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MAX8734A Fiches technique(PDF) 19 Page - Maxim Integrated Products |
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MAX8734A Fiches technique(HTML) 19 Page - Maxim Integrated Products |
19 / 33 page High-Efficiency, Quad-Output, Main Power- Supply Controllers for Notebook Computers ______________________________________________________________________________________ 19 tinuous and discontinuous inductor-current operation (also known as the critical conduction point): where K is the on-time scale factor (see the On-Time One-Shot (tON) section). The load-current level at which PFM/PWM crossover occurs, ILOAD(SKIP), is equal to 1/2 the peak-to-peak ripple current, which is a function of the inductor value (Figure 5). For example, in the MAX8732A Typical Application Circuit with VOUT2 = 5V, V+ = 12V, L = 7.6µH, and K = 5µs, switchover to pulse-skipping operation occurs at ILOAD = 0.96A or about 1/5 full load. The crossover point occurs at an even lower value if a swinging (soft-saturation) inductor is used. The switching waveforms may appear noisy and asyn- chronous when light loading causes pulse-skipping operation, but this is a normal operating condition that results in high light-load efficiency. Trade-offs in PFM noise vs. light-load efficiency are made by varying the inductor value. Generally, low inductor values produce a broader efficiency vs. load curve, while higher values result in higher full-load efficiency (assuming that the coil resistance remains fixed) and less output voltage ripple. Penalties for using higher inductor values include larger physical size and degraded load-tran- sient response (especially at low input-voltage levels). DC output accuracy specifications refer to the trip level of the error comparator. When the inductor is in continuous conduction, the output voltage has a DC regulation higher than the trip level by 50% of the ripple. In discontinuous conduction (SKIP = GND, light load), the output voltage has a DC regulation higher than the trip level by approxi- mately 1.5% due to slope compensation. Forced-PWM Mode The low-noise, forced-PWM (SKIP = VCC) mode dis- ables the zero-crossing comparator, which controls the low-side switch on-time. Disabling the zero-crossing detector causes the low-side, gate-drive waveform to become the complement of the high-side, gate-drive waveform. The inductor current reverses at light loads as the PWM loop strives to maintain a duty ratio of VOUT/V+. The benefit of forced-PWM mode is to keep the switching frequency fairly constant, but it comes at a cost: the no-load battery current can be 10mA to 50mA, depending on switching frequency and the external MOSFETs. Forced-PWM mode is most useful for reducing audio- frequency noise, improving load-transient response, providing sink-current capability for dynamic output voltage adjustment, and improving the cross-regulation of multiple-output applications that use a flyback trans- former or coupled inductor. Enhanced Ultrasonic Mode (25kHz (min) Pulse Skipping) Leaving SKIP unconnected or connecting SKIP to REF activates a unique pulse-skipping mode with a mini- mum switching frequency of 25kHz. This ultrasonic pulse-skipping mode eliminates audio-frequency mod- ulation that would otherwise be present when a lightly loaded controller automatically skips pulses. In ultra- sonic mode, the controller automatically transitions to fixed-frequency PWM operation when the load reaches the same critical conduction point (ILOAD(SKIP)) that occurs when normally pulse skipping. An ultrasonic pulse occurs when the controller detects that no switching has occurred within the last 28µs. Once triggered, the ultrasonic controller pulls DL high, turning on the low-side MOSFET to induce a negative inductor current. After the inductor current reaches the negative ultrasonic current threshold, the controller turns off the low-side MOSFET (DL pulled low) and trig- gers a constant on-time (DH driven high). When the on- time has expired, the controller reenables the low-side MOSFET until the controller detects that the inductor current dropped below the zero-crossing threshold. Starting with a DL pulse greatly reduces the peak out- put voltage when compared to starting with a DH pulse. The output voltage at the beginning of the ultrasonic pulse determines the negative ultrasonic current thresh- old, resulting in the following equation: where VFB > VREF and RON is the on-resistance of the synchronous rectifier (MAX8734A) or the current-sense resistor value (MAX8732A/MAX8733A). VI R V V ISONIC L ON REF FB == − () × 058 . I KV L VV V LOAD SKIP OUT OUT () __ = × × +− + 2 ILOAD = IPEAK / 2 ON-TIME 0 TIME -IPEAK L V+ - VOUT ∆i ∆t = Figure 5. Pulse-Skipping/Discontinuous Crossover Point |
Numéro de pièce similaire - MAX8734A |
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Description similaire - MAX8734A |
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