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LT3434 Datasheet(PDF) 8 Page - Linear Technology |
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LT3434 Datasheet(HTML) 8 Page - Linear Technology |
8 / 20 page LT3474/LT3474-1 8 3474fd APPLICATIONS INFORMATION Operation The LT3474 is a constant frequency, current mode regula- tor with an internal power switch capable of generating a constant 1A output. Operation can be best understood by referring to the Block Diagram. If the SHDN pin is tied to ground, the LT3474 is shut down and draws minimal current from the input source tied to VIN. If the SHDN pin exceeds 1.5V, the internal bias circuits turn on, including the internal regulator, reference, and oscillator. The switching regulator will only begin to operate when the SHDN pin exceeds 2.65V. The switcher is a current mode regulator. Instead of directly modulating the duty cycle of the power switch, the feedback loop controls the peak current in the switch during each cycle. Compared to voltage mode control, current mode control improves loop dynamics and provides cycle-by- cycle current limit. A pulse from the oscillator sets the RS flip-flop and turns on the internal NPN bipolar power switch. Current in the switch and the external inductor begins to increase. When this current exceeds a level determined by the voltage at VC, current comparator C1 resets the flip-flop, turning off the switch. The current in the inductor flows through the external Schottky diode and begins to decrease. The cycle begins again at the next pulse from the oscillator. In this way, the voltage on the VC pin controls the current through the inductor to the output. The internal error amplifier regulates the output current by continually adjusting the VC pin voltage. The threshold for switching on the VC pin is 0.8V, and an active clamp of 1.9V limits the output current. The voltage on the VADJ pin sets the current through the LED pin. The NPN Q2 pulls a current proportional to the voltage on the VADJ pin through the 100Ω resistor. The gm amplifier servos the VC pin to set the current through the 0.1Ω resistor and the LED pin. When the voltage drop across the 0.1Ω resistor is equal to the voltage drop across the 100Ω resistor, the servo loop is balanced. Tying the REF pin to the VADJ pin sets the LED pin current to 1A. Tying a resistor divider to the REF pin allows the programming of LED pin currents of less than 1A. LED pin current can also be programmed by tying the VADJ pin directly to a voltage source up to 1.25V. An LED can be dimmed with pulse width modulation us- ing the PWM pin and an external NFET. If the PWM pin is unconnected or pulled high, the part operates nominally. If the PWM pin is pulled low, the VC pin is disconnected from the internal circuitry and draws minimal current from the compensation capacitor. Circuitry drawing current from the OUT pin is also disabled. This way, the VC pin and the output capacitor store the state of the LED pin current until PWM is pulled high again. This leads to a highly linear relationship between pulse width and output light, allowing for a large and accurate dimming range. The RT pin allows programming of the switching frequency. For applications requiring the smallest external components possible, a fast switching frequency can be used. If very low or very high input voltages are required, a slower switching frequency can be programmed. During startup VOUT will be at a low voltage. The NPN Q2 can only operate correctly with sufficient voltage at VOUT, around 1.7V. A comparator senses VOUT and forces the VC pin high until VOUT rises above 2V, and Q2 is operating correctly. The switching regulator performs frequency foldback dur- ing overload conditions. An amplifier senses when VOUT is less than 2V and begins decreasing the oscillator frequency down from full frequency to 20% of the nominal frequency when VOUT = 0V. The OUT pin is less than 2V during startup, short circuit, and overload conditions. Frequency foldback helps limit switch current under these conditions. |
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