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LR8506 Datasheet(PDF) 7 Page - Leshan Radio Company |
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LR8506 Datasheet(HTML) 7 Page - Leshan Radio Company |
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7 / 10 page  MAIN CONTROL LOOP The LR8506 uses a constant frequency, current mode step-down architecture. Both the main (P-channel MOSFET) and synchronous (N-channel MOSFET) switches are internal. During normal operation, the internal top power MOSFET is turned on each cycle when the oscillator sets the RS latch, and turned off when the current comparator, ICOMP, resets the RS latch. The peak inductor current at which ICOMP resets the RS latch, is controlled by the output of error amplifier EA. When the load current increases, it causes a slight decrease in the feedback voltage, FB, relative to the 0.6V reference, which in turn, causes the EA amplifier’s output voltage to increase until the average inductor current matches the new load current. While the top MOSFET is off, the bottom MOSFET is turned on until either the inductor current starts to reverse, as indicated by the current reversal comparator IRCMP, or the beginning of the next clock cycle. The OVDET comparator controls output transient overshoots by turning the main switch off and keeping it off until the fault is removed. PULSE SKIPPING MODE At light loads, the inductor current may reach zero reverse on each pulse. The bottom MOSFET is turned off by the current reversal comparator, IRCMP, and the switch voltage will ring. This is discontinuous mode operation, and is normal behavior for the switching regulator. At very light loads, the LR8506 will automatically skip pulses in pulse skipping mode operation to maintain output regulation. SLOPE COMPENSATION Slope compensation provides stability in constant frequency architecture by preventing subharmonic oscillations at high duty cycles. It is accomplished internally by adding a compensating ramp to the inductor current signal at duty cycles in excess of 50%. This slope compensated current mode PWM control provides stable switching and cycle-by-cycle current limit for excellent load and line response. DROPOUT OPERATION As the input supply voltage decreases to a value approaching the output voltage, the duty cycle increases toward the maximum on-time. Further reduction of the supply voltage forces the main switch to remain on for more than one cycle until reaches 100% duty cycle. The output voltage will then be determined by the input voltage minus the voltage drop across the P-channel MOSFET and the inductor. An important detail to remember is that at low inputs supply voltages, the RDS(ON) of the P-channel switch increases. Therefore, the user should calculate the power dissipation when the LR8506 is used at 100% duty cycle with low input voltage. MAXIMUM LOAD CURRENT The LR8506 will operate with input voltage as low as 2.5V, however, the maximum load current decreases at lower input due to large IR drop on the main switch and synchronous rectifier. The slope compensation signal reduces the peak inductor current as a function of the duty cycle to prevent sub-harmonic oscillations at duty cycles greater than 50%.Conversely the current limit increase as the duty cycle decreases. OPERATION LESHAN RADIO COMPANY, LTD. 7/10 |
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