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LM3543 Folha de dados(PDF) 8 Page - National Semiconductor (TI) |
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LM3543 Folha de dados(HTML) 8 Page - National Semiconductor (TI) |
8 / 12 page Functional Description Power Switches The power switches that comprise the three ports of the LM3543 are N-Channel MOSFETs. They have a typical on-state drain-to-source resistance of 90 m Ω when the input voltage is 5 V. When enabled, each switch will supply a 500 mA minimum current to its load. In the unlikely event that a switch is enabled and the output voltage of that switch is pulled above the input voltage, the bi-directional nature of the switch results in current to flow from the output to the input. When a switch is disabled, current flow through the switch is prevented in both directions. Charge Pump and Driver The gate voltages of the high-side NFET power switches are supplied by an internal charge-pump and driver circuit combination. The charge pump is a low-current switched-capacitor circuit that efficiently generates voltages above the LM3543 input supply. The charge pump output is used to supply a transconductance amplifier driver circuit that controls the gate voltages of the power switches. Rise and fall times on the gates are typically kept between 2 ms and 4 ms to limit large current surges and associated electromagnetic interference (EMI). ENABLE (EN x or ENx) The LM3543 comes in two versions: an active-high enable version, LM3543-H, and an active-low enable version, LM3543-L. In the LM3543-H, the EN x pins are active-high logic inputs that, when asserted, turn on the associated power supply switch(es). Power supply switches are controlled by the EN x active-low logic inputs in the LM3543-L. With all three ports disabled on either version of the LM3543, less than 5 µA of supply current is consumed. Both types of enable inputs, active-high and active-low, are TTL and CMOS logic compatible. Input and Output The power supply to the control circuitry and the drains of the power-switch MOSFETs are connected to the two input pins, IN1 and IN2. These two pins are connected externally in most standard applications. The two ground nodes GND1 and GND2 must be connected externally in all applications. Pins OUT1, OUT2, and OUT3 are connections to the source nodes of the power-switch MOSFETs. In a typical application circuit, current flows through the switches from IN1 and IN2 to OUT x toward the load. Undervoltage Lockout (UVLO) Undervoltage Lockout (UVLO) prevents the MOSFET switches from turning on until the input voltage exceeds a typical value of 1.8V. If the input voltage drops below the UVLO threshold, the MOSFET switches are opened and fault flags are activated. UVLO flags function only when one or more of the ports is enabled. Due to the paired nature of the design, both FLAG1 and FLAG2 will assert if either port1 or port2 is enabled in a UVLO condition. Current Limit and Foldback The current limit circuit is designed to protect the system supply, the LM3543 switches, and the load from potential damage resulting from excessive currents. If a direct short occurs on an output of the LM3543, the input capacitor(s) rapidly discharge through the part, activating current limit circuitry. The threshold for activating current limiting is 2.0A (typ.). Protection is achieved by momentarily opening the MOSFET switch and then gradually turning it on. Turn-on is halted when the current through the switch reaches the current-limit level of 1.0A (typ.) The current is held at this level until either the excessive load/short is removed or the part overheats and thermal shutdown occurs (see Thermal Shutdown section, below). The fault flag of a switch is asserted whenever the switch is current limiting. If a port on the LM3543 is enabled into a short condition, the output current of that port will rise to the current-limit level and hold there. When a port is in a current-limit condition, the LM3543 senses the output voltage on that port and, if it is less than 1.0V (typ.), will reduce the output current through that port. This operation is shown in Figure 2, below. The current reduction, or foldback, reduces power dissipation through the overloaded MOSFET switch. An additional advantage of the foldback feature is the reduction of power required from the source supply when one or more output ports are shorted. Thermal Shutdown The LM3543 is internally protected against excessive power dissipation by a two-stage thermal protection circuit. If the device temperature rises to approximately 145˚C, the thermal shutdown circuitry turns off any switch that is current limited. Non-overloaded switches continue to function normally. If the die temperature rises above 160˚C, all switches are turned off and all three fault flag outputs are activated. Hysteresis ensures that a switch turned off by thermal shutdown will not be turned on again until the die temperature is reduced to 135˚C. Shorted switches will continue to cycle off and on, due to the rising and falling die temperature, until the short is removed. The thermal shutdown function is shown graphically in Figure 3 and Figure 4. 10125817 FIGURE 2. Short-Circuit Output Current (with Foldback) vs. Output Voltage www.national.com 8 |
Nº de peça semelhante - LM3543 |
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Descrição semelhante - LM3543 |
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