APPLICATION NOTES

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.

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 the regulator is at

one-half of its maximum rated power dissipation, which occurs

when the output current is at 1.5 amps.

Conditions for MSK 5332-5:

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)

= 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 - 7.0°C/W - 0.15°C/W

=

4.5°C/W

In this case the result is 4.5°C/W. Therefore, a heat sink with a

thermal resistance of no more than 4.5°C/W must be used in

this application to maintain the regulator junction temperature

under 125°C.

OVERLOAD SHUTDOWN

The MSK 5332 features both power and thermal overload

protection. When the maximum power dissipation is not ex-

ceeded, 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 rated die junction tem-

perature due to excessive ambient temperature, improper heat

sinking etc., the regulator will 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.

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 1 for connection dia-

gram.

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

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.

Low Dropout Negative Power Supply

MSK 5332 TYPICAL APPLICATION:

FIGURE 1

3

Rev. B 8/05