lm2575-5 ON Semiconductor, lm2575-5 Datasheet - Page 19

• Manufacturer Part Number: lm2575-5
• Description: 1.0 A, Adjustable Output Voltage, Step-down Switching Regulator
• Manufacturer: ON Semiconductor

I
V
V
I
turn−off can be neglected if proper type catch diode is used.
Packages Not on a Heatsink (Free−Standing)
the junction temperature can be determined by the following
expression:
where (R
caused by the dissipated power and T
ambient temperature.
Packages on a Heatsink
the selected safe operating junction temperature determined
in step 3, than a heatsink is required. The junction
temperature will be calculated as follows:
where
selected safe operating junction temperature, then a larger
heatsink is required.
Some Aspects That can Influence Thermal Design
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.
area, copper thickness, single− or double−sided, multilayer
board, the amount of solder on the board or even color of the
traces.
the board can also influence its effectiveness to dissipate
the heat.
Q
Load
in
O
The dynamic switching losses during turn−on and
For a free−standing application when no heatsink is used,
If the actual operating junction temperature is greater than
If the actual operating temperature is greater than the
It should be noted that the package thermal resistance and
Other factors are trace width, total printed circuit copper
The size, quantity and spacing of other components on
(quiescent current) and V
LM2575 data sheet,
is minimum input voltage applied,
is the regulator output voltage,
is the load current.
R
R
R
qJA
qJC
qCS
qSA
T
)(P
J
is the thermal resistance junction−case,
= P
is the thermal resistance case−heatsink,
is the thermal resistance heatsink−ambient.
D
) represents the junction temperature rise
D
T
J
(R
d +
= (R
qJA
t on
qJA
+ R
T
) (P
+
qCS
sat
V
V in
D
+ R
) + T
can be found in the
O
,
qSA
A
A
) + T
is the maximum
A
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19
ADDITIONAL APPLICATIONS
Inverting Regulator
is shown in Figure 25. 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.
−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.
0.35 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.
are higher than in the standard buck converter topology, the
available output current is lower.
require a larger amount of startup input current, even for
light loads. This may overload an input power source with
a current limit less than 1.5 A.
least 2.0 ms or more. The actual time depends on the output
voltage and size of the output capacitor.
this inverting regulator topology, the use of a delayed startup
or an undervoltage lockout circuit is recommended.
Unregulated
DC Input
12 V to 25 V
Figure 25. Inverting Buck−Boost Regulator Using the
An inverting buck−boost regulator using the LM2575−12
In this example the LM2575−12 is used to generate a
This circuit configuration is able to deliver approximately
Since the switch currents in this buck−boost configuration
This type of buck−boost inverting regulator can also
Such an amount of input startup current is needed for at
Because of the relatively high startup currents required by
100 mF
/50 V
C
in
LM2575−12 Develops −12 V @ 0.35 A
+V
in
1
3
LM2575−12
GND
5
ON/OFF
4
Output
2
Feedback
D1
1N5819
100 mH
L1
-12 V @ 0.35 A
Regulated
Output
C
1800 mF
/16 V
out