APPLICATION NOTES CONT.
THERMAL LIMITING
The MSK 5921RH 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.
TYPICAL APPLICATIONS CIRCUIT
HEAT SINK SELECTION
To select a heat sink for the MSK 5921RH, the follow-
ing formula for convective heat flow may be used.
Governing Equation:
T
J
=
P
D
X (R
θJC
+ R
θCS
+ R
θSA
) + T
A
Where
T
J
P
D
R
θJC
R
θCS
R
θSA
T
A
=
=
=
=
=
=
Junction Temperature
Total Power Dissipation
Junction to Case Thermal Resistance
Case to Heat Sink Thermal Resistance
Heat Sink to Ambient Thermal Resistance
Ambient Temperature
OUTPUT VOLTAGE SELECTION
As noted in the above typical applications circuit, the
formula for output voltage selection is
R1
V
OUT
=1.265 1+
R2
A good starting point for this output voltage selection is
to set R2=1K. By rearranging the formula it is simple to
calculate the final R1 value.
R1=R2
V
OUT
-1
1.265
Power Dissipation=(V
IN
-V
OUT
) x I
OUT
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 (R
θSA
).
Example:
An MSK 5921RH is connected for V
IN
=+3.3V and
V
OUT
=+2.5V. I
OUT
is a continuous 3A DC level. The am-
bient temperature is +25°C. The maximum desired junc-
tion temperature is +125°C.
R
θJC
=4.0°C/W and R
θCS
=0.15°C/W for most thermal
greases
Power Dissipation=(3.3V-2.5V) x (3A)
=2.4Watts
Solve for R
θSA:
R
θSA
= 125°C - 25°C -4.0°C/W - 0.15°C/W
2.4W
= 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.
Table 1 below lists some of the most probable resistor
combinations based on industry standard usage.
TABLE 1
OUTPUT
VOLTAGE
(V)
1.5
1.8
2.0
2.5
2.8
3.3
4.0
5.0
R2
Ω
(Ω)
1K
1K
1K
1K
1K
1K
1K
1K
R1 (nearest 1%)
(Ω)
Ω
187
422
576
976
1.21K
1.62K
2.15K
2.94K
4
Rev. C
1/06
MSK [ M.S. KENNEDY CORPORATION ]
