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MSK5900RH Fiches technique(PDF) 4 Page - M.S. Kennedy Corporation

No de pièce MSK5900RH
Description  RAD TOLERANT ULTRA LOW DROPOUT ADJUSTABLE POSITIVE LINEAR REGULATOR
Download  7 Pages
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Fabricant  MSK [M.S. Kennedy Corporation]
Site Internet  http://www.mskennedy.com
Logo MSK - M.S. Kennedy Corporation

MSK5900RH Fiches technique(HTML) 4 Page - M.S. Kennedy Corporation

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APPLICATION NOTES CONT.
4
Rev. F
2/06
HEAT SINK SELECTION
To select a heat sink for the MSK 5900RH, the follow-
ing formula for convective heat flow may be used.
Governing Equation:
TJ = PD X (R
θJC + RθCS + RθSA) + TA
Where
TJ
= Junction Temperature
PD
= Total Power Dissipation
R
θJC
= Junction to Case Thermal Resistance
R
θCS = Case to Heat Sink Thermal Resistance
R
θSA = 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 (R
θSA).
Example:
An MSK 5900RH is connected for VIN=+5V and
VOUT=+3.3V. IOUT is a continuous 1A DC level. The
ambient temperature is +25°C. The maximum desired
junction temperature is +125°C.
R
θJC=7.5°C/W and RθCS=0.15°C/W for most thermal
greases
Power Dissipation=(5V-3.3V) x (1A)
=1.7Watts
Solve for R
θSA:
125°C - 25°C
1.7W
= 51.2°C/W
In this example, a heat sink with a thermal resistance
of no more than 51°C/W must be used to maintain a
junction temperature of no more than 125°C.
THERMAL LIMITING
The MSK 5900RH 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
VOUT=1.265(1+R1/R2)
OUTPUT VOLTAGE SELECTION
As noted in the above typical applications circuit,
the formula for output voltage selection is
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.
Table 1 below lists some of the most probable resistor
combinations based on industry standard usage.
TABLE 1
R1
R2
1+
VOUT=1.265
VOUT
1.265
-1
R1=R2
1.5
1.8
2.0
2.5
2.8
3.3
4.0
5.0
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
R
θSA=
-7.5°C/W - 0.15°C/W
OUTPUT
VOLTAGE
(V)


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