LM2596 Просмотр технического описания (PDF) - ON Semiconductor
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LM2596 Datasheet PDF : 25 Pages
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LM2596
heat dissipation significantly. If further thermal
improvements are needed, double sided or multilayer PC
boards with large copper areas should be considered. In
order to achieve the best thermal performance, it is highly
recommended to use wide copper traces as well as large
areas of copper in the printed circuit board layout. The only
exception to this is the OUTPUT (switch) pin, which should
not have large areas of copper (see page 8 ‘PCB Layout
Guideline’).
Thermal Analysis and Design
The following procedure must be performed to determine
whether or not a heatsink will be required. First determine:
1. PD(max) maximum regulator power dissipation in the
application.
2. TA(max) maximum ambient temperature in the
application.
3. TJ(max) maximum allowed junction temperature
(125°C for the LM2596). For a conservative
design, the maximum junction temperature
should not exceed 110°C to assure safe
operation. For every additional +10°C
temperature rise that the junction must
withstand, the estimated operating lifetime
of the component is halved.
4. RqJC
5. RqJA
package thermal resistance junction−case.
package thermal resistance junction−ambient.
(Refer to Maximum Ratings on page 2 of this data sheet or
RqJC and RqJA values).
The following formula is to calculate the approximate
total power dissipated by the LM2596:
PD = (Vin x IQ) + d x ILoad x Vsat
where d is the duty cycle and for buck converter
d
+
ton
T
+
VO
Vin
,
IQ (quiescent current) and Vsat can be found in the
LM2596 data sheet,
Vin is minimum input voltage applied,
VO is the regulator output voltage,
ILoad is the load current.
The dynamic switching losses during turn−on and
turn−off can be neglected if proper type catch diode is used.
Packages Not on a Heatsink (Free−Standing)
For a free−standing application when no heatsink is used,
the junction temperature can be determined by the following
expression:
TJ = (RqJA) (PD) + TA
where (RqJA)(PD) represents the junction temperature rise
caused by the dissipated power and TA is the maximum
ambient temperature.
Packages on a Heatsink
If the actual operating junction temperature is greater than
the selected safe operating junction temperature determined
in step 3, than a heatsink is required. The junction
temperature will be calculated as follows:
TJ = PD (RqJA + RqCS + RqSA) + TA
where RqJC is the thermal resistance junction−case,
RqCS is the thermal resistance case−heatsink,
RqSA is the thermal resistance heatsink−ambient.
If the actual operating temperature is greater than the
selected safe operating junction temperature, then a larger
heatsink is required.
Some Aspects That can Influence Thermal Design
It should be noted that the package thermal resistance and
the junction temperature rise numbers are all approximate,
and there are many factors that will affect these numbers,
such as PC board size, shape, thickness, physical position,
location, board temperature, as well as whether the
surrounding air is moving or still.
Other factors are trace width, total printed circuit copper
area, copper thickness, single− or double−sided, multilayer
board, the amount of solder on the board or even color of the
traces.
The size, quantity and spacing of other components on the
board can also influence its effectiveness to dissipate the heat.
12 to 40 V
Feedback
R4
Unregulated
DC Input
+Vin
LM2596−ADJ
L1
33 mH
Cin
100 mF/50 V
ON/OFF GND
D1
Cout
1N5822 220 mF
R3
−12 V @ 0.7 A
Regulated
Output
Figure 22. Inverting Buck−Boost Develops −12 V
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