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LM5021MMX-1 Datasheet(PDF) 8 Page - Texas Instruments |
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LM5021MMX-1 Datasheet(HTML) 8 Page - Texas Instruments |
8 / 23 page UPPER LOWER INTERNAL BIAS GENERATOR VCC UVLO S R Q + HV VIN VCC REGULATOR VCC Rstart CVIN VIN UVLO CVCC Enable Driver TRANSFORMER BIAS WINDING LM5021 SNVS359D – MAY 2005 – REVISED MARCH 2013 www.ti.com DETAILED OPERATING DESCRIPTION START UP CIRCUIT Referring to Figure 9, the input capacitor CVIN is trickle charged through the start-up resistor Rstart, when the rectified ac input voltage HV is applied. The VIN current consumed by the LM5021 is only 18 µA (nominal) while the capacitor CVIN 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 enabled. 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 secondary transformer winding(s). A bias winding, shown in Figure 9, 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 bias winding should always be connected to the VIN pin as shown in Figure 9. Do not connect the bias winding to the VCC pin. The start-up sequence 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 LM5021 is designed for ultra-low start-up current into the VIN pin. To accomplish this very low start-up current, the VCC regulator of the LM5021 is unique as compared to the VCC regulator used in other controllers of the LM5xxx family. The LM5021 is designed specifically for applications with the bias winding connected to the VIN pin as shown in Figure 9. It is not recommended that the bias winding be connected to the VCC pin of the LM5021. 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. Figure 9. Start-Up Circuit Block Diagram RELATIONSHIP BETWEEN INPUT CAPACITOR CIN & VCC CAPACITOR CVCC The internal VCC linear regulator is enabled when VIN reaches 20V. The drop in VIN due to charge transfer from CVIN to CVCC after the regulator is enabled can be calculated from the following equations where VIN' is the voltage on CVIN immediately after the VCC regulator charges CVCC. ΔVIN x CVIN = ΔVCC x CVCC (1) (20V – VIN') CVIN = 8.5V CVCC (2) 8 Submit Documentation Feedback Copyright © 2005–2013, Texas Instruments Incorporated Product Folder Links: LM5021 |
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