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.
Figure 4. Setting Output Voltage
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)
V
OUT
= V
REF
(1+R2/R1) +I
ADJ
R2
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.
•
Linear Dimensions, Inc.
•
445 East Ohio Street, Chicago IL 60611 USA
•
tel 312.321.1810
•
fax 312.321.1830
•
•
LINEAR_DIMENSIONS [ LINEAR DIMENSIONS SEMICONDUCTOR ]
