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MIC2537-2BM Datasheet(PDF) 8 Page - Micrel Semiconductor |
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MIC2537-2BM Datasheet(HTML) 8 Page - Micrel Semiconductor |
8 / 12 page MIC2537 Micrel December 1999 8 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 V OUT 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 (V OUTx > 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. 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. |
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