10
LT1580/LT1580-2.5
If the LT1580 is connected as a single supply device with
the V
CONTROL
and V
POWER
input pins shorted together the
internal diode between the V
OUT
and the V
POWER
input pin
will protect the V
CONTROL
input pin.
Like any other regulator exceeding the maximum input to
output differential can cause the internal transistors to
break down and none of the internal protection circuitry is
then functional.
Thermal Considerations
The LT1580 has internal current and thermal limiting
designed to protect the device under overload conditions.
For continuous normal load conditions maximum junction
temperature ratings must not be exceeded. It is important
to give careful consideration to all sources of thermal
resistance from junction to ambient. This includes junc-
tion-to-case, case-to-heat sink interface and heat sink
resistance itself. Thermal resistance specifications are
given in the electrical characteristics for both the Control
section and the Power section of the device. The thermal
resistance of the Control section is given as 0.65°C/W and
junction temperature of the Control section is allowed to
run at up to 125°C. The thermal resistance of the Power
section is given as 2.7°C/W and the junction temperature
of the Power section is allowed to run at up to 150°C. The
difference in thermal resistances between Control and
Power sections is due to thermal gradients between the
power transistor and the control circuitry.
Virtually all of the power dissipated by the device is
dissipated in the power transistor. The temperature rise in
the power transistor will be greater than the temperature
rise in the Control section so the effective thermal resis-
tance, temperature rise per watt dissipated, will be lower
in the Control section. At power levels below 12W the
temperature gradient will be less than 25°C and the
maximum ambient temperature will be determined by the
junction temperature of the Control section. This is due to
the lower maximum junction temperature in the Control
section. At power levels greater than 12W the temperature
gradient will be greater than 25°C and the maximum
ambient temperature will be determined by the Power
section. For both cases the junction temperature is deter-
mined by the total power dissipated in the device. For most
APPLICATIONS INFORMATION
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A protection diode between the V
OUT
pin and the V
POWER
pin is usually not needed. An internal diode between the
V
OUT
pin and the V
POWER
pin on the LT1580 can handle
microsecond surge currents of 50A to 100A. Even with
large value output capacitors it is difficult to obtain those
values of surge currents in normal operation. Only with
large values of output capacitance, such as 1000µF to
5000µF, and with the V
POWER
pin instantaneously shorted
to ground can damage occur. A crowbar circuit at the
power input can generate those levels of current, and a
diode from output to power input is then recommended.
This is shown in Figure 6. Normal power supply cycling or
system “hot plugging and unplugging” will not do any
damage.
V
OUT
SENSE
ADJ
R2
1580 F06
LT1580
V
POWER
V
POWER
V
CONTROL
V
CONTROL
R1
V
OUT
D1*
D2*
*OPTIONAL DIODES: 1N4002
+
+
+
Figure 6. Optional Clamp Diodes Protect Against
Input Crowbar Circuits
A protection diode between the V
OUT
pin and the V
CONTROL
pin is usually not needed. An internal diode between the
V
OUT
pin and the V
CONTROL
pin on the LT1580 can handle
microsecond surge currents of 1A to 10A. This can only
occur if the V
CONTROL
pin is instantaneously shorted to
ground with a crowbar circuit with large value output
capacitors. Since the V
CONTROL
pin is usually a low current
supply, this condition is unlikely. A protection diode from
the V
OUT
pin to the V
CONTROL
pin is recommended if the
V
CONTROL
pin can be instantaneously shorted to ground.
This is shown in Figure 6. Normal power supply cycling or
system “hot plugging and unplugging” will not do any
damage.
