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LM2614ATL Datasheet(PDF) 10 Page - National Semiconductor (TI) |
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LM2614ATL Datasheet(HTML) 10 Page - National Semiconductor (TI) |
10 / 16 page Device Information (Continued) PWM OPERATION While in PWM (Pulse Width Modulation) mode, the output voltage is regulated by switching at a constant frequency and then modulating the energy per cycle to control power to the load. Energy per cycle is set by modulating the PFET switch on-time pulse-width to control the peak inductor cur- rent. This is done by comparing the signal from the current-sense amplifier with a slope compensated error sig- nal from the voltage-feedback error amplifier. At the begin- ning of each cycle, the clock turns on the PFET switch, causing the inductor current to ramp up. When the current sense signal ramps past the error amplifier signal, the PWM comparator turns off the PFET switch and turns on the NFET synchronous rectifier, ending the first part of the cycle. If an increase in load pulls the output voltage down, the error amplifier output increases, which allows the inductor current to ramp higher before the comparator turns off the PFET. This increases the average current sent to the output and adjusts for the increase in the load. Before going to the PWM comparator, the error signal is summed with a slope compensation ramp from the oscillator for stability of the current feedback loop. During the second part of the cycle, a zero crossing detector turns off the NFET synchronous rectifier if the inductor current ramps to zero. The minimum on time of the PFET in PWM mode is about 200ns. PFM OPERATION Connecting the SYNC/MODE to SGND sets the LM2614 to hysteretic PFM operation. While in PFM (Pulse Frequency Modulation) mode, the output voltage is regulated by switch- ing with a discrete energy per cycle and then modulating the cycle rate, or frequency, to control power to the load. This is done by using an error comparator to sense the output voltage. The device waits as the load discharges the output filter capacitor, until the output voltage drops below the lower threshold of the PFM error-comparator. Then the device initiates a cycle by turning on the PFET switch. This allows current to flow from the input, through the inductor to the output, charging the output filter capacitor. The PFET is turned off when the output voltage rises above the regulation threshold of the PFM error comparator. Thus, the output voltage ripple in PFM mode is proportional to the hysteresis of the error comparator. In PFM mode, the device only switches as needed to service the load. This lowers current consumption by reducing power consumed during the switching action in the circuit, due to transition losses in the internal MOSFETs, gate drive cur- rents, eddy current losses in the inductor, etc. It also im- proves light-load voltage regulation. During the second half of the cycle, the intrinsic body diode of the NFET synchro- nous rectifier conducts until the inductor current ramps to zero. OPERATING MODE SELECTION The LM2614 is designed for digital control of the operating modes by the system controller. This prevents the spurious switch over from low-noise PWM mode between transmis- sion intervals in mobile phone applications that can occur in other products. The SYNC/MODE digital input pin is used to select the operating mode. Setting SYNC/MODE high (above 1.3V) selects 600kHz current-mode PWM operation. PWM mode is optimized for low-noise, high-power operation for use when the load is active. Setting SYNC/MODE low (below 0.4V) selects hysteretic voltage-mode PFM operation. PFM mode is optimized for reducing power consumption and PWM Mode Switching Waveform 20036725 A: Inductor Current, 500mA/div B: SW Pin, 2V/div C: VOUT, 10mV/div, AC Coupled FIGURE 6. PFM Mode Switching Waveform 20036726 A: Inductor Current, 500mA/div B: SW Pin, 2V/div C: VOUT, 50mV/div, AC Coupled FIGURE 7. www.national.com 10 |
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