5921RH Datasheet, PDF (4/7 Pages) M.S. Kennedy Corporation – RAD TOLERANT ULTRA LOW DROPOUT ADJUSTABLE POSITIVE LINEAR REGULATOR

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:
TJ = PD X (RθJC + RθCS + RθSA) + TA
Where
TJ
PD
RθJC
RθCS
RθSA
TA
= Junction Temperature
= Total Power Dissipation
= Junction to Case Thermal Resistance
= Case to Heat Sink Thermal Resistance
= Heat Sink to Ambient Thermal Resistance
= 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 5921RH 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)
=2.4Watts
Solve for RθSA:
RθSA= ^{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.
OUTPUT VOLTAGE SELECTION
As noted in the above typical applications circuit, the
formula for output voltage selection is
VOUT=1.265
1+
R1
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
VOUT
1.265
-1
Table 1 below lists some of the most probable resistor
combinations based on industry standard usage.
OUTPUT
VOLTAGE
(V)
1.5
1.8
2.0
2.5
2.8
3.3
4.0
5.0
TABLE 1
R2
(Ω)
1K
1K
1K
1K
1K
1K
1K
1K
R1 (nearest 1%)
(Ω)
187
422
576
976
1.21K
1.62K
2.15K
2.94K
4
Rev. C 1/06