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LT3496IUFD-TRPBF Datasheet(PDF) 10 Page - Linear Technology |
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LT3496IUFD-TRPBF Datasheet(HTML) 10 Page - Linear Technology |
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10 / 20 page  LT3496 10 3496fe dimming waveforms and the start-up time should be checked across all operating conditions. Open-LED Protection The LT3496 has open-LED protection for all the three converters. As shown in Figure 1, the OVP1 pin receives the output voltage (the voltage across the output capacitor) feedback signal from an external resistor divider. OVP1 voltage is compared with a 1V internal voltage reference by comparator A6. In the event the LED string is disconnected or fails open, converter 1 output voltage will increase, caus- ing OVP1 voltage to increase. When OVP1 voltage exceeds 1V, the power switch Q1 will turn off, and cause the output voltage to decrease. Eventually, OVP1 will be regulated to 1V and the output voltage will be limited. In the event one of the converters has an open-LED protection, the other converters will continue functioning properly. Switching Frequency and Soft-Start The LT3496 switching frequency is controlled by FADJ pin voltage. Setting FADJ voltage to be less than 1V will reduce switching frequency. If FADJ voltage is higher than 1V, the default switch- ing frequency is 2.1MHz. In general, a lower switching frequency should be used where either very high or very low switch duty cycle is required or higher efficiency is desired. Selection of a higher switching frequency will allow use of low value external components and yield a smaller solution size and profile. Connecting FADJ pin to a lowpass filter (R5 and C4 in Figure 1) from the REF pin provides a soft-start function. During start-up, FADJ voltage increases slowly from 0V to the setting voltage. As a result, the switching frequency increases slowly to the setting frequency. This function limits the inrush current during start-up. Undervoltage Lockout The LT3496 has an undervoltage lockout circuit that shuts down all the three converters when the input volt- age drops below 2.4V. This prevents the converter from switching in an erratic mode when powered from a low supply voltage. Input Capacitor Selection For proper operation, it is necessary to place a bypass capacitor to GND close to the VIN pin of the LT3496. A 1μF or greater capacitor with low ESR should be used. A ceramic capacitor is usually the best choice. In the buck mode configuration, the capacitor at PVIN has large pulsed currents due to the current returned though the Schottky diode when the switch is off. For the best reliability, this capacitor should have low ESR and ESL and have an adequate ripple current rating. The RMS input current is: I IN(RMS) = ILED •1– D ()•D where D is the switch duty cycle. A 1μF ceramic type ca- pacitor placed close to the Schottky diode and the ground plane is usually sufficient for each channel. Output Capacitor Selection The selection of output filter capacitor depends on the load and converter configuration, i.e., step-up or step-down. For LED applications, the equivalent resistance of the LED is typically low, and the output filter capacitor should be large enough to attenuate the current ripple. To achieve the same LED ripple current, the required filter capacitor value is larger in the boost and buck-boost mode applications than that in the buck mode applications. For the LED buck mode applications, a 0.22μF ceramic capacitor is usually sufficient for each channel. For the LED boost and buck-boost applications, a 1μF ceramic capacitor is usually sufficient for each channel. If higher LED current ripple can be tolerated, a lower output capacitance can be selected to reduce the capacitor’s cost and size. Use only ceramic capacitors with X7R or X5R dielectric, as they are good for temperature and DC bias stability of the capacitor value. All ceramic capacitors exhibit loss of capacitance value with increasing DC voltage bias, so it may be necessary to choose a higher value capacitor to get the required capacitance at the operation voltage. Always check that the voltage rating of the capacitor is sufficient. Table 1 shows some recommended capacitor vendors. APPLICATIONS INFORMATION |
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