Datasheet LT3475, LT3475-1 (Analog Devices) - 7

LT3475/LT3475-1
OPERATION
The LT3475 is a dual constant frequency, current mode programming of LED pin currents of less than 1.5A. LED regulator with internal power switches capable of gen- pin current can also be programmed by tying the VADJ pin erating constant 1.5A outputs. Operation can be best directly to a voltage source. understood by referring to the Block Diagram. An LED can be dimmed with pulse width modulation If the SHDN pin is tied to ground, the LT3475 is shut using the PWM pin and an external NFET. If the PWM down and draws minimal current from the input source pin is unconnected or is pulled high, the part operates tied to VIN. If the SHDN pin exceeds 1V, the internal bias nominally. If the PWM pin is pulled low, the VC pin is dis- circuits turn on, including the internal regulator, reference connected from the internal circuitry and draws minimal and oscillator. The switching regulators will only begin to current from the compensation capacitor. Circuitry draw- operate when the SHDN pin exceeds 2.6V. ing current from the OUT pin is also disabled. This way, the V The switcher is a current mode regulator. Instead of directly C pin and the output capacitor store the state of the LED pin current until the PWM is pulled high again. modulating the duty cycle of the power switch, the feedback This leads to a highly linear relationship between pulse loop controls the peak current in the switch during each width and output light, allowing for a large and accurate cycle. Compared to voltage mode control, current mode dimming range. control improves loop dynamics and provides cycle-by- cycle current limit. The RT pin allows programming of the switching frequency. For applications requiring the smallest external components A pulse from the oscillator sets the RS fl ip-fl op and turns possible, a fast switching frequency can be used. If low on the internal NPN bipolar power switch. Current in the dropout or very high input voltages are required, a slower switch and the external inductor begins to increase. When switching frequency can be programmed. this current exceeds a level determined by the voltage at V During startup V C, current comparator C1 resets the fl ip-fl op, turning OUT will be at a low voltage. The NPN, off the switch. The current in the inductor fl ows through Q3, can only operate correctly with suffi cient voltage the external Schottky diode and begins to decrease. The of ≈1.7V at VOUT, A comparator senses VOUT and forces cycle begins again at the next pulse from the oscillator. the VC pin high until VOUT rises above 2V, and Q3 is op- In this way, the voltage on the V erating correctly. C pin controls the current through the inductor to the output. The internal error The switching regulator performs frequency foldback amplifi er regulates the output current by continually during overload conditions. An amplifi er senses when adjusting the VC pin voltage. The threshold for switching VOUT is less than 2V and begins decreasing the oscillator on the VC pin is 0.8V, and an active clamp of 1.8V limits frequency down from full frequency to 15% of the nominal the output current. frequency when VOUT = 0V. The OUT pin is less than 2V The voltage on the V during startup, short circuit, and overload conditions. ADJ pin sets the current through the LED pin. The NPN, Q3, pulls a current proportional to the Frequency foldback helps limit switch current under these voltage on the V conditions. ADJ pin through the 100Ω resistor. The gm amplifi er servos the VC pin to set the current through the The switch driver operates either from VIN or from the 0.067Ω resistor and the LED pin. When the voltage drop BOOST pin. An external capacitor and Schottky diode across the 0.067Ω resistor is equal to the voltage drop are used to generate a voltage at the BOOST pin that across the 100Ω resistor, the servo loop is balanced. is higher than the input supply. This allows the driver Tying the REF pin to the V to saturate the internal bipolar NPN power switch for ADJ pin sets the LED pin current to 1.5A. Tying a resistor divider to the REF pin allows the effi cient operation. 3475fb 7