APPLICATION NOTES

FIGURE 2

Avoiding Ground Loops

BYPASS CAPACITORS:

For most applications a 47uF, tantalum capacitor should

be attached as close to the regulator's output as possible. This

will effectively lower the regulator's output impedance, improve

transient response and eliminate any oscillations that may be

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 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. If 20 gauge wire were used as an example, 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. Refer

to Figure 2 for connection diagram.

MSK5202 TYPICAL APPLICATION:

Low Dropout Positive and Negative Power Supply

FIGURE 1

3

OVERLOAD SHUTDOWN:

The regulators feature both current and thermal overload

protection. 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 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.

WHERE

Tj = Junction Temperature

Pd = Total Power Dissipation

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. The negative regulator

is worst case due to the larger thermal resistance.

Conditions for MSK5202:

VIN = ±7.0V; Iout = ±1.5A

1.) Assume 45° heat spreading model.

2.) Find regulator power dissipation:

Pd = (VIN - VOUT)(Iout)

Pd = (7-5)(1.5)

Pd = 3.0W

3.) For conservative design, set Tj = +125°C Max.

4.) For this example, worst case Ta = +90°C.

= (125°C - 90°C)/3.0W - (4.7°C/W) - ( 0.15°C/W)

= 6.8°C/W

The same exercise must be performed for the negative regulator.

In this case the result is 6.82°C/W. Therefore, a heat sink with a

thermal resistance of no more than 6.8°C/W must be used in this

application to maintain both regulator circuit junction temperatures

under 125°C.

8548-161 Rev. J 12/16