Datasheet LT3478, LT3478-1 (Analog Devices) - 9

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