold which is typically 1.6A. Refer to “Func-

tional Characteristics: Current-Limit Response” photo for

details.

Fault Flag

FLG is an N-channel, open-drain MOSFET output. The fault-

flag is active (low) for one or more of the following conditions:

undervoltage (while 2V < V

shutdown. The flag output MOSFET is capable of sinking a

10mA load to typically 100mV above ground. Multiple FLG

pins may be “wire NORed” to a common pull-up resistor.