HEAT SINKING
Combining equations (1) and (2) gives:
Most applications require a heat sink to assure that the
maximum operating junction temperature (125°C) is not
exceeded. In addition, the junction temperature should be
kept as low as possible for increased reliability. Junction
temperature can be determined according to the equation:
TJ = TA + PD(θJC + θCH + θHA
)
(3)
TJ, TA, and PD are given. θJC is provided in the specification
table, 2.5°C/W (dc). θCH can be obtained from the heat sink
manufacturer. Its value depends on heat sink size, area, and
material used. Semiconductor package type, mounting screw
torque, insulating material used (if any), and thermal
joint compound used (if any) also affect θCH. A typical θCH
for a TO-220 mounted package is 1°C/W. Now we can solve
TJ = TA + PDθJA
(1)
(2)
where, θJA = θJC + θCH + θHA
TJ = Junction Temperature (°C)
TA = Ambient Temperature (°C)
for θHA
:
PD = Power Dissipated (W)
θJC = Junction-to-Case Thermal Resistance (°C/W)
θCH = Case-to-Heat Sink Thermal Resistance (°C/W)
TJ – TA
PD
θHA
θHA
=
=
– θ + θCH
JC
(
)
125°C – 40°C
θHA
= Heat Sink-to-Ambient Thermal Resistance (°C/W)
– 2.5°C/W +1°C/W = 13.5°C/W
(
)
5W
θJA = Junction-to-Air Thermal Resistance (°C/W)
Figure 7 shows maximum power dissipation versus ambient
temperature with and without the use of a heat sink. Using
a heat sink significantly increases the maximum power
dissipation at a given ambient temperature as shown.
To maintain junction temperature below 125°C, the heat
sink selected must have a θHA less than 14°C/W. In other
words, the heat sink temperature rise above ambient must be
less than 67.5°C (13.5°C/W x 5W). For example, at 5 Watts
Thermalloy model number 6030B has a heat sink
temperature rise of 66°C above ambient (θHA = 66°C/5W =
13.2°C/W), which is below the 67.5°C required in this
example. Figure 7 shows power dissipation versus ambient
temperature for a TO-220 package with a 6030B heat sink.
The difficulty in selecting the heat sink required lies in
determining the power dissipated by the OPA548. For dc
output into a purely resistive load, power dissipation is
simply the load current times the voltage developed across
the conducting output transistor, PD = IL(Vs–VO). Other
loads are not as simple. Consult Application Bulletin AB-
039 for further insight on calculating power dissipation.
Once power dissipation for an application is known, the
proper heat sink can be selected.
Another variable to consider is natural convection vs forced
convection air flow. Forced-air cooling by a small fan can
lower θCA (θCH + θHA) dramatically. Heat sink manufactures
provide thermal data for both of these cases. For additional
information on determining heat sink requirements, consult
Application Bulletin AB-038.
MAXIMUM POWER DISSIPATION
vs AMBIENT TEMPERATURE
10
As mentioned earlier, once a heat sink has been selected the
complete design should be tested under worst-case load and
signal conditions to ensure proper thermal protection.
PD = (TJ (max) – TA) /θJA
TO-220 with Thermalloy
6030B Heat Sink
TJ (max) = 150°C
8
6
4
2
0
θ
= 16.7°C/W
JA
With infinite heat sink
ENABLE/STATUS (E/S) PIN
(
θJA = 2.5°C/W),
max PD = 50W at TA = 25°C.
The Enable/Status Pin provides two functions: forcing this
pin low disables the output stage, or, E/S can be monitored
to determine if the OPA548 is in thermal shutdown. One or
both of these functions can be utilized on the same device
using single or dual supplies. For normal operation (output
enabled), the E/S pin can be left open or pulled high (at least
2.4V above the negative rail). A small value capacitor
connected between the E/S pin and V– may be required for
noisy applications.
DDPAK
= 26°C/W
(3 in one oz
θ
JA
2
copper mounting pad)
DDPAK or TO-220
= 65°C/W (no heat sink)
θ
JA
0
25
50
75
100
125
Ambient Temperature (°C)
FIGURE 7. Maximum Power Dissipation vs Ambient
Temperature.
Output Disable
A unique feature of the OPA548 is its output disable capa-
bility. This function not only conserves power during idle
periods (quiescent current drops to approximately 6mA) but
also allows multiplexing in low frequency (f<20kHz), mul-
tichannel applications. Signals greater than 20kHz may
cause leakage current to increase in devices that are shut-
down. Figure 18 shows the two OPA548s in a switched
amplifier configuration. The on/off state of the two amplifi-
ers is controlled by the voltage on the E/S pin.
Heat Sink Selection Example
A TO-220 package is dissipating 5 Watts. The maximum
expected ambient temperature is 40°C. Find the proper heat
sink to keep the junction temperature below 125°C (150°C
minus 25°C safety margin).
®
11
OPA548
BB [ BURR-BROWN CORPORATION ]
