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LM5021 数据表(PDF) 7 Page - National Semiconductor (TI) |
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LM5021 数据表(HTML) 7 Page - National Semiconductor (TI) |
7 / 15 page Typical Performance Characteristics Unless otherwise specified: T J = 25˚C. (Continued) Output Switching Frequency vs RT 20144210 Detailed Operating Description START UP CIRCUIT Referring to Figure 2, the input capacitor C VIN is trickle charged through the start-up resistor Rstart, when the recti- fied ac input voltage HV is applied. The VIN current con- sumed by the LM5021 is only 18 µA (nominal) while the capacitor C VIN is initially charged to the start-up threshold. When the input voltage, VIN reaches the upper VIN UVLO threshold of 20V, the internal VCC linear regulator is en- abled. The VCC regulator will remain on until VIN falls to the lower UVLO threshold of 7.25V (12.5V hysteresis). When the VCC regulator is turned on, the external capacitor at the VCC pin begins to charge. The PWM controller, soft-start circuit and gate driver are enabled when the VCC voltage reaches the VCC UVLO upper threshold of 7V. The VCC UVLO has 1.2V hysteresis between the upper and lower thresholds to avoid chattering during transients on the VCC pin. When the VCC UVLO enables the switching power supply, energy is transferred from the primary to the second- ary transformer winding(s). A bias winding, shown in Figure 2, delivers power to the VIN pin to sustain the VCC regulator. The voltage supplied should be from 11V (VCC regulated voltage maximum plus VCC regulator dropout voltage) to 30V (maximum operating VIN voltage). The start-up se- quence is completed and normal operation begins when the voltage from the bias winding is sufficient to maintain VCC level greater than the VCC UVLO threshold (5.8V typical). The size of the start-up resistor Rstart not only affects power supply start-up time, but also power supply efficiency since the resistor dissipates power in normal operation. The ultra low start-up current of the LM5021 allows a large value Rstart resistor (up to 3 M Ω) for improved efficiency with reasonable start-up time. 20144211 FIGURE 2. Start-Up Circuit Block Diagram www.national.com 7 |
类似零件编号 - LM5021 |
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类似说明 - LM5021 |
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