TC2575 Datasheet, PDF (16/25 Pages) TelCom Semiconductor, Inc – 1.0A Step-Down Switching Regulator

1.0A Step-Down Switching Regulator
TC2575
Thermal Analysis and Design
The following procedure must be performed to deter-
mine whether or not a heatsink will be required. 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 TC2575). 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 this data sheet
or Θ_JC and Θ_JA values).
The following formula is to calculate the approximate
total power dissipated by the TC2575:
P_D = (V_IN x I_Q ) + d x I_LOAD x V_SAT
where d is the duty cycle and for buck converter
d=
t_ON
T
=
V_OUT
V_IN
I_Q (quiescent current) and V_SAT can be found in the
TC2575 data sheet,
V_IN is minimum input voltage applied,
V_OUT is the regulator output voltage,
I_LOAD is the load current.
12 to 25V
Unregulated
DC Input
C_IN
100µF
/50V
Feedback
+VIN
1
3
TC2575
(12V)
GND 5
4
L1
100µH
Output
2
ON/OFF
D1
1N5819
COUT
1800µF/
16V
–12V @ 0.35A
Regulated
Output
Figure 8. Inverting Buck-Boost Regulator Using the TC2575 (12V)
Develops –12V @ 0.35A
The dynamic switching losses during turn–on and turn–
off can be neglected if a proper type catch diode is used.
Packages (Free–Standing)
For a free–standing application when no heatsink is
used, the junction temperature can be determined by the
following 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 maxi-
mum 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.
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.
TC2575-1 3/13/00
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