5805RHRF Datasheet, PDF (4/11 Pages) M.S. Kennedy Corporation – RAD HARD ULTRA LOW DROPOUT ADJUSTABLE POSITIVE LINEAR REGULATOR

APPLICATION NOTES CONT'D
THERMAL LIMITING
The MSK5805RH control circuitry has a thermal shutdown
temperature of approximately 150°C. This thermal shutdown
can be used as a protection feature, but for continuous op-
eration, the junction temperature of the pass transistor must
be maintained below 150°C. Proper heat sink selection is
essential to maintain these conditions. Exceeding the ther-
mal limit activates the latch feature of the MSK5805RH. See
LATCH pin description for instructions to reset the latch or
disable the latch feature.
TYPICAL APPLICATIONS CIRCUIT
HEAT SINK SELECTION
To select a heat sink for the MSK5805RH, the following
formula for convective heat flow may be used.
Governing Equation:
TJ = PD X (RθJC + RθCS + RθSA) + TA
Where
TJ = Junction Temperature
PD = Total Power Dissipation
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 tempera-
ture. The absolute maximum allowable junction temperature
is 150°C. The equation may now be rearranged to solve for
the required heat sink to ambient thermal resistance (RθSA).
Example:
An MSK5805RH is connected for VIN=+5V and VOUT-
=+3.3V. IOUT is a continuous 0.5A DC level. The ambient
temperature is +25°C. The maximum desired junction tem-
perature is +125°C.
RθJC=7.5°C/W and RθCS=0.15°C/W for most thermal
greases
Power Dissipation=(5V-3.3V) x (0.5A)
=0.85Watts
Solve for RθSA:
RθSA=
125°C - 25°C
0.85W
- 7.5°C/W - 0.15°C/W
= 110°C/W
In this example, a heat sink with a thermal resistance of
no more than 110°C/W must be used to maintain a junction
temperature of no more than 125°C.
VOUT=1.265(1+R1/R2)
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
START UP CURRENT
The MSK5805RH requires less starup current than other
RH1573 based regulators in this series. LDO regulators sink
increased current during startup to bring up the output volt-
age. The MSK5805RH was designed to require less starup
current making it ideal for lower current applications. The
startup current can be further reduced by placing a resistor
(RADJ) between the LADJ pins for lower current applications.
The use of RADJ decreases the saturated start up current and
the current limit of the device. Reference the "Current Limit
vs. RADJ" graph and "Saturated Start Up Current vs. Input
Volatge" graph in the typical performance curves section of
this data sheet. See AN 024 "Understanding Start Up Surge
Current With MS Kennedy's RH1573 based Rad Hard LDO
Regulators" in the application notes section of MSK's web
site for additional information.
 
http://www.anaren.com/msk
TOTAL DOSE RADIATION TEST
PERFORMANCE
Radiation performance curves for TID testing will be gener-
ated for all radiation testing performed by MSK. These curves
show performance trends throughout the TID test process
and can be located in the MSK5805RH radiation test report.
The complete radiation test report will be available in the RAD
HARD PRODUCTS section on the MSK website.
4
8548-111 Rev. F 5/15