MSK5150 Ver la hoja de datos (PDF) - M.S. Kennedy

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MSK5150

VERY HIGH CURRENT, LOW DROPOUT VOLTAGE REGULATORS

M.S. Kennedy 

APPLICATION NOTES

REGULATOR PROTECTION:

The MSK 5150 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.

OUTPUT CAPACITOR:

The output voltage ripple of the MSK 5150 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 20µF

is recommended for optimum performance. This capacitor need

not be an expensive low ESR type: aluminum electrolytics are

adequate. In fact, extremely low ESR capacitors may contrib-

ute to instability. Tantalum capacitors are recommended for

systems where fast load transient response is important. Tran-

sient load response can also be improved by placing a capaci-

tor directly across the load.

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 5150 series requires a mini-

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

ENABLE PIN:

The MSK 5150 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 approximately 10µA of quiescent current.

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

to the input.

FLAG OUTPUT PIN:

All of the fixed output voltage versions of the MSK 5150 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.

HEAT SINK SELECTION:

To select a heat sink for the MSK 5150, the following formula

for convective heat flow may be used.

Governing Equation:

Tj = Pd x (Rθjc + Rθcs + Rθsa) + Ta

WHERE:

Tj = Junction Temperature

Pd = Total Power Dissipation

Rθjc = Junction to Case Thermal Resistance

Rθcs = Case to Heat Sink Thermal Resistance

Rθsa = Heat Sink to Ambient Thermal Resistance

Ta = Ambient Temperature

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

heat sink to ambient thermal resistance (Rθsa).

EXAMPLE:

An MSK 5150-3.3 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.

Rθjc = 1.5°C/W and Rθcs = 0.15°C/W for most thermal greases

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

= 1.7 Watts

Solve for Rθsa:

Rθsa = 125°C - 25°C - 1.5°C/W - 0.15°C/W

1.7W

= 57.17°C/W

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

more than 57°C/W must be used to maintain a junction tem-

perature of no more than 125°C.

MSK5150-00 OUTPUT ADJUSTMENT:

The MSK 5150-00 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 5150 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.

3

Rev. F 1/06

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