MSK5009_15 Datasheet, PDF (3/7 Pages) M.S. Kennedy Corporation – Dual Low Dropout Voltage

APPLICATION NOTES
BYPASS CAPACITORS
For most applications a 33uF 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 oscil-
lations that are normally associated with low dropout regulators.
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.
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 diagram.
MSK5002 TYPICAL APPLICATION:
Low Dropout Positive and Negative Power Supply
OVERLOAD SHUTDOWN
The regulators feature both power and thermal overload protec-
tion. 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 am-
bient temperature, improper heat sinking etc., the regulators also
shutdown until an appropriate junction temperature 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 condition.
HEAT SINKING
To determine if a heat sink is required for your application and if so,
what type, refer to the thermal model and governing 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 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 MSK5002:
VIN = ±7.0V; Iout = ±1.5A
FIGURE 1
Avoiding Ground Loops
1.) Assume 45° heat spreading model.
2.) Find positive regulator power dissipation:
Pd = (VIN - VOUT)(Iout)
Pd = (7-5)(1.5)
= 3.0W
3.) For conservative design, set Tj = +125°C Max.
4.) For this example, worst case Ta = +90°C.
5.) Rθjc = 4.5°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)/3.0W - 4.5°C/W - 0.15°C/W
= 7.0°C/W
The same exercise must be performed for the negative regulator. In
this case the result is 7.0°C/W. Therefore, a heat sink with a thermal
resistance of no more than 7.0°C/W must be used in this application
to maintain both regulator circuit junction temperatures under 125°C.
FIGURE 2
3
8548-81 Rev. K 2/15