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LM5020MM-1 Datasheet(PDF) 7 Page - National Semiconductor (TI) |
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LM5020MM-1 Datasheet(HTML) 7 Page - National Semiconductor (TI) |
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7 / 12 page  Detailed Operating Description The LM5020 High Voltage PWM controller contains all of the features needed to implement single ended primary power converter topologies. The LM5020 includes a high-voltage startup regulator that operates over a wide input range to 100V. The PWM controller is designed for high speed capa- bility including an oscillator frequency range to 1MHz and total propagation delays less than 100ns. Additional features include an error amplifier, precision reference, line under- voltage lockout, cycle-by-cycle current limit, slope compen- sation, softstart, oscillator sync capability and thermal shut- down. The functional block diagram of the LM5020 is shown in Figure 1. The LM5020 is designed for current-mode con- trol power converters, which require a single drive output, such as Flyback and Forward topologies. The LM5020 pro- vides all of the advantages of current-mode control including line feed-forward, cycle-by-cycle current limiting and simpli- fied loop compensation . High Voltage Start-Up Regulator The LM5020 contains an internal high voltage startup regu- lator, that allows the input pin (Vin) to be connected directly to line voltages as high as 100V. The regulator output is internally current limited to 15mA. When power is applied, the regulator is enabled and sources current into an external capacitor connected to the V CC pin. The recommended ca- pacitance range for the Vcc regulator is 0.1µF to 100µF. When the voltage on the V CC pin reaches the regulation level of 7.7V, the controller output is enabled. The controller will remain enabled until V CC falls below 6V. In typical applications, a transformer auxiliary winding is connected through a diode to the V CC pin. This winding should raise the V CC voltage above 8V to shut off the internal startup regulator. Powering V CC from an auxiliary winding improves conversion efficiency while reducing the power dissipated in the controller. The external V CC capacitor must be selected such that the capacitor maintains the Vcc volt- age greater than the V CC UVLO falling threshold (6V) during the initial start-up. During a fault condition when the con- verter auxiliary winding is inactive, external current draw on the V CC line should be limited such that the power dissipated in the start-up regulator does not exceed the maximum power dissipation capability of the controller. An external start-up or other bias rail can be used instead of the internal start-up regulator by connecting the V CC and the Vin pins together and feeding the external bias voltage (8- 15V) to the two pins. Line Under Voltage Detector The LM5020 contains a line Under Voltage Lock Out (UVLO) circuit. An external set-point voltage divider from Vin to GND sets the operational range of the converter. The resistor divider must be designed such that the voltage at the UVLO pin is greater than 1.25V when Vin is in the desired operating range. If the under voltage threshold is not met, all functions of the controller are disabled and the controller remains in a low power standby state. UVLO hysteresis is accomplished with an internal 20µA current source that is switched on or off into the impedance of the set-point divider. When the UVLO threshold is ex- ceeded, the current source is activated to instantly raise the voltage at the UVLO pin. When the UVLO pin voltage falls below the 1.25V threshold the current source is turned off, causing the voltage at the UVLO pin to fall. The UVLO pin can also be used to implement a remote enable / disable function. If an external transistor pulls the UVLO pin below the 1.25V threshold, the converter will be disabled. Error Amplifier An internal high gain error amplifier is provided within the LM5020. The amplifier’s non-inverting input is internally set to a fixed reference voltage of 1.25V. The inverting input is connected to the FB pin. In non-isolated applications, the power converter output is connected to the FB pin via volt- age scaling resistors. Loop compensation components are connected between the COMP and FB pins. For most iso- lated applications the error amplifier function is implemented on the secondary side of the converter and the internal error amplifier is not used. The internal error amplifier is config- ured as an open drain output and can be disabled by con- necting the FB pin to ground. An internal 5K pull-up resistor between a 5V reference and COMP can be used as the pull-up for an optocoupler in isolated applications. Current Limit/Current Sense The LM5020 provides a cycle-by-cycle over current protec- tion function. Current limit is accomplished by an internal current sense comparator. If the voltage at current sense comparator input exceeds 0.5V, the output will be immedi- ately terminated. A small RC filter, located near the control- ler, is recommended to filter noise from the current sense signal. The CS input has an internal MOSFET which dis- charges the CS pin capacitance at the conclusion of every cycle. The discharge device remains on an additional 50ns after the beginning of the new cycle to attenuate the leading edge spike on the current sense signal. The LM5020 current sense and PWM comparators are very fast, and may respond to short duration noise pulses. Layout considerations are critical for the current sense filter and sense resistor. The capacitor associated with the CS filter must be located very close to the device and connected directly to the pins of the controller (CS and GND). If a current sense transformer is used, both leads of the trans- former secondary should be routed to the sense resistor and the current sense filter network. A sense resistor located in the source of the primary power MOSFET may be used for current sensing, but a low inductance resistor is required. When designing with a current sense resistor all of the noise sensitive low power ground connections should be con- nected together local to the controller and a single connec- tion should be made to the high current power ground (sense resistor ground point). www.national.com 7 |
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