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LT1580IT7-2.5 Datasheet(PDF) 7 Page - Linear Technology |
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LT1580IT7-2.5 Datasheet(HTML) 7 Page - Linear Technology |
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7 / 16 page  7 LT1580/LT1580-2.5 APPLICATIONS INFORMATION The LT1580 has improved frequency compensation which permits the use of capacitors with very low ESR. This is critical in addressing the needs of modern, low voltage, high speed microprocessors. Current generation micro- processors cycle load current from several hundred mil- liamperes to several amperes in tens of nanoseconds. Output voltage tolerances are tighter and include transient response as part of the specification. The LT1580 is designed to meet the fast current load step requirements of these microprocessors and saves total cost by needing less output capacitance to maintain regulation. Careful design 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 the 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 LT1580 can also be operated as a single supply device by tying the control and power inputs together. Dropout in single supply operation will be determined by the mini- mum control voltage. The LT1580 includes several innovative features that require additional pins over the traditional 3-terminal regulator. Both the fixed and adjustable devices have remote SENSE pins, permitting very accurate regulation of output voltage at the load, where it counts, rather than at the regulator. As a result the typical load regulation over an output current range of 100mA to 7A with a 2.5V output is typically less than 1mV. For the fixed voltage devices the ADJ pin is also brought out. This allows the user to improve transient response by bypassing the internal resistor divider. In the past fixed output voltage devices did not provide this capability. Bypassing the ADJ pin with a capacitor in the range of 0.1 µF to 1µF will provide optimum transient response. The value chosen will de- pend on the amount of output capacitance in the system. In addition to the enhancements mentioned above the reference accuracy has been improved by a factor of two with a guaranteed initial tolerance of ±0.6% at 25°C. Temperature drift is also very well controlled. When com- bined with ratiometrically accurate internal divider resis- tors the part can easily hold 1% output accuracy over the full temperature range and load current range, guaran- teed, while operating with an input/output differential of well under 1V. Typical applications for the LT1580 include 3.3V to 2.5V conversion with a 5V control supply, 5V to 4.2V conver- sion with a 12V control supply or 5V to 3.6V conversion with a 12V control supply. It is easy to obtain dropout voltages of less than 0.5V at 4A along with excellent static and dynamic specifications. The LT1580 is capable of 7A of output current with a maximum dropout of 0.8V. The LT1580 has fast transient response that allows it to handle the large current changes associated with today’s micro- processors. The device is fully protected against overcurrent and overtemperature conditions. Both fixed voltage (2.5V) and adjustable output versions are avail- able. The device is available in a multilead TO-220 package with five leads for the adjustable device and seven leads for the fixed voltage device. Grounding and Output Sensing The LT1580 allows true Kelvin sensing for both the high and low side of the load. This means that the voltage regulation at the load can be easily optimized. Voltage drops due to parasitic resistances between the regulator and the load which would normally degrade regulation can be placed inside the regulation loop of the LT1580. Figures 1 through 3 illustrate the advantages of remote sensing. Figure 1 shows the LT1580 connected as a conventional 3-terminal regulator with the SENSE lead connected di- rectly to the output of the device. RP represents the parasitic resistance of the connections between the LT1580 and the load. The load is typically a microprocessor and RP is made up of the PC traces and/or connector resis- tances, in the case of a modular regulator, between the regulator and the processor. The effect of RP can be seen in trace A of Figure 3. Very small resistances cause significant load regulation steps. For example, at 7A out- put current the output voltage will shift by 7mV for every 0.001 Ω of resistance. In Figure 2 the LT1580 is connected to take advantage of the remote sense feature. The SENSE |
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