LM2576
Thermal Analysis and Design
If the actual operating temperature is greater than the
selected safe operating junction temperature, then a larger
heatsink is required.
The following procedure must be performed to determine
whether or not a heatsink will be required. First determine:
1. P
maximum regulator power dissipation in the
application.
maximum ambient temperature in the
application.
maximum allowed junction temperature
(125°C for the LM2576). 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.
D(max)
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 colour of
the traces.
2. T
)
A(max
J(max)
3. T
The size, quantity and spacing of other components on
the board can also influence its effectiveness to dissipate
the heat.
4. R
5. R
package thermal resistance junction–case.
package thermal resistance junction–ambient.
θJC
θJA
(Refer to Absolute Maximum Ratings on page 2 of this data
sheet or R and R values).
θJC θJA
Figure 26. Inverting Buck–Boost Develops –12 V
The following formula is to calculate the approximate total
power dissipated by the LM2576:
12 to 40 V
Feedback
Unregulated
DC Input
L1
68
+V
4
in
1
P
= (V x I ) + d x I
in
x V
sat
LM2576–12
µH
D
Q
Load
Output
C
µ
in
where d is the duty cycle and for buck converter
2
100
F
D1
1N5822
C
3
Gnd
5
ON/OFF
out
2200
V
V
t
on
T
O
in
µF
d
,
I
(quiescent current) and V
LM2576 data sheet,
is minimum input voltage applied,
is the regulator output voltage,
is the load current.
can be found in the
–12 V @ 0.7 A
Regulated
Output
Q
sat
V
V
I
in
O
Load
ADDITIONAL APPLICATIONS
Inverting Regulator
The dynamic switching losses during turn–on and turn–off
An inverting buck–boost regulator using the LM2576–12 is
shown in Figure 26. This circuit converts a positive input
voltage to a negative output voltage with a common ground
by bootstrapping the regulators ground to the negative output
voltage. By grounding the feedback pin, the regulator senses
the inverted output voltage and regulates it.
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:
T = (R
) (P ) + T
In this example the LM2576–12 is used to generate a
–12 V output. The maximum input voltage in this case
cannot exceed +28 V because the maximum voltage
appearing across the regulator is the absolute sum of the
input and output voltages and this must be limited to a
maximum of 40 V.
This circuit configuration is able to deliver approximately
0.7 A to the output when the input voltage is 12 V or higher. At
lighter loads the minimum input voltage required drops to
approximately 4.7 V, because the buck–boost regulator
topology can produce an output voltage that, in its absolute
value, is either greater or less than the input voltage.
J
θJA
D A
where (R
θJA
)(P ) represents the junction temperature rise
D
caused by the dissipated power and T is the maximum
A
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:
T = P (R
+ R
+ R
) + T
θSA A
J
D
θJA
θCS
where
R
R
R
is the thermal resistance junction–case,
is the thermal resistance case–heatsink,
is the thermal resistance heatsink–ambient.
θJC
θCS
θSA
18
MOTOROLA ANALOG IC DEVICE DATA
MOTOROLA [ MOTOROLA ]
