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Moteur de recherche de fiches techniques de composants électroniques |
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Moteur de recherche de fiches techniques de composants électroniques |
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LM2793 Fiches technique(PDF) 8 Page - National Semiconductor (TI) |
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LM2793 Fiches technique(HTML) 8 Page - National Semiconductor (TI) |
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8 / 10 page  Application Information (Continued) TABLE 2. White LED Selection Manufacturer Contact Osram www.osram-os.com Nichia www.nichia.com PARALLEL DX OUTPUTS FOR INCREASED CURRENT DRIVE Outputs D 1 and D2 may be connected together to drive a single LED. In such a configuration, two parallel current sources of equal value drive the single LED. The voltage on SD-BRGT should be chosen so that the current through each of the outputs is programmed to 50% of the total desired LED current. For example, if 30mA is the desired drive current for the single LED, SD-BRGT should be se- lected so that the current through each of the outputs is 15mA. Connecting the outputs in parallel does not affect internal operation of the LM2793 and has no impact on the Electrical Characteristics and limits previously presented. The available diode output current, maximum diode voltage, and all other specifications provided in the Electrical Char- acteristics table apply to this parallel output configuration, just as they do to the standard 2-LED application circuit. P OUT P OUT uses pre-regulation to loosely regulate the output of the LM2793 1.5x charge pump. Pre-regulation uses the voltage present at P OUT to limit the gate drive of the 1.5x switched capacitor charge pump. Pre-regulation helps to reduce input current noise and large input current spikes normally associated with switched capacitor charge pumps. At voltages below 3.3V (typ.), the LM2793 acts as an open loop charge pump. When the device is in open loop, the voltage at P OUT is 1.5 times the input voltage. At input voltages higher than 3.3V (typ.) P OUT is loosely regulated to 4.9V (typ.). POWER EFFICIENCY Figure 2 shows the efficiency of the LM2793. Efficiency (E) of the LM2793 is defined here as the ratio of the power consumed by LEDs (P LED) to the power drawn from the input source (P IN). In the equations below, IQ is the quiescent current of the LM2793, I LED is the current flowing through one LED, and V LED is the forward voltage at that LED current. In the input power calculation, the 1.5 multiplier reflects the 3/2 switched capacitor gain of the LM2793. P LED =NxVLED xILED P IN =VIN xIIN P IN =VIN x(1.5xNxILED +IQ) E=(P LED ÷PIN) It is also worth noting that efficiency as defined here is in part dependent on LED voltage. Variation in LED voltage does not affect power consumed by the circuit and typically does not relate to the brightness of the LED. For an advanced analysis, it is recommended that power consumed by the circuit (V IN xIIN) be evaluated rather than power efficiency. Power consumption of the LM2793 Typical Application Cir- cuit is shown in Figure 3. THERMAL PROTECTION When the junction temperature exceeds 150˚C, the LM2793 internal thermal protection circuitry disables the part. This feature protects the device from damage due to excessive power dissipation. The device will recover and operate nor- mally when the junction temperature falls below 125˚C. It is important to have good thermal conduction with a proper layout to reduce thermal resistance. POWER DISSIPATION When operating within specified operating ratings, the peak power dissipation (P DISSIPATION) of the LM2793 occurs at an input voltage of 5.5V. Assuming a typical junction-to-ambient thermal resistance ( θJA) for the LLP-10 package of 55˚C/W, a LED forward voltage (V DX) of 3.6V, and a total load (ILOAD) of 32mA for two White LEDs connected to D 1 and D2, the power dissipation and junction temperature (T J) are calcu- lated below for a part operating at the maximum rated am- bient temperature (T A) of 85˚C. In the equations below, VIN is the input voltage to the LM2793, P IN is the power generated by the 1.5x charge pump, and P LED is the power consumed by the LEDs. P DISSIPATION =PIN -PLED = (1.5V IN −VDX)xILOAD = ((1.5 x 5.5V) - 3.6V) x 0.032A = 149mW 20063612 FIGURE 2. Efficiency vs. V IN 2 LEDs, V LED = 3.6V, ILED = 16mA 20063613 FIGURE 3. I LED current vs. PIN 2 LEDs, 2.5 ≤ V LED ≤ 3.9V, I LED = 16mA www.national.com 8 |
Numéro de pièce similaire - LM2793 |
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Description similaire - LM2793 |
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