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MAX767CAP Datasheet(PDF) 11 Page - Maxim Integrated Products |
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MAX767CAP Datasheet(HTML) 11 Page - Maxim Integrated Products |
11 / 20 page Oscillator Frequency The SYNC input controls the oscillator frequency. Connecting SYNC to GND or to VCC selects 200kHz operation; connecting it to REF selects 300kHz opera- tion. SYNC can also be driven with an external 240kHz to 350kHz CMOS/TTL source to synchronize the inter- nal oscillator. Normally, 300kHz operation is chosen to minimize the inductor and output filter capacitor sizes, but 200kHz operation may be chosen for a small (about 1%) increase in efficiency at heavy loads. Internal Reference The internal 3.3V bandgap reference (REF) remains active, even when the switching regulator is turned off. It can furnish up to 5mA, and can be used to supply memory keep-alive power or for other purposes. Bypass REF to GND with 0.22µF, plus 1µF/mA of load current. Applications Information and __________________Design Procedure Most users will be able to work with one of the standard application circuits; others may want to implement a circuit with an output current rating that lies between or beyond the standard values. If you want an output current level that lies between two of the standard application circuits, you can interpolate many of the component values from the values given for the two circuits. These components include the input and output filter capacitors, the inductor, and the sense resistor. The capacitors must meet ESR and rip- ple current requirements (see Input Filter Capacitor and Output Filter Capacitor sections). The inductor must meet the required current rating (see Inductor section). You may use the rectifier and MOSFETs specified for the circuit with the greater output current capability, or choose a new rectifier and MOSFETs according to the requirements detailed in the Rectifier and MOSFET Switches sections. For more complete information, or for output currents in excess of 10A, refer to the design information in the following sections. Inductor, L1 Three inductor parameters are required: the inductance value (L), the peak inductor current (ILPEAK), and the coil resistance (RL). The inductance is: 1.32 L1 = ______________ f x IOUT x LIR where: f = switching frequency, normally 300kHz IOUT = maximum 3.3V DC load current (A) LIR = ratio of inductor peak-to-peak AC current to average DC load current, typically 0.3. A higher LIR value allows smaller inductance, but results in higher losses and ripple. The highest peak inductor current (ILPEAK) equals the DC load current (IOUT) plus half the peak-to-peak AC inductor current (ILPP). The peak-to-peak AC inductor current is typically chosen as 30% of the maximum DC load current, so the peak inductor current is 1.15 x IOUT. The peak inductor current at any load is given by: 1.32 ILPEAK = IOUT + __________ 2 x f x L1 The coil resistance should be as low as possible, preferably in the low milliohms. The coil is effectively in series with the load at all times, so the wire losses alone are approximately: Power Loss = IOUT2 x RL In general, select a standard inductor that meets the L, ILPEAK, and RL requirements. If a standard inductor is unavailable, choose a core with an LI2 parameter greater than L x ILPEAK2, and use the largest wire that will fit the core. Current-Sense Resistor, R1 The current-sense resistor must carry the peak current in the inductor, which exceeds the full DC load current. The internal current limiting starts when the voltage across the sense resistors exceeds 100mV nominally, 80mV minimum. Use the minimum value to ensure ade- quate output current capability: R1 = 80mV / ILPEAK. The low VIN/VOUT ratio creates a potential problem with start-up under full load or with load transients from no- load to full load. If the supply is subjected to these con- ditions, reduce the sense resistor: 70mV R1 = ——— ILPEAK Since the sense-resistance values are similar to a few centimeters of narrow traces on a printed circuit board, trace resistance can contribute significant errors. To prevent this, Kelvin connect the CS and FB pins to the sense resistors; i.e., use separate traces not carrying any of the inductor or load current, as shown in Figure 2. 5V-to-3.3V, Synchronous, Step-Down Power-Supply Controller ______________________________________________________________________________________ 11 |
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