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поискавой системы для электроныых деталей |
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поискавой системы для электроныых деталей |
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MAX8772 датащи(PDF) 39 Page - Maxim Integrated Products |
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MAX8772 датащи(HTML) 39 Page - Maxim Integrated Products |
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39 / 47 page  MAX8770/MAX8771/MAX8772 Dual-Phase, Quick- PWM Controller for IMVP-6+ CPU Core Power Supplies ______________________________________________________________________________________ 39 CONFIDENTIAL INFORMATION – RESTRICTED TO INTEL® IMVP-6 LICENSEES • Switching frequency: This choice determines the basic trade-off between size and efficiency. The opti- mal frequency is largely a function of maximum input voltage due to MOSFET switching losses that are proportional to frequency and VIN2. The optimum fre- quency is also a moving target, due to rapid improvements in MOSFET technology that are mak- ing higher frequencies more practical. • Inductor operating point: This choice provides trade-offs between size vs. efficiency and transient response vs. output noise. Low inductor values pro- vide better transient response and smaller physical size, but also result in lower efficiency and higher output noise due to increased ripple current. The minimum practical inductor value is one that causes the circuit to operate at the edge of critical conduc- tion (where the inductor current just touches zero with every cycle at maximum load). Inductor values lower than this grant no further size-reduction benefit. The optimum operating point is usually found between 20% and 50% ripple current. Inductor Selection The switching frequency and operating point (% ripple current or LIR) determine the inductor value as follows: where ηTOTAL is the total number of phases. Find a low-loss inductor having the lowest possible DC resistance that fits in the allotted dimensions. Ferrite cores are often the best choice, although powdered iron is inexpensive and can work well at 200kHz. The core must be large enough not to saturate at the peak inductor current (IPEAK): Transient Response The inductor ripple current impacts transient-response performance, especially at low VIN - VOUT differentials. Low inductor values allow the inductor current to slew faster, replenishing charge removed from the output fil- ter capacitors by a sudden load step. The amount of output sag is also a function of the maximum duty fac- tor, which can be calculated from the on-time and mini- mum off-time. For a dual-phase controller, the worst-case output sag voltage may be determined by: where tOFF(MIN) is the minimum off-time (see the Electrical Characteristics table). The amount of overshoot due to stored inductor energy can be calculated as: where ηTOTAL is the total number of active phases. Setting the Current Limit The minimum current-limit threshold must be high enough to support the maximum load current when the current limit is at the minimum tolerance value. The val- ley of the inductor current occurs at ILOAD(MAX) minus half the ripple current; therefore: where ηTOTAL is the total number of active phases, and ILIMIT(LOW) equals the minimum current-limit threshold voltage divided by the current-sense resistor (RSENSE). For the 22.5mV default setting, the minimum current-limit threshold is 19.5mV. I I LIR LIMIT LOW LOAD MAX TOTAL () () > ⎛ ⎝⎜ ⎞ ⎠⎟ − ⎛ ⎝⎜ ⎞ ⎠⎟ η 1 2 V IL CV SOAR LOAD MAX TOTAL OUT OUT () ≈ () ∆ 2 2 η V VT V t CV VV T V t C VT V t SAG OUT SW IN OFF MIN OUT OUT IN OUT SW IN OFF MIN OUT OUT SW IN OFF MIN () () () = () ⎛ ⎝⎜ ⎞ ⎠⎟ + ⎡ ⎣ ⎢ ⎢ ⎤ ⎦ ⎥ ⎥ − () ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ − ⎡ ⎣ ⎢ ⎢ ⎤ ⎦ ⎥ ⎥ + ⎛ ⎝⎜ ⎞ ⎠⎟ + ⎡ ⎣ LI I LOAD(MAX) 2 LOAD(MAX) ∆ ∆ 2 2 2 2 ⎢⎢ ⎢ ⎤ ⎦ ⎥ ⎥ I I LIR PEAK LOAD MAX TOTAL () = ⎛ ⎝⎜ ⎞ ⎠⎟ + ⎛ ⎝⎜ ⎞ ⎠⎟ η 1 2 L VV f I LIR V V TOTAL IN OUT SW LOAD MAX OUT IN = − ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ ⎛ ⎝⎜ ⎞ ⎠⎟ η () |
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