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LT3782AIFE-PBF Datasheet(PDF) 8 Page - Linear Technology |
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LT3782AIFE-PBF Datasheet(HTML) 8 Page - Linear Technology |
8 / 20 page LT3782A 8 3782af Operation The LT3782A is a two phase constant frequency current mode boost controller. Switching frequency can be pro- grammed up to 500kHz. During normal switching cycles, the two channels are controlled by internal flip-flops and are 180 degrees out-of-phase. Referring to the Block Diagram, the LT3782A’s basic func- tions include a transconductance amplifier (gm) to regulate the output voltage and to control the current mode PWM current loop. It also includes the necessary logic and flip- flop to control the PWM switching cycles, two high speed gate drivers to drive high power N-channel MOSFETs, and 2-phase control signals to drive external gate drivers for optional synchronous operation. In normal operation, each switching cycle starts with a switch turn-on. The inductor current of each channel is sampled through the current sense resistor and amplified then compared to the error amplifier output VC to turn the switch off. The phase delay of the second channel is controlled by the divide-by-two D flip-flop and is exactly 180 degrees out-of-phase of the first channel. With a re- sistor divider connected to the FB pin, the output voltage is programmed to the desired value. The 10V gate drivers are sufficient to drive most high power N-channel MOSFET in many industrial applications. Additional important features include shutdown, cur- rent limit, soft-start, synchronization and programmable maximum duty cycle. Additional slope compensation can be added also. Output Voltage Programming With a 2.44V feedback reference voltage VREF, the output VOUT is programmed by a resistor divider as shown in the Block Diagram. VOUT = 2.44 1+ RF1 RF2 Soft-Start and Shutdown During soft-start, the voltage on the SS pin (VSS) controls the output voltage. The output voltage thus ramps up fol- lowing VSS. The effective range of VSS is from 0V to 2.44V. The typical time for the output to reach the programmed level is t = C • 2.44V 10μA C is the capacitor connected from the SS pin to GND. Undervoltage Lockout and Shutdown Only when VRUN is higher than 2.45V VGBIAS will be active and the switching enabled. The LT3782A goes into low current shutdown when VRUN is below 0.3V. A resistor divider can be used on RUN pin to set the desired VCC undervoltage lockout voltage. 80mV of hysteresis is built in on RUN pin thresholds. Oscillation Frequency Setting and Synchronization The switching frequency of LT3782A can be set up to 500kHz by a resistor RFREQ from pin RSET to ground. For fSET = 250kHz, RFREQ = 80k Once the switching frequency fSET is chosen, RFREQ can be found from the Switching Frequency vs RFREQ graph found under the Typical Performance Characteristics section. Note that because of the 2-phase operation, the internal oscillator is running at twice the switching frequency. To synchronize the LT3782A to the system frequency fSYSTEM, the synchronizing frequency fSYNC should be two times fSYSTEM, and the LT3782A switching frequency fSET should be set below 80% of fSYSTEM. fSYNC = 2fSYSTEM and fSET < (fSYSTEM • 0.8) For example, to synchronize the LT3782A to 200kHz sys- tem frequency fSYSTEM, fSYNC needs to be set at 400kHz and fSET needs to be set at 160kHz. From the Switching Frequency vs RFREQ graph found under the Typical Per- formance Characteristics section, RFREQ = 130k. APPLICATIONS INFORMATION |
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