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LT1581/LT1581-2.5
APPLICATIONS INFORMATION
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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 0.1µF 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-
rectly to the output of the device. R
P
represents the
parasitic resistance of the connections between the LT1581
and the load. The load is typically a microprocessor and
R
P
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 R
P
can be seen
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 0.001Ω 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
divider is connected to the bottom of the load. R
P
is now
effectively connected inside the regulating loop of the
LT1581 and the load regulation at the load will be negli-
gible for reasonable values of R
P
. Trace B of Figure 3
