March 2000

9

MIC2536

MIC2536

Micrel

Functional Description

The MIC2536-1 and MIC2536-2 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 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 V

to IN during an on-condition since the MOSFET is bidirec-

tional when enabled.

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

disabled. In this situation, the MIC2536 prevents reverse

current flow.

Thermal Shutdown

Each output MOSFET has its own thermal sensor. If either or

both channels reach 135

°C, affected channel(s) will be shut

down and flag(s) asserted. 10

°C of hysteresis prevents the

switches from turning on until the die temperature drops to

125

°C. Overtemperature detection functions only when at

least one switch is enabled.

The MIC2536 will automatically reset its output when the die

temperature cools to approximately 125

°C. The MIC2536

output and FLG signal will continue to cycle on and off until the

device is disabled or the fault is removed.

Depending on PCB layout, package, ambient temperature,

etc., it may take several hundred milliseconds from the

occurrence of the fault to the output MOSFET being shut off.

Delay to reach thermal shutdown will be shortest with a dead

short on the output.

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 275mA. 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, ther-

mal 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 temperature, board layout, and load

impedance, but is typically several seconds. The USB con-

troller must therefore recognize a fault and disable the

appropriate channel within this time.

Power Dissipation

Power dissipation depends on several factors such as the

load, PCB layout, ambient temperature and package type.

Equations that can be used to calculate power dissipation

and die temperature are found below:

Calculation of power dissipated by each channel can be

accomplished by the following equation:

P

2

Total power dissipation of the device will be the summation

of P

temperature, the following equation can be used:

T

where:

T

T

θ

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 150mA 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 enabled into a heavy load or short circuit, the

switch immediately goes into a constant-current mode, re-

ducing the output voltage. The FLG is asserted indicating an

overcurrent condition.

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 maximum short-circuit current-limit

specification.

Current-Limit Response Ramped Load

The MIC2536 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 MIC2536 will deliver load current up to the

current-limit threshold before entering current-limited opera-

tion.

Fault Flag

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

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

the case where an overcurrent condition occurs, FLG will be

asserted only after the flag response delay time, t

elapsed. This ensures that FLG is asserted only upon valid

overcurrent conditions and that erroneous error reporting is

eliminated. False overcurrent conditions can occur during

hot-plug events when a highly capacitive load is connected

and causes a high transient inrush current that exceeds the

current-limit threshold. The flag response delay time is typi-

cally 12ms.