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LM3402 Datasheet(PDF) 2 Page - National Semiconductor (TI) |
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LM3402 Datasheet(HTML) 2 Page - National Semiconductor (TI) |
2 / 18 page 30061602 FIGURE 2. LM3402/04 PSOP Thermal PAD and Via Layout Connecting to LED Array The LM3402 / 04 evaluation board includes two standard 94 mil turret connectors for the cathode and anode connections to a LED array. Low Power Shutdown The LM3402/04 can be placed into a low power shutdown state (I Q typically 90 µA) by grounding the DIM terminal. Dur- ing normal operation this terminal should be left open-circuit. Constant On Time Overview The LM3402 and LM3404 are buck regulators with a wide in- put voltage range and a low voltage reference. The controlled on-time (COT) architecture is a combination of hysteretic mode control and a one-shot on-timer that varies inversely with input voltage. With the addition of a PNP transistor, the on-timer can be made to be inversely proportional to the input voltage minus the output voltage. This is one of the application improvements made to this demonstration board that will be discussed later (improved average LED current circuit). The LM3402 / 04 were designed with a focus of controlling the current through the load, not the voltage across it. A con- stant current regulator is free of load current transients, and has no need for output capacitance to supply the load and maintain output voltage. Therefore, in this demonstration board in order to demonstrate the fast transient capabilities, I have chosen to omit the output capacitor. With any Buck reg- ulator, duty cycle (D) can be calculated with the following equations. The average inductor current equals the average LED current whether an output capacitor is used or not. 30061604 FIGURE 3. Buck Converter Inductor Current Waveform A voltage signal, V SNS, is created as the LED current flows through the current setting resistor, R SNS, to ground. VSNS is fed back to the CS pin, where it is compared against a 200 mV reference (V REF). A comparator turns on the power MOS- FET when V SNS falls below VREF. The power MOSFET con- ducts for a controlled on-time, t ON, set by an external resistor, R ON. 30061605 FIGURE 4. V SNS Circuit SETTING THE AVERAGE LED CURRENT Knowing the average LED current desired and the input and output voltages, the slopes of the currents within the inductor can be calculated. The first step is to calculate the minimum inductor current (LED current) point. This minimum level needs to be determined so that the average LED current can be determined. www.national.com 2 |
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