APPLICATION NOTES
BYPASS CAPACITORS
For most applications a 10uF minimum, low ESR (0.5-2 ohm)
tantalum capacitor should be attached as close to the regulator's
output as possible. This will effectively lower the regulator's
output impedance, increase transient response and eliminate any
oscillations that are normally associated with low dropout regu-
lators. Additional bypass capacitors can be used at the remote
load locations to further improve regulation. These can be either
of the tantalum or the electrolytic variety. Unless the regulator
is located very close to the power supply filter capacitor(s), a
4.7uF minimum low ESR (0.5-2 ohm) tantalum capacitor should
also be added to the regulator's input. An electrolytic may also
be substituted if desired. When substituting electrolytic in place
of tantalum capacitors, a good rule of thumb to follow is to
increase the size of the electrolytic by a factor of 10 over the
tantalum value.
OVERLOAD SHUTDOWN
The regulators feature both power and thermal overload pro-
tection. When the maximum power dissipation is not exceeded,
the regulators will current limit slightly above their 3 amp rating.
As the Vin-Vout voltage increases, however, shutdown occurs in
relation to the maximum power dissipation curve. If the device
heats enough to exceed its rated die junction temperature due to
excessive ambient temperature, improper heat sinking etc., the
regulators also shutdown until an appropriate junction tempera-
ture is maintained. It should also be noted that in the case of an
extreme overload, such as a sustained direct short, the device
may not be able to recover. In these instances, the device must
be shut off and power reapplied to eliminate the shutdown con-
dition.
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 each regulator
is at one-half of its maximum rated power dissipation, which
occurs when the output currents are at 1.5 amps each.
Conditions for MSK 5002:
Vin = ±7.0V; Iout = ±1.5A
1.) Assume 45° heat spreading model.
2.) Find positive regulator power dissipation:
Pd = (Vin - Vout)(Iout)
Pd = (7-5)(1.5)
= 3.0W
3.)
4.)
5.)
6.)
7.)
For conservative design, set Tj = +125°C Max.
For this example, worst case Ta = +90°C.
R
θ
jc = 2.5°C/W from the Electrical Specification Table.
R
θ
cs = 0.15°C/W for most thermal greases.
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)/3.0W - (2.5°C/W - 0.15°C/W)
= 9.32°C/W
The same exercise must be performed for the negative regula-
tor. In this case the result is 9.32°C/W. Therefore, a heat sink
with a thermal resistance of no more than 9.3°C/W must be
used in this application to maintain both regulator circuit junc-
tion temperatures under 125°C.
Rev.E 7/00
LOAD REGULATION
For best results the ground pin should be connected directly to
the load as shown below, this effectively reduces the ground
loop effect and eliminates excessive voltage drop in the sense
leg. It is also important to keep the output connection between
the regulator 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 .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. Refer to Figure 2 for connection dia-
gram.
MSK 5002 TYPICAL APPLICATION:
Low Dropout Positive and Negative Power Supply
FIGURE 1
Avoiding Ground Loops
FIGURE 2
3
MSK [ M.S. KENNEDY CORPORATION ]
