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LM27313XMFX Datasheet(PDF) 8 Page - Texas Instruments |
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LM27313XMFX Datasheet(HTML) 8 Page - Texas Instruments |
8 / 20 page Duty Cycle = VOUT + VDIODE - VIN VOUT + VDIODE - VSW LM27313 SNVS487D – DECEMBER 2006 – REVISED APRIL 2013 www.ti.com Figure 13. Recommended PCB Component Layout Some additional guidelines to be observed: 1. Keep the path between L1, D1, and C2 extremely short. Parasitic trace inductance in series with D1 and C2 will increase noise and ringing. 2. The feedback components R1, R2 and CF must be kept close to the FB pin of the LM27313 to prevent noise injection on the high impedance FB pin. 3. If internal ground planes are available (recommended) use vias to connect directly to the LM27313 ground at device pin 2, as well as the negative sides of capacitors C1 and C2. SETTING THE OUTPUT VOLTAGE The output voltage is set using the external resistors R1 and R2 (see Typical Application Circuits). A value of 13.3 k Ω is recommended for R2 to establish a divider current of approximately 92 µA. R1 is calculated using the formula: R1 = R2 x ( (VOUT / VFB) − 1 ) (2) DUTY CYCLE The maximum duty cycle of the switching regulator determines the maximum boost ratio of output-to-input voltage that the converter can attain in continuous mode of operation. The duty cycle for a given boost application is defined as: (3) This applies for continuous mode operation. The equation shown for calculating duty cycle incorporates terms for the FET switch voltage and diode forward voltage. The actual duty cycle measured in operation will also be affected slightly by other power losses in the circuit such as wire losses in the inductor, switching losses, and capacitor ripple current losses from self-heating. Therefore, the actual (effective) duty cycle measured may be slightly higher than calculated to compensate for these power losses. A good approximation for effective duty cycle is : DC (eff) = (1 - Efficiency x (VIN / VOUT)) (4) Where the efficiency can be approximated from the curves provided. 8 Submit Documentation Feedback Copyright © 2006–2013, Texas Instruments Incorporated Product Folder Links: LM27313 |
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