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LT3439 Datasheet(PDF) 6 Page - Linear Technology |
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LT3439 Datasheet(HTML) 6 Page - Linear Technology |
6 / 12 page 6 LT3439 sn3439 3439fs vary from 2.8V to 17.5V with very little change in device performance. When the part is in shutdown mode, the internal regulator is turned off, drawing less than 20 µA of current from VIN. Overcurrent Protection A linearly controlled current limit circuit is provided to protect the circuit from excessive currents and to facilitate start-up into a highly capacitive load. Upon reaching cur- rent limit, the switching cycle is not terminated, instead the base drive to the output transistor is regulated to maintain the maximum current over the entire switch cycle. Very high power dissipation in the switches occurs during this mode of operation. If the current limit is enabled for a long enough period of time, over temperature protection shutdown will be enabled to protect the device. OPERATIO Overtemperature Protection When the IC has exceeded the maximum temperature the part will trigger the overtemperature protection circuit where both output drivers are turned off. Undervoltage Lockout Protection When VIN is below 2.55V the part will go into undervoltage lockout mode where both output drivers are turned off. No Load Operation The operation of the supply is stable all the way down to zero load and a preload is not required. APPLICATIO S I FOR ATIO Reducing EMI from switching power supplies has tradi- tionally invoked fear in designers. Many switchers are designed solely on efficiency and, as such, produce wave- forms filled with high frequency harmonics that propagate through the rest of the supply. The LT3439 provides control of two of the primary vari- ables for controlling EMI while switching inductive loads: switch voltage slew rate and switch current slew rate. The use of this part will reduce noise and EMI over conven- tional switch mode controllers. Because these variables are under control, a supply built with this part will exhibit far less tendency to create EMI and less chance of running into problems during production. It is beyond the scope of this data sheet to get into EMI fundamentals. AN70, “A Monolithic Switching Regulator with 100 µV Output Noise” contains much information concerning noise in switching regulators and should be consulted. Oscillator Frequency The internal oscillator generates the switching frequency that determines the fundamental positioning of the harmonics. Using quality external components is impor- tant to ensure oscillator frequency stability. A current defined by external resistor RT charges and discharges the capacitor CT creating a saw tooth waveform where the outputs’ states change at the peak. The frequency of each output is one half of the frequency of the oscillator. By having both components external, the user has greater flexibility in setting the frequency and the frequency is less susceptible to any temperature variations in the device. The external capacitance CT is chosen by: CT(nF) = 1183/[fOSC(kHz) • RT(kΩ)] where fOSC is the desired oscillator frequency. For RT equal to 16.9k, this simplifies to: CT(nF) = 70/fOSC(kHz) e.g., CT = 1nF for fOSC = 70kHz Nominally, RT should be set to 16.9k. Low tolerance and low temperature coefficient compo- nents are recommended. |
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