MIC2537 查看數據表（PDF） - Micrel

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MIC2537

Quad Power Distribution Switch

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MIC2537

Functional Description

The MIC2537-1 and MIC2537-2 are quad 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 each output MOSFET. OUTx (output) is the

source of each respective MOSFET. In a typical circuit,

current flows through the switch from IN to OUTx toward the

load. If VOUT is greater than VIN, current will 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 (VOUTx > VIN) when the output is off. In

this situation, the MIC2537 prevents reverse current flow.

Thermal Shutdown

Thermal shutdown shuts off the affected output MOSFET

and signals the corresponding fault flags if the die tempera-

ture 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 limits output current to ap-

proximately 200mA. The resulting increase in power dissipa-

tion may cause the shorted channel to go into thermal

shutdown. In addition, even though individual channels are

thermally isolated, it is possible they may shut down when an

adjacent channel is shorted. When this is undesirable, shut-

down of the channels not shorted can be avoided by exter-

nally 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 temperature, board

layout, and load impedance, but is typically several seconds.

The USB controller must therefore recognize a fault and

disable the appropriate channel within this time.

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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 0.15A 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 the following three sce-

narios:

Switch Enabled into Short Circuit

If a switch is powered on or enabled into an excessive load or

short circuit, the switch immediately goes into a constant-

current mode, slowly increasing the output voltage. The fault

flag goes low until the load is reduced.

Short Circuit Applied to Output

When a heavy load or short circuit is applied to an enabled

switch, a large transient current may flow until the current-

limit circuitry responds. Once this occurs, the device limits

current to less than the short circuit current-limit specification.

See the “Functional Characteristics: Current-Limit Transient

Response graph for details.

Current-Limit Response

The MIC2537 current-limit profile exhibits a small foldback

effect of approximately 100mA. 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 MIC2537 will deliver load current up to the

current-limit threshold. See the “Functional Characteristics:

Current-Limit Response” graph for details.

Fault Flag

FLGx is an open-drain N-channel MOSFET output. Fault

flags are active (low) for current limit or thermal shutdown.

Each flag output MOSFET is capable of sinking a 10mA load

to approximately 200mV above ground. Several FLGx pins

may be wired-NOR connected to a common pull-up resistor.

December 1999

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MIC2537

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