MSK5231 Datasheet, PDF (3/5 Pages) M.S. Kennedy Corporation – Hermetic Surface Mount Package

APPLICATION NOTES
OVERLOAD SHUTDOWN
The MSK 5231 features both power and thermal over-
load protection. When the maximum power dissipation
is not exceeded, the regulator will current limit slightly
above its 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 related die junction temperature due to exces-
sive ambient temperature, improper heat sinking etc.,
the regulator shuts down until an appropriate junction
temperature is maintained. It should also be noted that
in the case of an extreme overload, such as 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 applica-
tion and if so, what type, refer to the thermal model and
governing equation below.
Governing Equation: Tj=Pd x (Rθjc + Rθcs +Rθsa)+ Ta
In this case the result is 10.0°C/W. Therefore, a heat
sink with a thermal resistance of no more than 10.0°C/
W must be used in this application to maintain regulator
circuit junction temperature under 125°C.
STABILITY
For stable operation in most applications it is recom-
mended that a 10μf tantalum capacitor be used at the
input of the regulator. In addition, if the regulator is not
located very close to the power supply filter capacitors,
a 4.7μf low ESR tantalum capacitor should be added to
the regulator's input. Electrolytic capacitors may also be
used. When substituting an electrolytic in place of a tan-
talum the value should be increased by a factor of ten
over the tantalum value.
The output of the regulator should be bypassed using
a minimum 10μf tantalum capacitor. To ensure good tran-
sient response under heavy load conditions the output
capacitor may be increased without limit. Larger capaci-
tance values will further improve stability and transient
response.
WHERE
Tj = Junction Temperatre
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:
LOAD REGULATION
Load regulation will be limited by the resistance of the
wire connecting the regulator to the load. For example,
if 20 gauge wire were used which has a resistance of
approximately 0.008 ohms per foot, this would result in
a voltage drop of 8mV/ft at 1 amp of load current. Also,
best load regulation will be seen when the adjust resis-
tor, R1, is connected directly to the regulator and not to
the load. In addition, it is important to follow the capaci-
tor selection guidelines for stability and load regulation
performance.
This example demonstrates an analysis where the regu-
lator is at one-half of its maximum rated power dissipa-
tion, which occurs when the output current is at 1.5
amps.
VIN=+7.0V VOUT=+5.0V
IOUT=1.5A
1.) Assume 45° heat spreading model.
2.) Find power dissipation:
OUTPUT VOLTAGE
The MSK 5231 requires two external resistors to set
the output voltage. A 1.25 volt reference voltage is de-
veloped between the output and adjust pins. Resistor,
R1, is placed between these pins with a second resistor,
R2, placed from the adjust pin to ground. A constant
current will flow from the output through these two
resitors to set the output voltage. The series resistance
of R1 & R2 should be selected to provide a minimum
load current of 10mA.
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=1.5°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 - 1.5°C/W - 0.15°C/W
= 10.0°C/W
3
8548-21 Rev. A 2/12