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LT3478-1 Datasheet(PDF) 9 Page - Linear Technology |
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LT3478-1 Datasheet(HTML) 9 Page - Linear Technology |
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9 / 24 page  LT3478/LT3478-1 9 34781f OPERATIO The LT3478/LT3478-1 are high powered LED drivers with a 42V, 4.5A internal switch and the ability to drive LEDs with up to 1050mA for LT3478-1 and up to 105mV/RSENSE for LT3478. The LT3478/LT3478-1 work similarly to a conventional current mode boost converter but use LED current (instead of output voltage) as feedback for the control loop. The Block Diagram in Figure 1 shows the major functions of the LT3478/LT3478-1. For the part to turn on, the VIN pin must exceed 2.8V and the ⎯S⎯H⎯D⎯N pin must exceed 1.4V. The ⎯S⎯H⎯D⎯N pin threshold allows programming of an undervoltage lockout (UVLO) threshold for the system input supply using a simple resistor divider. A 10µA current flows into the ⎯S⎯H⎯D⎯N pin before part turn on and is removed after part turn on. This current hysteresis allows programming of hysteresis for the UVLO threshold. See “Shutdown Pin and Programming Undervoltage Lockout” in the Applications Information Section. For micropower shutdown the ⎯S⎯H⎯D⎯N pin at 0V reduces VIN supply current to approximately 3µA. Each LED driver is a current mode step-up switch- ing regulator. A regulation point is achieved when the boosted output voltage VOUT across the output LED(s) is high enough to create current in the LED(s) equal to the programmed LED current. A sense resistor connected in series with the LED(s) provides feedback of LED current to the converter loop. The basic loop uses a pulse from an internal oscillator to set the RS flip-flop and turn on the internal power NPN switch Q1 connected between the switch pin, SW, and ground. Current increases in the external inductor until switch current limit is exceeded or until the oscillator reaches its maximum duty cycle. The switch is then turned off, causing inductor current to lift the SW pin and turn on an external Schottky diode connected to the output. Inductor current flows via the Schottky diode charging the output capacitor. The switch is turned back on at the next reset cycle of the internal oscillator. During normal operation the VC voltage controls the peak switch current limit and hence the inductor current available to the output LED(s). As with all current mode converters, slope compensation is added to the control path to ensure stability. The CTRL1 pin is used to program maximum LED current via Q2. The CTRL2 pin can be used to program a decrease in LED current versus temperature for maximum reliability and utilization of the LED(s). A CTRL2 voltage with negative temperature coefficient can be created using an external resistor divider from VREF with temperature dependant resistance. Unused CTRL2 is tied to VREF. For True Color PWM dimming, the LT3478/LT3478-1 provide up to a 3000:1 wide PWM dimming range by al- lowing the duty cycle of the PWM pin (connected to the IC and an external N-channel MOSFET in series with the LED(s)) to be reduced from 100% to as low as 0.033% for a PWM frequency of 100Hz. Dimming by PWM duty cycle, allows for constant LED color to be maintained over the entire dimming range. For robust operation, the LT3478/LT3478-1 monitor system performance for any of the following faults : VIN or ⎯S⎯H⎯D⎯N pin voltages too low and/or inductor current too high and/or boosted output voltage too high. On detection of any of these faults, the LT3478/LT3478-1 stop switching immediately and a soft-start latch is set discharging the SS pin (see Timing Diagram for SS pin in Figure 11). All faults are detected internally and do not require external components. When all faults no longer exist, an internal 12µA supply charges the SS pin with a timing programmed using a single external capacitor. A gradual ramp up of SS pin voltage limits switch current during startup. For optimum component sizing, duty cycle range and ef- ficiency the LT3478/LT3478-1 allow for a separate inductor supply VS and for switching frequency to be programmed from 200kHz up to 2.25MHz using a resistor from the RT pin to ground. The advantages of these options are covered in the Applications Informations section. |
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