LND1117

Linear Dimensions, Inc. 445

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 1µF for bypassing the Adjust pin. In

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.

between the device and the load.Trace A of

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

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 • www.lineardimensions.com •