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LM3481MM датащи(PDF) 9 Page - Texas Instruments |
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LM3481MM датащи(HTML) 9 Page - Texas Instruments |
9 / 30 page 'Vsamp1 = - M2 M1 D 1-D 'Vsamp0 'Vsamp0 = - PWM Comparator Oscillator Sets the RS Latch PWM Comparator resets the RS latch Blank-Out prevents false reset + _ VSL Tmin (on) Blank-Out time LM3481 www.ti.com SNVS346E – NOVEMBER 2007 – REVISED APRIL 2012 Figure 18. Basic Operation of the PWM Comparator OVER VOLTAGE PROTECTION The LM3481 has over voltage protection (OVP) for the output voltage. OVP is sensed at the feedback pin (FB). If at anytime the voltage at the feedback pin rises to VFB + VOVP, OVP is triggered. See the electrical characteristics section for limits on VFB and VOVP. OVP will cause the drive pin (DR) to go low, forcing the power MOSFET off. With the MOSFET off, the output voltage will drop. The LM3481 will begin switching again when the feedback voltage reaches VFB + (VOVP - VOVP(HYS)). See the electrical characteristics section for limits on VOVP(HYS). The internal bias of the LM3481 comes from either the internal bias voltage generator as shown in the block diagram or directly from the voltage at the VIN pin. At input voltages lower than 6V the internal IC bias is the input voltage and at voltages above 6V the internal bias voltage generator of the LM3481 provides the bias. SLOPE COMPENSATION RAMP The LM3481 uses a current mode control scheme. The main advantages of current mode control are inherent cycle-by-cycle current limit for the switch and simpler control loop characteristics. It is easy to parallel power stages using current mode control since current sharing is automatic. However there is a natural instability that will occur for duty cycles, D, greater than 50% if additional slope compensation is not addressed as described below. The current mode control scheme samples the inductor current, IL, and compares the sampled signal, Vsamp, to a internally generated control signal, Vc. The current sense resistor, RSEN, as shown in Figure 22, converts the sampled inductor current, IL, to the voltage signal, Vsamp, that is proportional to IL such that: Vsamp = IL x RSEN The rising and falling slopes, M1 and −M2 respectively, of Vsamp are also proportional to the inductor current rising and falling slopes, Mon and −Moff respectively. Where Mon is the inductor slope during the switch on-time and −Moff is the inductor slope during the switch off-time and are related to M1 and −M2 by: M1 = Mon x RSEN −M2 = −Moff x RSEN For the boost topology: Mon = VIN / L −Moff = (VIN − VOUT) / L M1 = [VIN / L] x RSEN −M2 = [(VIN − VOUT) / L] x RSEN M2 = [(VOUT − VIN) / L] x RSEN Current mode control has an inherent instability for duty cycles greater than 50%, as shown in Figure 19, where the control signal slope, MC, equals zero. In Figure 19, a small increase in the load current causes the sampled signal to increase by ΔVsamp0. The effect of this load change, ΔVsamp1, at the end of the first switching cycle is : Copyright © 2007–2012, Texas Instruments Incorporated Submit Documentation Feedback 9 Product Folder Links: LM3481 |
Аналогичный номер детали - LM3481MM |
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Аналогичное описание - LM3481MM |
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