LND1117
APPLICATION INFORMATION
The LND1117 is designed to make use of
multiple power supplies, 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 NPN output transistor. This allows the
NPN output transistor to be driven into
saturation. For the control voltage the
current requirement is small and 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 trying the control and power inputs
together the LND1117 can also be operated
as a single supply device. In single supply
operation the dropout will be determined by
the minimum control voltage.
Both the 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 800mA,
the typical load regulation is less than1mV.
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
Dimensions, Inc. 445
the Adjust pin. In
Linear
1µF for bypassing
addition to the enhancements mentioned, the
reference accuracy has been improved a
factor or 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 LND1117 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 LND1117 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 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.
R
P
is the parasitic resistanceof the connections
between the device and the load.Trace A of
figure 3, illiustrates the effect of R
P
. 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 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 LND1117 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 R
P.
Stability
The circuit design used in the LND1117 series
requires the use of an output capacitor as part
of the device frequency compensation. The
addition of 150µF aluminum electrolytic or a
22µF solid tantalum on the output will ensure
stability for all operating conditions.
•
Linear Dimensions, Inc.
•
445 East Ohio Street, Chicago IL 60611 USA
•
tel 312.321.1810
•
fax 312.321.1830
•
•
LINEAR_DIMENSIONS [ LINEAR DIMENSIONS SEMICONDUCTOR ]
