APPLICATION NOTES CONT.
THERMAL LIMITING
TYPICAL APPLICATIONS CIRCUIT
The MSK 5920RH control circuitry has a thermal shut-
down temperature of approximately 150°C. This ther-
mal shutdown can be used as a protection feature, but
for continuous operation, the junction temperature of the
pass transistor must be maintained below 150°C. Proper
heat sink selection is essential to maintain these condi-
tions.
HEAT SINK SELECTION
To select a heat sink for the MSK 5920RH, the follow-
ing formula for convective heat flow may be used.
Governing Equation:
TJ = PD X (RθJC + RθCS + RθSA) + TA
Where
TJ
PD
= Junction Temperature
= Total Power Dissipation
FIGURE 2
RθJC = Junction to Case Thermal Resistance
RθCS = Case to Heat Sink Thermal Resistance
RθSA = Heat Sink to Ambient Thermal Resistance
TA
= Ambient Temperature
Power Dissipation=(VIN-VOUT) x IOUT
Next, the user must select a maximum junction tem-
perature. The absolute maximum allowable junction tem-
perature is 150°C. The equation may now be rearranged
to solve for the required heat sink to ambient thermal
resistance (RθSA).
Example:
An MSK 5920-2.5RH is connected for VIN=+3.3V and
VOUT=+2.5V. IOUT is a continuous 3A DC level. The am-
bient temperature is +25°C. The maximum desired junc-
tion temperature is +125°C.
RθJC=4.0°C/W and RθCS=0.15°C/W for most thermal
greases
Power Dissipation=(3.3V-2.5V) x (3A)
=2.4 Watts
Solve for RθSA:
125°C - 25°C
2.4W
RθSA=
-4.0°C/W - 0.15°C/W
= 36.5°C/W
In this example, a heat sink with a thermal resistance
of no more than 36.5°C/W must be used to maintain a
junction temperature of no more than 125°C.
Rev. G 1/06
4
MSK [ M.S. KENNEDY CORPORATION ]
