lt3085ems8e Linear Technology Corporation, lt3085ems8e Datasheet - Page 15

• Manufacturer Part Number: lt3085ems8e
• Description: Adjustable 500ma Single Resistor Low Dropout Regulator
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
Internal Parasitic Diodes and Protection Diodes
In normal operation, the LT3085 does not require protection
diodes. Older three-terminal regulators require protection
diodes between the VOUT pin and the input pin or between
the ADJ pin and the VOUT pin to prevent die overstress.
On the LT3085, internal resistors and diodes limit current
paths on the SET pin. Even with bypass capacitors on the
SET pin, no protection diode is needed to ensure device
safety under short-circuit conditions. The SET pin handles
±10V (either transient or DC) with respect to OUT without
any device degradation.
Internal parasitic diodes exist between OUT and the two
inputs. Negative input voltages are transferred to the output
and may damage sensitive loads. Reverse-biasing either
input to OUT will turn on these parasitic diodes and allow
current fl ow. This current fl ow will bias internal nodes
of the LT3085 to levels that possibly cause errors when
suddenly returning to normal operating conditions and
expecting the device to start and operate. Prediction of
results of a bias fault is impossible, immediate return to
normal operating conditions can be just as diffi cult after
a bias fault. Suffi ce it to say that extra wait time, power
cycling, or protection diodes may be needed to allow the
LT3085 to return to a normal operating mode as quickly
as possible.
Figure 7. Connections for Best Load Regulation
V
CONTROL
IN
SET R
LT3085
+
–
SET
OUT
RESISTANCE
PARASITIC
R
R
R
P
P
P
3085 F07
LOAD
Protection diodes are not otherwise needed between
the OUT pin and IN pin. The internal diodes can handle
microsecond surge currents of up to 50A. Even with
large output capacitors, obtaining surge currents of those
magnitudes is diffi cult in normal operation. Only with large
output capacitors, such as 1000μF to 5000μF , and with
IN instantaneously shorted to ground will damage occur.
A crowbar circuit at IN is capable of generating those
levels of currents, and then protection diodes from OUT
to IN are recommended. Normal power supply cycling or
system “hot plugging and unplugging” does not do any
damage.
A protection diode between OUT and V
not needed. The internal parasitic diode on V
the LT3085 handles microsecond surge currents of 1A to
10A. Again, this only occurs when using crowbar circuits
with large value output capacitors. Since the V
pin is usually a low current supply, this is unlikely. Still,
a protection diode is recommended if V
instantaneously shorted to ground. Normal power supply
cycling or system “hot plugging and unplugging” does
not do any damage.
If the LT3085 is confi gured as a three-terminal (single supply)
regulator with IN and V
diode of the IN pin will protect the V
Like any other regulator, exceeding the maximum input-
to-output differential causes internal transistors to break
down and then none of the internal protection circuitry
is functional.
Thermal Considerations
The LT3085 has internal power and thermal limiting cir-
cuitry designed to protect it under overload conditions.
For continuous normal load conditions, maximum junc-
tion temperature must not be exceeded. It is important
to give consideration to all sources of thermal resistance
CONTROL
shorted together, the internal
CONTROL
CONTROL
CONTROL
LT3085
pin.
CONTROL
is usually
CONTROL
15
can be
3085f
of