6

LT1581/LT1581-2.5

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 LT1581 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

LT1581 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 LT1581 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 10A with a 2.5V

output is typically less than 1mV. For the fixed voltage

devices the ADJUST 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

ADJUST pin with a capacitor in the range of 0.1

µF to 1µF

will provide optimum transient response. The value cho-

sen will depend 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 LT1581 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 0.4V at 4A along with excellent static

and dynamic specifications. The LT1581 is capable of 10A

of output current with a maximum dropout of 0.7V. The

LT1581 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 available. The device is

available in a 7-lead TO-220 package.

Grounding and Output Sensing

The LT1581 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 LT1581. Figures

1 through 3 illustrate the advantages of remote sensing.

Figure 1 shows the LT1581 connected as a conventional

3-terminal regulator with the SENSE lead connected di-

parasitic resistance of the connections between the LT1581

and the load. The load is typically a microprocessor and

tances, in the case of a modular regulator, between the

in trace A of Figure 3. Very small resistances cause

significant load regulation steps. For example, at 10A

output current the output voltage will shift by 10mV for

every 0.001

Ω of resistance. In Figure 2 the LT1581 is

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

effectively connected inside the regulating loop of the

LT1581 and the load regulation at the load will be negli-