LND1117

Output Voltage

The LND1117(adjustable version) develops a

1.25V reference voltage between the sense

pin and the adjust pin ( figure 4). Placing a

resistor between these two terminals causes

a constant current to flow though R1 and

down though R2 to set the output voltage. In

general R1 is chosen so that this current is

the specified minimum load current of 10mA.

The current out of the Adjust pin is small,

typically 50µA and it adds to the current from

R1. For best regulation the top of the resistor

divider should be connected directly to the

Sense pin.

Protection Diodes

In normal operation LND1117 family does not

need any protection diodes between the

adjustment pin and the output and from the

output to the input to prevent die overstress.

Internal resistors are limiting the internal current

paths on the ADJ pin. Therefore even with

bypass capacitors on the adjust pin no

protection diode is needed to ensure device

safety under short-circuit conditions. The adjust

pin can be driver on a transient basis ± 7V with

respect to the output without any device

degradation. A protection diode between the

Output pin and V power pin is not usually

needed. Microsecond surge currents of pin is

not usually needed. Microsecond surge currents

of 50A to 100A can be handled by the internal

diode between the output pin and Vpower pin of

the device. In normal operations it is difficult to

get those values of surge currents even with

high value output capacitors, such as 1000µF to

5000µF and the Vpower pin is instantaneously

shorted to ground, damage can occur. A diode

from output to input is recommended(Figure 5)

Thermal Considerations

The LND1117 series have internal power

and thermal limiting circuitry designed to

protect the device under overload

conditions. However, maximum junction

temperature ratings should not be

exceeded under continuous normal load

conditions. Careful consideration must be

given to all sources of thermal resistance

from junction to ambient, including junction-

to-case, case-to-heat sink interface and

heat sink resistance itself. Junction

temperature of the Power section can run

up to 150ºC.

Figure 5. Optional Clamp Diodes Protect against

Input Crowbar Circuits

If LND1117 is connected as a single supply

device with the control and power input pins

shorted together the internal diode between the

output and the power input pin will protect the

control input pin.

Figure 4. Setting Output Voltage

• Linear Dimensions, Inc. • 445 East Ohio Street, Chicago IL 60611 USA • tel 312.321.1810 • fax 312.321.1830 • www.lineardimensions.com •