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LM3519MK-20 Datenblatt(PDF) 5 Page - National Semiconductor (TI) |
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LM3519MK-20 Datenblatt(HTML) 5 Page - National Semiconductor (TI) |
5 / 14 page Circuit Description The LM3519 is a step-up converter for white LED applica- tions that uses a unique and proprietary pulse frequency modulation (PFM) architecture to optimize high efficiency at high frequency operation. Unlike most PFM architecture implementations, the LM3519’s unique architectural imple- mentation results in non-pulse skipping variable frequency operation. The regulator is forced to operate at the edge of Continous Conduction Mode (CCM). The error amplifier will set the end of the on-time (I PEAK of inductor) based on the load (LEDs) current. During this operation, the inductor cur- rent ramps up and reaches a peak current at end of the on-time. At this point, the internal power switch is turned off until the inductor current reaches zero, and the cycle repeats again. The switching frequency is set based on the charge (on-time) and discharge(off-time) of the inductor current. The frequency can range between 2MHz to 8MHz over the op- erating input range. The LM3519 operation can be best understood through an examination of the block diagram in Figure 2. When LED current is out of regulation, the LED_rtn voltage falls below or rises above the internal reference voltage (VREF). The error amplifier will output a signal to increase or decrease the proper on-time duration of N1 power FET. This correction allows the inductor’s stored energy to increase or decrease to a sufficient level that when transferred to the load will bring the LED_rtn current back into regulation. During steady-state operation for a typical switching cycle, the oscillator sets the driver logic and turns on N1 power device. N1 conducts current through the inductor and re- verse biases the external diode. The LED current is supplied by the output capacitor when N1 is conducting. Once N1 on-time period is concluded, the internal power device is turned off and the external diode is forward baised. The inductor current then flows through the diode to the LED load to replenish the output capacitor and keep the LED current regulated at the trimmed target. www.national.com 5 |
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