0.5A Step-Down Switching Regulator
TC2574
exception to this is the output (switch) pin, which should not
have large areas of copper in order to minimize coupling to
sensitive circuitry.
IQ (quiescent current) and VSAT can be found in the
TC2574 data sheet,
Additional improvement in heat dissipation can be
achieved even by using of double sided or multilayer boards
which can provide even better heat path to the ambient.
Using a socket for the 8-Pin PDIP (narrow) package is
not recommended because socket represents an additional
thermal resistance, and as a result the junction temperature
will be higher.
Since the current rating of the TC2574 is only 0.5 A, the
totalpackagepowerdissipationforthisswitcherisquitelow,
ranging from approximately 0.1Ω up to 0.75Ω under varying
conditions. In a carefully engineered printed circuit board,
the through–hole PDIP package can easily dissipate up to
0.75Ω, evenatambienttemperaturesof60°C, andstillkeep
the maximum junction temperature below 125°C.
V
IN is minimum input voltage applied,
VO is the regulator output voltage,
ILOAD is the load current.
8.0 to 25 V
Unregulated
+V
Feedback
1
L1
DC Input
IN
5
68
mH
TC2574
(12V)
Output
C
*
IN
22µF
7
D1
MBR150
4
Pwr
GND
2
Sig
GND
3
ON/OFF
C
OUT
680µF
–12 V @ 100mA
Regulated
Output
Thermal Analysis and Design
Figure 8. Inverting Buck-Boost Develops –12V
The following procedure must be performed to deter-
mine the operating junction temperature. First determine:
The dynamic switching losses during turn-on and
turn-offcanbeneglectedifapropertypecatchdiodeisused.
The junction temperature can be determined by the follow-
ing expression:
1. PD(max) – maximum regulator power dissipation in the
application.
TJ = (ΘJA )(PD ) + TA
2. TA(max) – maximum ambient temperature in the
application.
where (ΘJA )(PD ) represents the junction temperature rise
caused by the dissipated power and TA is the maximum
ambient temperature.
3. TJ (max) – maximum allowed junction temperature
(125°C for the TC2574). For a conservative design, the
maximum junction temperature should not exceed 110°C
to assure safe operation. For every additional +10°C tem-
perature rise that the junction must withstand, the estimated
operating lifetime of the component is halved.
Some Aspects That can Influence
Thermal Design
It should be noted that the package thermal resistance
and the junction temperature rise numbers are all approxi-
mate, and there are many factors that will affect these
numbers, suchasPCboardsize, shape, thickness, physical
position,location,boardtemperature,aswellaswhetherthe
surrounding air is moving or still. At higher power levels the
thermal resistance decreases due to the increased air
current activity.
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.
4. ΘJC – package thermal resistance junction–case.
5. ΘJA – package thermal resistance junction–
ambient.
(Refer to Absolute Maximum Ratings on page 2 of this
data sheet or ΘJC and ΘJA values).
The following formula is to calculate the approximate
total power dissipated by the TC2574:
The size, quantity and spacing of other components on
the board can also influence its effectiveness to dissipate
the heat. Some of them, like the catch diode or the inductor
will enerate some additional heat.
PD = (VIN x IQ ) + d x ILOAD x VSAT
where d is the duty cycle and for buck converter
tON VO
d =
=
T
VIN
15
TC2574-1 1/6/00
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