Datasheet LT3474, LT3474-1 (Analog Devices) - 8

LT3474/LT3474-1
APPLICATIONS INFORMATION Operation
across the 0.1Ω resistor is equal to the voltage drop across the 100Ω resistor, the servo loop is balanced. The LT3474 is a constant frequency, current mode regula- tor with an internal power switch capable of generating Tying the REF pin to the VADJ pin sets the LED pin current a constant 1A output. Operation can be best understood to 1A. Tying a resistor divider to the REF pin allows the by referring to the Block Diagram. programming of LED pin currents of less than 1A. LED pin current can also be programmed by tying the V If the SHDN pin is tied to ground, the LT3474 is shut ADJ pin directly to a voltage source up to 1.25V. down and draws minimal current from the input source tied to VIN. If the SHDN pin exceeds 1.5V, the internal bias An LED can be dimmed with pulse width modulation us- circuits turn on, including the internal regulator, reference, ing the PWM pin and an external NFET. If the PWM pin is and oscillator. The switching regulator will only begin to unconnected or pulled high, the part operates nominally. operate when the SHDN pin exceeds 2.65V. If the PWM pin is pulled low, the VC pin is disconnected from the internal circuitry and draws minimal current from The switcher is a current mode regulator. Instead of directly the compensation capacitor. Circuitry drawing current from modulating the duty cycle of the power switch, the feedback the OUT pin is also disabled. This way, the V loop controls the peak current in the switch during each C pin and the output capacitor store the state of the LED pin current cycle. Compared to voltage mode control, current mode until PWM is pulled high again. This leads to a highly control improves loop dynamics and provides cycle-by- linear relationship between pulse width and output light, cycle current limit. allowing for a large and accurate dimming range. A pulse from the oscillator sets the RS fl ip-fl op and turns The R on the internal NPN bipolar power switch. Current in the T pin allows programming of the switching frequency. For applications requiring the smallest external components switch and the external inductor begins to increase. When possible, a fast switching frequency can be used. If very this current exceeds a level determined by the voltage at low or very high input voltages are required, a slower VC, current comparator C1 resets the fl ip-fl op, turning switching frequency can be programmed. off the switch. The current in the inductor fl ows through the external Schottky diode and begins to decrease. The During startup VOUT will be at a low voltage. The NPN Q2 cycle begins again at the next pulse from the oscillator. can only operate correctly with suffi cient voltage at VOUT, In this way, the voltage on the VC pin controls the current around 1.7V. A comparator senses VOUT and forces the VC through the inductor to the output. The internal error pin high until VOUT rises above 2V, and Q2 is operating amplifi er regulates the output current by continually correctly. adjusting the VC pin voltage. The threshold for switching The switching regulator performs frequency foldback dur- on the VC pin is 0.8V, and an active clamp of 1.9V limits ing overload conditions. An amplifi er senses when VOUT is the output current. less than 2V and begins decreasing the oscillator frequency The voltage on the VADJ pin sets the current through the down from full frequency to 20% of the nominal frequency LED pin. The NPN Q2 pulls a current proportional to the when VOUT = 0V. The OUT pin is less than 2V during startup, voltage on the VADJ pin through the 100Ω resistor. The short circuit, and overload conditions. Frequency foldback gm amplifi er servos the VC pin to set the current through helps limit switch current under these conditions. the 0.1Ω resistor and the LED pin. When the voltage drop 3474fd 8