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LM3414SDX Datenblatt(PDF) 10 Page - Texas Instruments |
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LM3414SDX Datenblatt(HTML) 10 Page - Texas Instruments |
10 / 30 page VLED VIN 400 nS x fSW t ILED ILX IL1 1/fSW tON Time ILED = IL(AVERAGE) = Mid-point of ILX during tON LM3414, LM3414HV SNVS678F – JUNE 2010 – REVISED NOVEMBER 2015 www.ti.com 7.3 Feature Description 7.3.1 Pulse-Level-Modulation (PLM) Operation Principles The main control circuitry of the LM3414/HV is generally a Pulse-Width-Modulated (PWM) controller with the incorporation of the Pulse-Level-Modulation (PLM) technology. PLM is a technology that facilitates true output average current control without the need to sense the output current directly. In the LM3414/LM3414HV, the PLM circuit senses the current of the internal switch through integrated current sensing circuitry to realize average output current control. The use of PLM reduces the current sensing power losses as it needs current information only when the switch is turned ON. For proper operation of this control scheme, the converter must operate in CCM (continuous conduction mode), so the switching frequency and inductor value must be chosen to prevent the inductor current reaching 0 A during the switch OFF time each cycle. In general, for the LED drivers with current sensing resistor at the output, the power dissipation on the current sensing resistor is ILED 2 × R ISNS, where ILED is the average output current and RISNS is the resistance of the current sensing resistor. In the LM3414/LM3414HV, power dissipates on the internal RISNS only during ON period of the internal power switch. The power loss on RISNS(internal) becomes ILED 2 × R ISNS × D, where D is the switching duty cycle. For example, when the switching duty cycle, D of a converter is 0.5, the power loss on RISNS with PLM is half of those with conventional output current sensing resulting in increased efficiency. The Pulse-Level-Modulation is a patented method to ensure accurate average output current regulation without the need of direct output current sensing. Figure 14 shows the current waveforms of a typical buck converter under steady state, where, IL1 is the inductor current and ILX is the main switch current flowing into the LX pin. For a buck converter operating in steady state, the mid-point of the RAMP section of the main switch current is equal to the average level of the inductor current–hence the average output current. In short, by regulating the mid-point of the RAMP section of the main switch current with respect to a precise reference level, PLM achieves output current regulation by sensing the main switch current solely. Figure 14. Waveforms of a Floating Buck LED Driver With PLM 7.3.2 Minimum Switch ON-time As the LM3414 features a 400 ns minimum ON time, it is essential to make sure the ON time of the internal switch is not shorter than 400 ns when setting the LED driving current. If the switching ON time is shorter than 400 ns, the accuracy of the LED current may not maintain and exceed the rated current of the LEDs. The ratio of the LED forward voltage to input voltage is restricted by the following restriction, as shown in Equation 1. (1) 10 Submit Documentation Feedback Copyright © 2010–2015, Texas Instruments Incorporated Product Folder Links: LM3414 LM3414HV |
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