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LR645LG-G Datasheet(PDF) 4 Page - Supertex, Inc |
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LR645LG-G Datasheet(HTML) 4 Page - Supertex, Inc |
4 / 9 page 4 LR645 Figure 2: High Current SMPS Start-Up Circuit R2 R1 + 15V to 400V – + 5.0V – DN2540 C OUT C IN V CC V AUX = 12V Note: When used with the DN25, +V IN is not connected on the LR6. PWM IC LR645 V OUT GND GATE TRIM LR645: High Current SMPS Start-Up Circuit The 8 lead version of the LR645 has connections for an ex- ternal depletion-mode MOSFET for higher output current and external resistors for adjustable output voltage. As shown in Figure 2, the output current is increased to 150mA by using the Supertex 400V depletion-mode MOSFET DN2540. The maximum operating input voltage will be limited by the drain- to-source breakdown voltage of the external MOSFET, but cannot exceed the 450V rating of the LR645. The output voltage can be adjusted from 8V to 12V with 2ex- ternal resistors, R1 and R2. The ratio of R2/R1 determines the output voltage. R2 is connected between the V OUT and TRIM pins. R1 is connected between TRIM and GND pins. Figure 5 is a curve showing output voltage versus resistor ratio R2/R1. The optimum range for R1 + R2 is 200KΩ to 300KΩ. This minimizes loading and optimizes accuracy of the output voltage. Figure 5 uses an R1 + R2 of 250KΩ. LR645: Off Line Linear Regulator Circuits requiring low voltages to operate logic and analog circuits benefit from the LR645. The conventional use of step down transformers can be eliminated, thereby saving space and cost. Some examples of these applications are: proximity controlled light switches, street lamp controls, and low voltage power supplies for appliances such as washing machines, dishwashers, and refrigerators. The wide operating input voltage range of 15V to 450V as well as the ripple rejection ratio of 50dB minimum allows the use of a small, high voltage input capacitor. The input AC line can be either full-wave or half-wave rectified. A minimum output capacitance of 0.01µF is recommended for output stability. Figure 3 shows the LR645 as a pre-regulator to a precision- regulator for high precision regulation. Higher output cur- rent is also possible by using an external depletion-mode MOSFET DN2540N5 as shown in Figure 4. Power Dissipation Considerations The LR645 is a true linear regulator. Its power dissipation is- therefore a function of input voltage and output load current. Forexample, if the LR645 is providing a continuous load cur- rent of 3mAat 10V while its input voltage is 400V, total dis- sipation in the LR645 will be: P DISS= (VIN - VOUT) x (IOUT + IMAX QUIESCENT) = (400V - 10V) x (3.0mA + 150µA) = 1.23 Watts The 1.23 watts is for continuous operation. This is within the dissipation capabilities of the TO-220 and SOT-89 packag- es. See the thermal characteristics chart on page 2 for derat- ings. For SMPS start-up applications, the output current is usually required only during start-up. This duration depends upon the auxiliary supply output capacitor and C OUT, but is typically a few hundred milliseconds. All package types of the LR645 have been characterized for use with a C OUT of at least 10µF, and an AC line of 277V. 5.000V ± 0.002V @ 0 to 3mA AC Line 24V to 277V C OUT 0.1µF C IN 1.0µF LR6 Max 875 ACSA Figure 3: Cascading for Precision |
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