APPLICATION NOTES

3

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

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.

WHERE

Tj = Junction Temperatre

Pd = Total Power Dissipation

Tc = Case Temperature

Ta = Ambient Temperature

Ts = Heat Sink Temperature

EXAMPLE:

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.

1.) Assume 45° heat spreading model.

2.) Find power dissipation:

Pd=(7-5) (1.5)

=3.0W

= (125°C - 90°C)/3.0W - 1.5°C/W - 0.15°C/W

= 10.0°C/W

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.

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.

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.

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.

8548-21 Rev. A 2/12