MIC2526

Micrel

MIC2526

8

February 2000

Functional Description

The MIC2526-1 and MIC2526-2 are dual high-side switches

with active-high and active-low enable inputs, respectively.

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 making 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

flow from OUT to IN 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 MIC2526 avoids undesirable current flow

from OUT to IN. 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 MIC2526 to go into thermal shutdown, disabling affected

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 indication of a current

limit fault and thermal shutdown will vary with ambient tem-

perature, board layout, and load impedance, but is typically

several hundred milliseconds. The USB controller must there-

fore 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 Characteristics:

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 MIC2526 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 MIC2526 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.