LND5203
APPLICATION INFORMATION
The LND5203 series of adjustable and fixed
regulators are designed to power the new
generation of microprocessors. The LND5203 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 LND5203 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 3A, the typical load regulation are 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 LND5203 include
3.3V to 2.5V conversion with a 5V control
supply, 5V to 4.2 V conversion with a 12V
control supply. The device is fully protected
against overcurrent and overtemperature
conditions.
Grounding and Output Sensing
The LND5203 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
conventional 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 LND5203 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 LND5203
series requires the use of an output capacitor
as part of the device frequency
compensation. The addition of 150
µF
aluminum electrolytic or a 22
µF
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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LINEAR_DIMENSIONS [ LINEAR DIMENSIONS SEMICONDUCTOR ]
