LT1580 데이터 시트보기 (PDF) - Linear Technology
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LT1580 Datasheet PDF : 16 Pages
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LT1580/LT1580 -2.5
APPLICATIONS INFORMATION
A protection diode between the VOUT pin and the VPOWER
pin is usually not needed. An internal diode between the
VOUT pin and the VPOWER 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 VPOWER 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.
VCONTROL
+
VPOWER +
VCONTROL
VPOWER
VOUT
LT1580
SENSE
ADJ
D1* D2*
+
VOUT
R1
*OPTIONAL DIODES: 1N4002
R2
1580 F06
Figure 6. Optional Clamp Diodes Protect Against
Input Crowbar Circuits
A protection diode between the VOUT pin and the VCONTROL
pin is usually not needed. An internal diode between the
VOUT pin and the VCONTROL pin on the LT1580 can handle
microsecond surge currents of 1A to 10A. This can only
occur if the VCONTROL pin is instantaneously shorted to
ground with a crowbar circuit with large value output
capacitors. Since the VCONTROL pin is usually a low current
supply, this condition is unlikely. A protection diode from
the VOUT pin to the VCONTROL pin is recommended if the
VCONTROL 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.
If the LT1580 is connected as a single supply device with
the VCONTROL and VPOWER input pins shorted together the
internal diode between the VOUT and the VPOWER input pin
will protect the VCONTROL 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
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