APPLICATION NOTES

HEAT SINK SELECTION:

To select a heat sink for the MSK 5115, the following formula

for convective heat flow may be used.

Governing Equation:

WHERE:

Tj = Junction Temperature

Pd = Total Power Dissipation

Ta = Ambient Temperature

REGULATOR PROTECTION:

The MSK 5115 series is fully protected against reversed input

polarity, overcurrent faults, overtemperature conditions (Pd) and

transient voltage spikes of up to 60V. If the regulator is used

in dual supply systems where the load is returned to a negative

supply, the output voltage must be diode clamped to ground.

First, the power dissipation must be calculated as follows:

Power Dissipation = (Vin - Vout) x Iout

Next, the user must select a maximum junction temperature.

The absolute maximum allowable junction temperature is 125°C.

The equation may now be rearranged to solve for the required

EXAMPLE;

An MSK 5115-3.3 is configured for Vin=+5V and

Vout=+3.3V. Iout is a continuous 1A DC level. The ambient

temperature is +25°C. The maximum desired junction tem-

perature is 125°C.

Power Dissipation = (5V - 3.3V) x (1A)

= 1.7 Watts

- 2.6°C/W - 0.5°C/W

In this example, a heat sink with a thermal resistance of no

more than 56°C/W must be used to maintain a junction tem-

perature of no more than 125°C.

MSK5115-00 OUTPUT ADJUSTMENT:

The MSK 5115-00 is an adjustable version in the series of high

performance regulators. The diagram below illustrates proper

adjustment technique for the output voltage. The series resis-

tance of R1+R2 should be selected to pass the minimum regu-

lator output current requirement of 10mA.

DEVICE/CASE CONNECTION:

The MSK 5115 series are highly thermally conductive devices

and the thermal path from the package heat sink to the internal

junctions is very short. Since the case is electrically isolated

from the internal circuitry, the package can be directly con-

nected to a heat sink.

FLAG OUTPUT PIN:

All of the fixed output voltage versions of the MSK 5115 series

are equipped with a flag output pin. Since the flag pin is an

open collector configuration it can be pulled up to any voltage

between 3V and 26V. This feature allows direct interfacing to

practically any logic. This active low output has a typical level

of 0.22V when the flag comparator detects an "out of regula-

tion" condition. Flag states include low input voltage, out of

regulation and output current limit. Extremely high level input

voltage transients will also cause the flag output pin to acti-

vate.

ENABLE PIN

The MSK 5115 series of voltage regulators are equipped with a

TTL compatible ENABLE pin. A TTL high level on this pin acti-

vates the internal bias circuit and powers up the device. A TTL

low level on this pin places the controller in shutdown mode

and the device draws only 5µA of quiescent current. If the

enable function is not used, simply connect the enable pin to

the input.

LOAD CONNECTIONS

In voltage regulator applications where very large load cur-

rents are present, the load connection is very important. The

path connecting the output of the regulator to the load must be

extremely low impedance to avoid affecting the load regulation

specifications. Any impedance in this path will form a voltage

divider with the load. The MSK 5115 series requires a mini-

mum of 10mA of load current to stay in regulation.

OUTPUT CAPACITOR:

The output voltage ripple of the MSK 5115 series voltage regu-

lators can be minimized by placing a filter capacitor from the

output to ground. The optimum value for this capacitor may

vary from one application to the next, but a minimum of 10µF

is recommended for optimum performance. Transient load re-

sponse can also be improved by placing a capacitor directly

across the load.

1.7W

3

Rev. B 7/00

= 55.72°C/W