Datasheet LT1581, LT1581-2.5 (Analog Devices) - 6

LT1581/LT1581-2.5
U U W U APPLICATIONS INFORMATION
processors cycle load current from several hundred mil- teed, while operating with an input/output differential of liamperes to several amperes in tens of nanoseconds. well under 1V. Output voltage tolerances are tighter and include transient Typical applications for the LT1581 include 3.3V to 2.5V response as part of the specification. The LT1581 is conversion with a 5V control supply, 5V to 4.2V conver- designed to meet the fast current load step requirements sion with a 12V control supply or 5V to 3.6V conversion of these microprocessors and saves total cost by needing with a 12V control supply. It is easy to obtain dropout less output capacitance to maintain regulation. voltages of less than 0.4V at 4A along with excellent static Careful design has eliminated any supply sequencing and dynamic specifications. The LT1581 is capable of 10A issues associated with a dual supply system. The output of output current with a maximum dropout of 0.7V. The voltage will not turn on until both supplies are operating. LT1581 has fast transient response that allows it to handle If the control voltage comes up first, the output current will the large current changes associated with today’s micro- be limited to a few milliamperes until the power input processors. The device is fully protected against overcurrent voltage comes up. If the power input comes up first the and overtemperature conditions. Both fixed voltage (2.5V) output will not turn on at all until the control voltage comes and adjustable output versions are available. The device is up. The output can never come up unregulated. The available in a 7-lead TO-220 package. LT1581 can also be operated as a single supply device by tying the control and power inputs together. Dropout in
Grounding and Output Sensing
single supply operation will be determined by the mini- The LT1581 allows true Kelvin sensing for both the high mum control voltage. and low side of the load. This means that the voltage The LT1581 includes several innovative features that regulation at the load can be easily optimized. Voltage require additional pins over the traditional 3-terminal drops due to parasitic resistances between the regulator regulator. Both the fixed and adjustable devices have and the load which would normally degrade regulation can remote sense pins, permitting very accurate regulation of be placed inside the regulation loop of the LT1581. Figures output voltage at the load, where it counts, rather than at 1 through 3 illustrate the advantages of remote sensing. the regulator. As a result the typical load regulation over Figure 1 shows the LT1581 connected as a conventional an output current range of 100mA to 10A with a 2.5V 3-terminal regulator with the SENSE lead connected di- output is typically less than 1mV. For the fixed voltage rectly to the output of the device. RP represents the devices the ADJUST pin is also brought out. This allows parasitic resistance of the connections between the LT1581 the user to improve transient response by bypassing the and the load. The load is typically a microprocessor and internal resistor divider. In the past, fixed output voltage RP is made up of the PC traces and/or connector resis- devices did not provide this capability. Bypassing the tances, in the case of a modular regulator, between the ADJUST pin with a capacitor in the range of 0.1µF to 1µF regulator and the processor. The effect of RP can be seen will provide optimum transient response. The value cho- in trace A of Figure 3. Very small resistances cause sen will depend on the amount of output capacitance in the significant load regulation steps. For example, at 10A system. output current the output voltage will shift by 10mV for every 0.001Ω of resistance. In Figure 2 the LT1581 is In addition to the enhancements mentioned above, the connected to take advantage of the remote sense feature. reference accuracy has been improved by a factor of two The SENSE pin and the top of the resistor divider are with a guaranteed initial tolerance of ±0.6% at 25°C. connected to the top of the load. The bottom of the resistor Temperature drift is also very well controlled. When com- divider is connected to the bottom of the load. R bined with ratiometrically accurate internal divider resis- P is now effectively connected inside the regulating loop of the tors the part can easily hold 1% output accuracy over the LT1581 and the load regulation at the load will be negli- full temperature range and load current range, guaran- gible for reasonable values of RP. Trace B of Figure 3 6