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SP7650 Datasheet(PDF) 6 Page - Exar Corporation |
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SP7650 Datasheet(HTML) 6 Page - Exar Corporation |
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6 / 16 page  6 Date: /5/06 SP7650 Wide Input Voltage Range 3A, 300kHz, Buck Regulator © Copyright 2006 Sipex Corporation 6 Date: 2/16/06 SP7650 Wide Input Voltage Range 3A, 300kHz, Buck Regulator © Copyright 2006 Sipex Corporation THEORY OF OPERATION desired, then compensation can be as simple as an RC circuit to Ground. If a more complex compensation is required, then the amplifier has enough bandwidth (45° at 4 MHz) and enough gain (60dB) to run Type III compensation schemes with adequate gain and phase margins at crossover frequencies greater than 50kHz. The common mode output of the error amplifier is 0.9V to 2.2V. Therefore, the PWM voltage ramp has been set between 1.1V and 2.2V to ensure proper 0% to 100% duty cycle capability. The voltage loop also includes two other very important features. One is an asynchronous startup mode. Basically, the synchronous recti- fier cannot turn on unless the high side switch has attempted to turn on or the SS pin has exceeded 1.7V. This feature prevents the con- troller from “dragging down” the output voltage during startup or in fault modes. The second feature is a 100% duty cycle timeout that en- sures synchronized refreshing of the BST ca- pacitor at very high duty ratios. In the event that the high side NFET is on for 20 continuous clock cycles, a reset is given to the PWM flip- flop half way through the 21st cycle. This forces GL to rise for the cycle, in turn refreshing the BST capacitor. The boost capacitor is used to generate a high voltage drive supply for the high side switch, which is 5V above VIN. Power MOSFETs The SP7650 contains a pair of integrated low resistance N-channel switches designed to drive up to 3A of output current. Care should be taken to de-rate the output current based on the ther- mal conditions in the system such as ambient temperature, airflow and heat sinking. Maxi- mum output current could be limited by thermal limitations of a particular application by taking advantage of the integrated-over-temperature protective scheme employed in the SP7650. The SP7650 incorporates a built-in over-tem- perature protection to prevent internal overheat- ing. GH Voltage GL Voltage V(VIN) 0V -0V -V(Diode) V V(VIN)+V(VCC) BST Voltage V(VCC) TIME SWN Voltage VBST VSWN V(VCC) The SP7650 can be set to different output volt- ages. The relationship in the following formula is based on a voltage divider from the output to the feedback pin VFB, which is set to an internal reference voltage of 0.80V. Standard 1% metal film resistors of surface mount size 0603 are recommended. Vout = 0.80V ( R1 / R2 + 1 ) => R2 = R1 / [ ( Vout / 0.80V ) – 1 ] Where R1 = 68.1K and for Vout = 0.80V setting, simply remove R2 from the board. Furthermore, one could select the value of the R1 and R2 combination to meet the exact output voltage setting by restricting R1 resistance range such that 50K < R1 < 100K for overall system loop stability. Setting Output Voltages |
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