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SP7653 Datasheet(PDF) 5 Page - Sipex Corporation |
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SP7653 Datasheet(HTML) 5 Page - Sipex Corporation |
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5 / 15 page  5 Date: /20/06 SP7653 Wide Input Voltage Range, .3MHz, Buck Regulator © Copyright 2006 Sipex Corporation 5 Date: 2/17/06 SP7653 Wide Input Voltage Range, 1.3MHz, Buck Regulator © Copyright 2006 Sipex Corporation tures include thermal shutdown and short-cir- cuit detection. In the event that either a thermal, short-circuit, or UVLO fault is detected, the SP7653 is forced into an idle state where the output drivers are held off for a finite period before a restart is attempted. Soft Start “Soft Start” is achieved when a power converter ramps up the output voltage while controlling the magnitude of the input supply source cur- rent. In a modern step down converter, ramping up the positive terminal of the error amplifier controls soft start. As a result, excess source current can be defined as the current required to charge the output capacitor: IVIN = COUT * ( VOUT / TSOFT-START) The SP7653 provides the user with the option to program the soft start rate by tying a capacitor from the SS pin to GND. The selection of this capacitor is based on the 10uA pull up current present at the SS pin and the 0.8V reference voltage. Therefore, the excess source can be redefined as: IVIN = COUT * [ (DVOUT *10µA) /(CSS * 0.8V)] Under Voltage Lock Out (UVLO) The SP7653 contains two separate UVLO com- parators to monitor the bias (VCC) and conver- sion (VIN) voltages independently. The VCC UVLO threshold is internally set to 4.25V, whereas the VIN UVLO threshold is program- mable through the UVIN pin. When the voltage on the UVIN pin is greater than 2.5V, the SP7653 is permitted to start up pending the removal of all other faults. Both the VCC and VIN UVLO comparators have been designed with hysteresis to prevent noise from resetting a fault. Thermal and Short-Circuit Protection Because the SP7653 is designed to drive large output current, there is a chance that the power converter will become too hot. Therefore, an internal thermal shutdown (145°C) has been included to prevent the IC from malfunctioning at extreme temperatures. A short-circuit detection comparator has also been included in the SP7653 to protect against an accidental short at the output of the power converter. This comparator constantly monitors the positive and negative terminals of the error amplifier, and if the VFB pin falls more than 250mV (typical) below the positive reference, a short-circuit fault is set. Because the SS pin overrides the internal 0.8V reference during soft start, the SP7653 is capable of detecting short- circuit faults throughout the duration of soft start as well as in regular operation. Handling of Faults: Upon the detection of power (UVLO), thermal, or short-circuit faults, the SP7653 is forced into an idle state where the SS and COMP pins are pulled low and both switches are held off. In the event of UVLO fault, the SP7653 remains in this idle state until the UVLO fault is removed. Upon the detection of a thermal or short-circuit fault, an internal 200ms timer is activated. In the event of a short-circuit fault, a restart is at- tempted immediately after the 200ms timeout expires. Whereas, when a thermal fault is de- tected, the 200ms delay continuously recycles and a restart cannot be attempted until the ther- mal fault is removed and the timer expires. Error Amplifier and Voltage Loop The heart of the SP7653 voltage error loop compensation is a high performance, wide band- width transconductance amplifier. Because of the amplifier’s current limited (+/-150µA) transconductance, there are many ways to com- pensate the voltage loop or to control the COMP THEORY OF OPERATION |
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