Datasheet LT1580, LT1580-2.5 (Analog Devices) - 7

LT1580/LT1580-2.5
U U W U APPLICATIONS INFORMATION
The LT1580 has improved frequency compensation which bined with ratiometrically accurate internal divider resis- permits the use of capacitors with very low ESR. This is tors the part can easily hold 1% output accuracy over the critical in addressing the needs of modern, low voltage, full temperature range and load current range, guaran- high speed microprocessors. Current generation micro- teed, while operating with an input/output differential of processors cycle load current from several hundred mil- well under 1V. liamperes to several amperes in tens of nanoseconds. Typical applications for the LT1580 include 3.3V to 2.5V Output voltage tolerances are tighter and include transient conversion with a 5V control supply, 5V to 4.2V conver- response as part of the specification. The LT1580 is sion with a 12V control supply or 5V to 3.6V conversion designed to meet the fast current load step requirements with a 12V control supply. It is easy to obtain dropout of these microprocessors and saves total cost by needing voltages of less than 0.5V at 4A along with excellent static less output capacitance to maintain regulation. and dynamic specifications. The LT1580 is capable of 7A Careful design has eliminated any supply sequencing of output current with a maximum dropout of 0.8V. The issues associated with a dual supply system. The output LT1580 has fast transient response that allows it to handle voltage will not turn on until both supplies are operating. the large current changes associated with today’s micro- If the control voltage comes up first, the output current will processors. The device is fully protected against be limited to a few milliamperes until the power input overcurrent and overtemperature conditions. Both fixed voltage comes up. If the power input comes up first the voltage (2.5V) and adjustable output versions are avail- output will not turn on at all until the control voltage comes able. The device is available in a multilead TO-220 package up. The output can never come up unregulated. The with five leads for the adjustable device and seven leads for LT1580 can also be operated as a single supply device by the fixed voltage device. tying the control and power inputs together. Dropout in single supply operation will be determined by the mini-
Grounding and Output Sensing
mum control voltage. The LT1580 allows true Kelvin sensing for both the high The LT1580 includes several innovative features that and low side of the load. This means that the voltage require additional pins over the traditional 3-terminal regulation at the load can be easily optimized. Voltage regulator. Both the fixed and adjustable devices have drops due to parasitic resistances between the regulator remote SENSE pins, permitting very accurate regulation of and the load which would normally degrade regulation can output voltage at the load, where it counts, rather than at be placed inside the regulation loop of the LT1580. Figures the regulator. As a result the typical load regulation over 1 through 3 illustrate the advantages of remote sensing. an output current range of 100mA to 7A with a 2.5V output Figure 1 shows the LT1580 connected as a conventional is typically less than 1mV. For the fixed voltage devices the 3-terminal regulator with the SENSE lead connected di- ADJ pin is also brought out. This allows the user to rectly to the output of the device. RP represents the improve transient response by bypassing the internal parasitic resistance of the connections between the LT1580 resistor divider. In the past fixed output voltage devices and the load. The load is typically a microprocessor and did not provide this capability. Bypassing the ADJ pin with RP is made up of the PC traces and/or connector resis- a capacitor in the range of 0.1µF to 1µF will provide tances, in the case of a modular regulator, between the optimum transient response. The value chosen will de- regulator and the processor. The effect of RP can be seen pend on the amount of output capacitance in the system. in trace A of Figure 3. Very small resistances cause significant load regulation steps. For example, at 7A out- In addition to the enhancements mentioned above the put current the output voltage will shift by 7mV for every reference accuracy has been improved by a factor of two 0.001Ω of resistance. In Figure 2 the LT1580 is connected with a guaranteed initial tolerance of ±0.6% at 25°C. to take advantage of the remote sense feature. The SENSE Temperature drift is also very well controlled. When com- 7