Datasheet LT3439 (Analog Devices) - 6

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