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SP7651ER-L Datasheet(PDF) 4 Page - Exar Corporation |
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SP7651ER-L Datasheet(HTML) 4 Page - Exar Corporation |
4 / 15 page 4 Rev J: 3/4/07 SP765 Wide Input Voltage Range 3A, 900kHz, Buck Regulator © Copyright 2007 Sipex Corporation General Overview The SP7651 is a fixed frequency, voltage mode, synchronous PWM regulator opti- mized for high efficiency. The part has been designed to be especially attractive for split plane applications utilizing 5V to power the controller and 2.5V to 20V for step down conversion. The heart of the SP765 is a wide bandwidth transconductance amplifier designed to ac- commodate Type II and Type III compensa- tion schemes. A precision 0.8V reference, present on the positive terminal of the error amplifier, permits the programming of the output voltage down to 0.8V via the VFB pin. The output of the error amplifier, COMP, which is compared to a .V peak-to-peak ramp, is responsible for trailing edge PWM control.This voltage ramp, and PWM control logic are governed by the internal oscillator THEORY OF OPERATION that accurately sets the PWM frequency to 900kHz. The SP765 contains two unique control features that are very powerful in distributed applications.First,asynchronousdrivercon- trol is enabled during startup, to prohibit the low side NFET from pulling down the output until the high side NFET has attempted to turn on. Second, a 100% duty cycle timeout ensuresthatthelowsideNFETisperiodically enhanced during extended periods at 100% dutycycle.Thisguaranteesthesynchronized refreshing of the BST capacitor during very large duty ratios. The SP765 also contains a number of valuableprotectionfeatures.Programmable UVLO allows the user to set the exact VIN value at which the conversion voltage can PIN DESCRIPTION Pin # Pin Name Description -3 Pgnd Ground connection for the synchronous rectifier 4,8,9-2 GND Ground Pin. The control circuitry of the IC and lower power driver are refer- enced to this pin. Return separately from other ground traces to the (-) terminal of Cout. 5 Vfb Feedback Voltage and Short Circuit Detection pin. It is the inverting input of the Error Amplifier and serves as the output voltage feedback point for the Buck Converter. The output voltage is sensed and can be adjusted through an exter- nal resistor divider. Whenever Vfb drops 0.25V below the positive reference, a short circuit fault is detected and the IC enters hiccup mode. 6 COMP Output of the Error Amplifier. It is internally connected to the inverting input of the PWM comparator. An optimal filter combination is chosen and connected to this pin and either ground or Vfb to stabilize the voltage mode loop. 7 UVIN UVLO input for Vin voltage. Connect a resistor divider between Vin and UVin to set minimum operating voltage. 9 SS Soft Start. Connect an external capacitor between SS and GND to set the soft start rate based on the 0µA source current. The SS pin is held low via a mA (min) current during all fault conditions. 0-3 Vin Input connection to the high side N-channel MOSFET. Place a decoupling capacitor between this pin and Pgnd. 4-6,23-26 LX Connect an inductor between this pinand Vout 7 NC No Connect 8 BST High side driver supply pin. Connect BST to the external boost diode and ca- pacitor as shown in the Typical Application Circuit on page . High side driver is connected between BST pin and SWN pin. 22 Vcc Input for external 5V bias supply |
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