TC2574 Datasheet, PDF (15/25 Pages) TelCom Semiconductor, Inc – 0.5A Step-Down Switching Regulator

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.
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
total package power dissipation for this switcher is quite low,
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
0.75 Ω, even at ambient temperatures of 60°C, and still keep
the maximum junction temperature below 125°C.
Thermal Analysis and Design
The following procedure must be performed to deter-
mine the operating junction temperature. First determine:
1. P_D(max) – maximum regulator power dissipation in the
application.
2. T_A(max) – maximum ambient temperature in the
application.
3. T_{J (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.
4. Θ_JC – package thermal resistance junction–case.
5. Θ_JA – package thermal resistance junction–
ambient.
(Refer to Absolute Maximum Ratings on page 2 of this
data sheet or Θ_JC and Θ_JA values).
The following formula is to calculate the approximate
total power dissipated by the TC2574:
P_D = (V_IN x I_Q ) + d x I_LOAD x V_SAT
where d is the duty cycle and for buck converter
I_Q (quiescent current) and V_SAT can be found in the
TC2574 data sheet,
V_IN is minimum input voltage applied,
V_O is the regulator output voltage,
I_LOAD is the load current.
8.0 to 25 V
Unregulated
DC Input ^+VIN
C_IN*
5
22µF
4
TC2574
(12V)
Pwr 2 Sig 3
GND GND
Feedback
1
L1
68 mH
Output
7
ON/OFF D1
MBR150
COUT
680µF
–12 V @ 100mA
Regulated
Output
Figure 8. Inverting Buck-Boost Develops –12V
The dynamic switching losses during turn-on and
turn-off can be neglected if a proper type catch diode is used.
The junction temperature can be determined by the follow-
ing expression:
T_J = (Θ_JA )(P_D ) + T_A
where (Θ_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
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, 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.
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. Some of them, like the catch diode or the inductor
will enerate some additional heat.
d=
t_ON
T
=
V_O
V_IN
TC2574-1 1/6/00
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