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LM3475MFX Fiches technique(PDF) 7 Page - National Semiconductor (TI) |
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LM3475MFX Fiches technique(HTML) 7 Page - National Semiconductor (TI) |
7 / 14 page Block Diagram 20070103 Operation Description OVERVIEW The LM3475 is a buck (step-down) DC-DC controller that uses a hysteretic control architecture, which results in Pulse Frequency Modulated (PFM) regulation. The hysteretic con- trol scheme does not utilize an internal oscillator. Switching frequency depends on external components and operating conditions. Operating frequency decreases at light loads, resulting in excellent efficiency compared to PWM architec- tures. Because switching is directly controlled by the output conditions, hysteretic control provides exceptional load tran- sient response. HYSTERETIC CONTROL CIRCUIT The LM3475 uses a comparator-based voltage control loop. The voltage on the feedback pin is compared to a 0.8V reference with 21mV of hysteresis. When the FB input to the comparator falls below the reference voltage, the output of the comparator goes low. This results in the driver output, PGATE, pulling the gate of the PFET low and turning on the PFET. With the PFET on, the input supply charges C OUT and supplies current to the load through the PFET and the induc- tor. Current through the inductor ramps up linearly, and the output voltage increases. As the FB voltage reaches the upper threshold (reference voltage plus hysteresis) the out- put of the comparator goes high, and the PGATE turns the PFET off. When the PFET turns off, the catch diode turns on, and the current through the inductor ramps down. As the output voltage falls below the reference voltage, the cycle repeats. The resulting output, inductor current, and switch node waveforms are shown in Figure 1. The LM3475 operates in discontinuous conduction mode at light load current and continuous conduction mode at heavy load current. In discontinuous conduction mode, current through the inductor starts at zero and ramps up to the peak, then ramps down to zero. The next cycle starts when the FB voltage reaches the reference voltage. Until then, the induc- tor current remains zero. Operating frequency is low, as are switching losses. In continuous conduction mode, current always flows through the inductor and never ramps down to zero. SOFT-START The LM3475 includes an internal soft-start function to protect components from excessive inrush current and output volt- age overshoot. As V IN rises above 2.7V (typical), the internal bias circuitry becomes active. When EN goes high, the 20070104 FIGURE 1. Hysteretic Waveforms www.national.com 7 |
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