APPLICATION NOTES
CAPACITOR SELECTION
POSITIVE REGULATOR
INPUT CAPACITOR:
An input bypass capacitor is recommenced when using the
MSK 5901 series regulators. This is especially true if the regu-
lator is located farther than 6 inches from the power supply filter
capacitors. For most applications a 1µF solid tantalum capacitor
will be suitable.
TYPICAL APPLICATION CIRCUIT
OUTPUT CAPACITOR:
A minimum of a 1µF solid tantalum capacitor should also be
used at the output to insure stability. Any increase of this out-
put capacitor larger than 10µF will only improve output imped-
ance.
LOAD REGULATION
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.
+C
COMP
CAPACITOR:
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.
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 (R
θ
jc + R
θ
cs + R
θ
sa) + Ta
WHERE
Tj = Junction Temperature
Pd = Total Power Dissipation
R
θ
jc = Junction to Case Thermal Resistance
R
θ
cs = Case to Heat Sink Thermal Resistance
R
θ
sa = 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 5901RH:
+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.) R
θ
jc = 10.5°C/W from the Electrical Specification Table.
6.) R
θ
cs = 0.15°C/W for most thermal greases.
7.) Rearrange governing equation to solve for R
θ
sa:
R
θ
sa= ((Tj - Ta)/Pd) - (R
θ
jc) - (R
θ
cs)
= (125°C-90°C)/1.5W - 10.5°C/W - 0.15°C/W
= 12.7°C/W
The same exercise must be performed for the negative regula-
tor.
NEGATIVE REGULATOR
INPUT CAPACITOR:
Once again, if the regulator will be farther than 6 inches from
power supply filter capacitors, then an input capacitor will be
required on the negative regulator. It is recommended that a
1µF solid tantalum capacitor be used.
OUTPUT CAPACITOR:
A minimum of a 1µF solid tantalum capacitor should also be
used at the output to insure stability. Any increase of this out-
put capacitor larger than 10µF will only improve output imped-
ance.
-C
COMP
CAPACITOR:
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
Rev. D 7/05
MSK [ M.S. KENNEDY CORPORATION ]
