LND5201
APPLICATIONS INFORMATION
The LND5201 series of adjustable and fixed
regulators are designed to power the new
generation of microprocessors. The LND5201 is
designed to make use of multiple power
supplies, present in most systems, to reduce the
dropout voltage. One of the advantages of the
two supply approach is maximizing the
efficiency.
The second supply is at least 1V greater than
output voltage and is providing the power for the
control circuitry and supplies the drive current to
the NPN output transistor. This allows the NPN
output transistor to be driven into saturation. For
the control voltage the current requirement is
small equal to about 1% of the output current.
This drive current becomes part of the output
current. The maximum voltage on the Control
pin is 12V. The maximum voltage at the Power
pin is 7V. By tying the control and power inputs
together the LND5201 can also be operated as
a single supply device. In single supply
operation the dropout will be determined by the
minimum control voltage.
Both fixed and adjustable versions have remote
sense pins, permitting very accurate regulation
of output voltage. As a result, over an output
current range of 100mA to 1A, the typical load
regulation is less than 1mV. For the fixed
voltages the adjust pin is brought out allowing
the user to improve transient response by
bypassing the internal resistor divider. Optimum
transient response is provided using a capacitor
in the range of 0.1µF to 1µF for bypassing the
Adjust pin. In addition to the enhancements
mentioned, the reference accuracy has been
improved by a factor of two with a guaranteed
initial tolerance of ±1% at 25ºC and 1.6%
accuracy over the full temperature and load
current range.
Typical applications for the LND5201 include
3.3V to 2.5V conversion with a 5V control
supply, 5V to 4.2V conversion with a 12V
control supply. The device is fully protected
against overcurrent and overtemperature
conditions.
Grounding and Output Sensing
The LND5201 allows true Kelvin sensing for
both the high and low side of the load. As a
result the voltage regulation at the load can be
easily optimized. Voltage drops due to
parasitic resistance’s between the regulator
and the load can be placed inside the
regulation loop. The advantages of remote
sensing are illustrated in figures 1 through 3.
Figure 1 shows the device connected as a
convential 3 terminal regulator with the sense
lead connected directly to the output of the
device. Rp is the parasitic resistance of the
connections between the device and the load.
Trace A of figure 3 illustrates the effect of Rp.
Figure 2 shows the device 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. The effect on output
regulation can be seen in trace B of figure 3.
It is important to note that the voltage drops
due to Rp are not eliminated. They will add to
the dropout voltage of the regulator regardless.
The LND5201 can control the voltage at the
load as long as the input-output voltage is
greater than the total of the dropout voltage of
the device plus the voltage drop across Rp.
Stability
The circuit design used in the LND5201 series
requires the use of an output capacitor as part
of the device frequency compensation. The
addition of 150µF aluminnum electrolytic or a
22µF solid tantalum on the output will ensure
stability for all operating conditions
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Linear Dimensions, Inc.
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445 East Ohio Street, Chicago IL 60611 USA
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tel 312.321.1810
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fax 312.321.1830
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