DRV101 Datasheet, PDF (13/19 Pages) Burr-Brown (TI) – PWM SOLENOID/VALVE DRIVER

HEAT SINKING
Most applications will not require a heat sink to assure that
the maximum operating junction temperature (125°C) is not
exceeded. However, junction temperature should be kept as
low as possible for increased reliability. Junction tempera-
ture can be determined according to the equation:
T_J = T_A + P_Dθ_JA
(1)
where, θ_JA = θ_JC + θ_CH + θ_HA
(2)
T_J = Junction Temperature (°C)
T_A = Ambient Temperature (°C)
P_D = Power Dissipated (W)
θ_JC = Junction-to-Case Thermal Resistance (°C/W)
θ_CH = Case-to-Heat Sink Thermal Resistance (°C/W)
θ_HA = Heat Sink-to-Ambient Thermal Resistance (°C/W)
θ_JA = Junction-to-Air Thermal Resistance (°C/W)
Figure 12 shows maximum power dissipation versus ambi-
ent 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.
MAXIMUM POWER DISSIPATION
vs AMBIENT TEMPERATURE
10
TO-220 with Thermalloy ^{PD = (TJ (max) – TA) /θ JA}
6030B Heat Sink
T_J (max) = 125°C
8
θ_JA = 16.7°C/W
With infinite heat sink
6
DDPAK
4
(^θ3^JAin=²
26°C/W
one oz
copper mounting pad)
(θ_JA = 3°C/W),
max P_D = 33W
at T_A = 25°C
2
DDPAK or TO-220
θ _JA = 65°C/W (no heat sink)
0
0
25
50
75
100
125
Ambient Temperature (°C)
FIGURE 12. Maximum Power Dissipation vs Ambient
Temperature.
Heat Sink Selection Example
A TO-220 package is dissipating 5 Watts. The maximum
expected ambient temperature is 35°C. Find the proper heat
sink to keep the junction temperature below 125°C.
Combining Equations 1 and 2 gives:
T_J = T_A + P_D(θ_JC + θ_CH + θ_HA)
(3)
T_J, T_A, and P_D are given. θ_JC is provided in the specification
table, 3°C/W. θ_CH 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 θ_CH. A typical θ_CH
for a TO-220 mounted package is 1°C/W. Now we can solve
for θ_HA:
( ) θ_HA
=
T_J – T_A
P_D
–
θ_JC + θ_CH
(4)
θ _HA
=
125° C – 35° C
5W
–
(3° C/ W
+ 1° C/ W)
=
14° C/ W
To maintain junction temperature below 125°C, the heat
sink selected must have a θ_HA less than 14°C/W. In other
words, the heat sink temperature rise above ambient must be
less than 70°C (14°C/W x 5W). For example, at 5 Watts
Thermalloy model number 6030B has a heat sink
temperature rise of 66°C above ambient (θ_HA = 66°C/5W =
13.2°C/W), which is below the 70°C required in this ex-
ample. Figure 12 shows power dissipation versus ambient
temperature for a TO-220 package with a 6030B heat sink.
Another variable to consider is natural convection versus
forced convection air flow. Forced-air cooling by a small fan
can lower θ_CA (θ_CH + θ_HA) dramatically. Heat sink manufac-
turers provide thermal data for both of these cases. For
additional information on determining heat sink require-
ments, consult Application Bulletin AB-038.
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.
The difficulty in selecting the heat sink required lies in
determining the power dissipated by the DRV101. For dc
output into a purely resistive load, power dissipation is simply
the load current times the voltage developed across the
conducting output transistor times the duty cycle. 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.
®
13
DRV101