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LM5006 Datasheet(PDF) 11 Page - Texas Instruments |
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LM5006 Datasheet(HTML) 11 Page - Texas Instruments |
11 / 27 page FB SW L1 C2 RFB2 VOUT R3 LM5006 RFB1 DC = tON tON + tOFF = VOUT VIN F = VOUT x (Vin - 0.5V) 1.25 x 10 -10 x V IN x (RT + 500 :) F = VOUT 2 x L x 1.28 x 1020 RL x (RT) 2 LM5006 www.ti.com SNVS646B – FEBRUARY 2011 – REVISED MARCH 2013 If the flyback diode (D1 in the Block Diagram) is used, the LM5006 operates in discontinuous conduction mode at light load currents, and continuous conduction mode at heavy load current. In discontinuous conduction mode, current through the output inductor starts at zero and ramps up to a peak during the on-time, then ramps back to zero before the end of the off-time. The next on-time period starts when the voltage at FB falls below the internal reference - until then the inductor current remains zero. In this mode the operating frequency is lower than in continuous conduction mode, and varies with load current. Therefore at light loads the conversion efficiency is maintained, since the switching losses reduce with the reduction in load and frequency. The discontinuous operating frequency can be calculated as follows: where • RL = the load resistance (1) In continuous conduction mode, current flows continuously through the inductor and never ramps down to zero. In this mode the operating frequency is greater than the discontinuous mode frequency and remains relatively constant with load and line variations. The approximate continuous mode operating frequency can be calculated as follows: (2) The buck switch duty cycle is approximately equal to: (3) If a synchronous rectifier (Q1 in the Typical Application schematic on the front page) is used instead of a flyback diode, the LM5006 operates in continuous conduction mode for all values of load current. The switching frequency remains relatively constant with load and line variations, and can be calculated using Equation 2. The output voltage (VOUT) is programmed by two external resistors as shown in the Block Diagram. The regulation point can be calculated as follows: VOUT = 2.5 x (RFB1 + RFB2) / RFB1 (4) The LM5006 regulates the output voltage based on ripple voltage at the feedback input, requiring a minimum amount of ESR for the output capacitor C2. A minimum of 25mV to 50mV of ripple voltage at the feedback pin (FB) is required for the LM5006. In cases where the capacitor ESR is too small, additional series resistance may be required (R3 in the Block Diagram). For applications where lower output voltage ripple is required the output can be taken directly from a low ESR output capacitor, as shown in Figure 32. However, R3 slightly degrades the load regulation. Figure 32. Low Ripple Output Configuration Start-Up Regulator (VCC) The high voltage bias regulator is integrated within the LM5006. The input pin (VIN) can be connected directly to line voltages between 6V and 75V, with transient capability to 80V. The VCC output is regulated at 7.5V. The VCC regulator output current is limited at approximately 30 mA. Copyright © 2011–2013, Texas Instruments Incorporated Submit Documentation Feedback 11 Product Folder Links: LM5006 |
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