LM2574-12 Motorola Inc, LM2574-12 Datasheet - Page 18

• Manufacturer Part Number: LM2574-12
• Description: EASY SWITCHERE 0.5 A STEP-DOWN VOLTAGE REGULATOR
• Manufacturer: Motorola Inc

can be neglected if a proper type catch diode is used. The
junction temperature can be determined by the following
expression:
where (R JA )(P D ) represents the junction temperature rise
caused by the dissipated power and T A is the maximum
ambient temperature.
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. At higher power levels the
thermal resistance decreases due to the increased air
current activity.
area, copper thickness, single– or double–sided, multilayer
board, the amount of solder on the board or even color of the
traces.
board can also influence its effectiveness to dissipate the
heat. Some of them, like the catch diode or the inductor will
generate some additional heat.
ADDITIONAL APPLICATIONS
Inverting Regulator
shown in Figure 27. 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.
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.
0.1 A to the output when the input voltage is 8.0 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.
Unregulated
8.0 to 25 V
18
DC Input
The dynamic switching losses during turn–on and turn–off
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 the
An inverting buck–boost regulator using the LM2574–12 is
In this example the LM2574–12 is used to generate a –12 V
This circuit configuration is able to deliver approximately
22 F
Figure 27. Inverting Buck–Boost Develops –12 V
C in
+V in
5
4
Pwr
Gnd
LM2574–12
T J = (R JA )(P D ) + T A
2
Sig
Gnd
3
ON/OFF
1
Output
7
Feedback
D1
MBR150
68 H
L1
–12 V @ 100 mA
Regulated
Output
C out
680 F
LM2574
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 0.6 A.
this inverting regulator topology, the use of a delayed startup
or an undervoltage lockout circuit is recommended.
capacitor can charge up to a higher voltage before the
switch–mode regulator begins to operate.
supplied by the input capacitor C in .
Design Recommendations:
the buck converter and so a different design procedure has to
be used to select the inductor L1 or the output capacitor C out .
normally required for buck converter designs. Low input
voltages or high output currents require a large value output
capacitor (in the range of thousands of F).
inverting converter design is between 68 H and 220 H. To
select an inductor with an appropriate current rating, the
inductor peak current has to be calculated.
current:
conditions, the worst case occurs when V in is minimal.
situations, the delayed startup or the undervoltage lockout
features could be very useful. A delayed startup circuit
Unregulated
12 to 25 V
22 F
DC Input
/50 V
Since the switch currents in this buck–boost configuration
This type of buck–boost inverting regulator can also
Because of the relatively high startup currents required by
While using a delayed startup arrangement, the input
The high input current needed for startup is now partially
The inverting regulator operates in a different manner than
The output capacitor values must be larger than what is
The recommended range of inductor values for the
The following formula is used to obtain the peak inductor
where t on
Under normal continuous inductor current operating
It has been already mentioned above, that in some
C in
Figure 28. Inverting Buck–Boost Regulator with
0.1 F
C1
I
peak
+V in
+
47 k
R1
5
V
3 ON/OFF
MOTOROLA ANALOG IC DEVICE DATA
[
in
|V
I
R2
47 k
)
Load
Delayed Startup
LM2574–12
O
|V
|
O
V
|
4
V
in
x 1.0
in
Pwr
Gnd
)
f osc
|V
2
O
, and f osc = 52 kHz.
Sig
Gnd
|
1
Output
7
Feedback
)
V
D1
MBR150
in
68 H
2L 1
L1
x t on
–12 V @ 100 mA
Regulated
Output
C out
680 F
/16 V