
APPLICATION NOTES
HEAT SINKING
To determine if a heat sink is required for your application
and if so, what type, refer to the thermal model and govern-
ing equation below.
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
Tc = Case Temperature
Ta = Ambient Temperature
Ts = Heat Sink Temperature
EXAMPLE:
This example demonstrates an analysis where the output cur-
rents are at 0.5 amp each and both inputs are 8V.
Conditions for MSK 5911RH:
+Vin = +8.0V; Iout = 0.5A Positive Regulator
1.) Assume 45° heat spreading model.
2.) Find positive regulator power dissipation:
Pd = (Vin - Vout)(Iout)
Pd = (+8V-5V)(0.5A)
Pd = 1.5W
3.) For conservative design, set Tj = +125°C Max.
4.) For this example, worst case Ta = +90°C.
5.) Rθjc = 11.5°C/W from the Electrical Specification Table.
6.) Rθcs = 0.15°C/W for most thermal greases.
7.) Rearrange governing equation to solve for Rθsa:
Rθsa= ((Tj - Ta)/Pd) - (Rθjc) - (Rθcs)
= (125°C-90°C)/1.5W - 11.5°C/W - 0.15°C/W
= 11.7°C/W
The same exercise must be performed for the negative regula-
tor.
CAPACITOR SELECTION
INPUT CAPACITOR:
Input bypass capacitors are recommenced when using the MSK
5911 series regulators. This is especially true if the regulator is
located farther than 6 inches from the power supply filter capaci-
tors. For most applications a 1μF solid tantalum capacitor for
each input will be suitable.
OUTPUT CAPACITOR:
A minimum of a 1μF solid tantalum capacitor should also be
used at both outputs to insure stability. Any increase of this
output capacitor larger than 10μF will only improve output im-
pedance.
+CCOMP CAPACITOR; POSITIVE REGULATOR:
For improved ripple rejection, +Ccomp can be bypassed to
ground with a 10μF tantalum capacitor. This bypass capacitor
will provide 80dB ripple rejection. Increased capacitance above
10μF does not improve the ripple rejection at frequencies above
120Hz. If the Ccomp bypass capacitor is used, it may be neces-
sary to add a protection diode to protect the regulator from ca-
pacitor discharge damage. See Typical Applications Circuit for
clarification. If the bypass capacitor is not used, it should be left
open since it is internally connected to the regulator.
3
-CCOMP CAPACITOR; NEGATIVE REGULATOR:
For improved ripple rejection, -Ccomp can be bypassed to
ground with a 10μF tantalum capacitor. This bypass capacitor
will provide 66dB ripple rejection. Increased capacitance above
10μF does not improve the ripple rejection at frequencies above
120Hz. If the Ccomp bypass capacitor is used, it may be neces-
sary to add a protection diode to protect the regulator from ca-
pacitor discharge damage. See Typical Applications Circuit for
clarification. If the bypass capacitor is not used, it should be left
open since it is internally connected to the regulator.
It is important to keep the output connection between the regu-
lator and the load as short as possible since this directly affects
the load regulation. For example, if 20 gauge wire were used
which has a resistance of about 0.008 ohms per foot, this would
result in a drop of 8mV/ft at 1Amp of load current. It is also
important to follow the capacitor selection guidelines to achieve
best performance.
TYPICAL APPLICATION CIRCUIT
LOAD REGULATION
Rev. D 5/11
Radiation performance curves for TID testing have been gener-
ated for all radiation testing performed by MS Kennedy. These
curves show performance trends throughout the TID test pro-
cess and can be located in the MSK 5911RH radiation test re-
port. The complete radiation test report is available in the RAD
HARD PRODUCTS section on the MSK website.
TOTAL DOSE RADIATION TEST
PERFORMANCE
http://www.mskennedy.com/store.asp?pid=9951&catid=19680
ADDITIONAL APPLICATION INFORMATION
For additional applications information, please reference Linear
Technology Corporation's® LT117, RH117, LT137 and RH137
data sheets.