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MIC2545A-2BTS Datasheet(PDF) 11 Page - Micrel Semiconductor |
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MIC2545A-2BTS Datasheet(HTML) 11 Page - Micrel Semiconductor |
11 / 16 page June 2000 11 MIC2545A/2549A MIC2545A/2549A Micrel Functional Description The MIC2545A and MIC2549A are high-side N-channel switches available with active-high or active-low enable in- puts. Fault conditions turn off or inhibit turn-on of the output transistor and activate the open-drain error flag transistor making it sink current to ground. Input and Output IN is the power supply connection to the logic circuitry and the drain of the output MOSFET. OUT is the source of the output MOSFET. In a typical circuit, current flows from IN to OUT toward the load. If V OUT is greater than VIN, current will flow from OUT to IN since the switch is bidirectional when en- abled. 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 OUT > VIN) when the switch is disabled. In this situation, the MIC2545A/49A avoids undesirable current flow from OUT to IN. Both IN pins must be connected together, and both OUT pins must be con- nected together. Thermal Shutdown Thermal shutdown shuts off the output MOSFET and signals the fault flag 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 the switch is enabled. The MIC2549A features an internal latch which causes the part to remain off after thermal shutdown until a reset pulse is provided via the enable pin (pin 1). While in current-limit, the thermal shutdown latch prevents on/off cycling of the output. Refer to Figure 2 for timing diagram. The flag remains low until reset. Enable Input EN must be driven logic high or logic low, or be pulled high or low for a clearly defined input. Floating the input may cause unpredictable operation. EN should not be allowed to go negative with respect to GND. Current-Limit Operation The current limit is user adjustable with an external set resistor. Current limiting in the range of 500mA to 3A is available with a set point accuracy of better than ±30% (±20% for I SET 1 to 2.5A). The current-limit circuit prevents damage to the output MOSFET and external load. The current-limit response of the MIC2545A/49A is based on the type of load that is applied to the output and is defined in three parts: 1. The first mode of operation is where the device enters a constant-current mode preventing further increases in output current. The value of this current, I LIMIT, is defined by the value of R SET as explained further in this section. 2. When a short circuit is applied to the output of an enabled device the output current immedi- ately folds back to a value less than I LIMIT called I SHORT. This further protects the load and reduces device power dissipation. Refer to the “Typical Characteristics: Output Current vs. Output Voltage” graph for details. 3. When a load is increased, the output current will proportionally increase up to the current-limit threshold, I TH, as shown in the “Functional Characteristics: Current-Limit Response” graph. The device in this case will supply current slightly higher than the current-limit set point defined by R SET. As the load is increased further the current folds back to I LIMIT. The nominal current-limit value, I LIMIT, is set with an external resistor between ILIM (pin 4) and GND (pin 3). For a desired current-limit, the value of the external set resistor is given by: R = I SET LIMIT 230 where: 76.8 Ω < R SET < 459Ω For example, to set a 1A nominal current-limit, R SET is calculated as: 230 1A =Ω 230 Refer to the “Typical Characteristics: Current-Limit vs R SET” graph for more details. Current through R SET increases with output current. The voltage across R SET could be monitored with a high imped- ance comparator to provide an indication of output current. R SET should be between 76.8Ω and 459Ω (1% resister value). Fault Flag FLG is an N-channel, open-drain MOSFET output. The fault- flag is active (low) for current-limit or thermal shutdown conditions. The flag output MOSFET is capable of sinking a 10mA load to typically 100mV above ground. For applications with V IN ≤ 3.6V, it is recommended that flag current be limited to 5mA or less. |
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