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AMS1505CMV-2.5 Datasheet(PDF) 4 Page - Advanced Monolithic Systems |
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AMS1505CMV-2.5 Datasheet(HTML) 4 Page - Advanced Monolithic Systems |
4 / 8 page AMS1505V APPLICATION HINTS The AMS1505V series of adjustable and fixed regulators are designed to power the new generation of microprocessors. The AMS1505V is designed to make use of multiple power supplies, existing in most systems, to reduce the dropout voltage. One of the advantages of the two supply approach is maximizing the efficiency. The second supply is at least 0.35V greater than output voltage and is providing the power for the control circuitry and supplies the drive current to the NPN output transistor. This allows the NPN to be driven into saturation; thereby reducing the dropout voltage by a VBE compared to conventional designs. For the control voltage the current requirement is small equal to about 1% of the output current or approximately 50mA for a 5A load. Most of this current is drive current for the NPN output transistor. This drive current becomes part of the output current. The maximum voltage on the Control pin is 13V. The maximum voltage at the Power pin is 7V. Ground pin current for fixed voltage devices is typical 6mA and is constant as a function of load. Adjust pin current for adjustable devices is 60 µA at 25°C and varies proportional to absolute temperature. The improved frequency compensation of AMS1505V permits the use of capacitors with very low ESR. This is critical in addressing the needs of modern, low voltage high sped microprocessors. The new generation of microprocessors cycle load current from several hundred mA to several A in tens of nanoseconds. Output voltage tolerances are tighter and include transient response as part of the specification. Designed to meet the fast current load step requirements of the video-processors, the AMS1505V also saves total cost by needing less output capacitance to maintain regulation. Careful design of the AMS1505V has eliminated any supply sequencing issues associated with a dual supply system. The output voltage will not turn on until both supplies are operating. If the control voltage comes up first, the output current will be limited to a few milliamperes until the power input voltage comes up. If power input comes up first the output will not turn on at all until the control voltage comes up. The output can never come up unregulated. The new features of the AMS1505V require additional pins over the traditional 3-terminal regulator. Both the fixed and adjustable versions have remote sense pins, permitting very accurate regulation of output voltage at the load, rather than at the regulator. As a result, over an output current range of 100mA to 5A with a 2.5V output, the typical load regulation is less than 1mV. Optimum transient response is provided using a capacitor in the range of 0.1 µF to 1µF for bypassing the Adjust pin. The value chosen will depend on the amount of output capacitance in the system. In addition to the enhancements mentioned, the reference accuracy has been improved by a factor of two with a guaranteed initial tolerance of ±1% at 25°C. This device can hold 1% accuracy over the full temperature range and load current range, guaranteed, when combined with ratiometrically accurate internal divider resistors and operating with an input/output differential of well under 1V. Typical applications for the AMS1505V include 3.3V to 2.9V conversion with a 5V control supply, 5V to 4.7V conversion with a 12V control supply or 5V to 3.6V conversion with a 12V control supply. Capable of 5A of output current with a maximum dropout of 350mV the AMS1505V also has a fast transient response that allows it to handle large current changes associated with the new generation of video-processors. The device is fully protected against overcurrent and overtemperature conditions. Grounding and Output Sensing The AMS1505V allows true Kelvin sensing for both the high and low side of the load. As a result the voltage regulation at he load can be easily optimized. Voltage drops due to parasitic resistances between the regulator and the load can be placed inside the regulation loop of the AMS1505V. The advantages of remote sensing are illustrated in figures 1 through 3. Figure 1 shows the device connected as a conventional 3 terminal regulator with the Sense lead connected directly to the output of the device. RP is the parasitic resistance of the connections between the device and the load. Typically the load is a microprocessor and RP is made up of the PC traces and /or connector resistances (in the case of a modular regulator) between the regulator and the processor. Trace A of figure 3 illustrates the effect of RP. Very small resistances cause significant load regulation steps. Figure 2 shows the device connected to take advantage of the remote sense feature. The Sense pin and the top of the resistor divider are connected to the top of the load; the bottom of the resistor divider is connected to the bottom of the load. RP is now connected inside the regulation loop of the AMS1505V and for reasonable values of RP the load regulation at the load will be negligible. The effect on output regulation can be seen in trace B of figure 3. CONTROL POWER SENSE AMS1505V OUTPUT ADJ LOAD + - V OUT R P R P R1 R2 5V 3.3V Figure 1. Conventional Load Sensing Advanced Monolithic Systems, Inc. www.advanced-monolithic.com Phone (925) 443-0722 Fax (925) 443-0723 |
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