MSK5021(2015) Ver la hoja de datos (PDF) - M.S. Kennedy Corporation

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Fabricante

MSK5021
(Rev.:2015)

HIGH CURRENT, SUPER LOW DROPOUT ADJUSTABLE VOLTAGE REGULATOR

M.S. Kennedy Corporation 

APPLICATION NOTES

MINIMIZING OUTPUT RIPPLE:

The output voltage ripple of the MSK5021 voltage regulator 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 and is best determined by experimentation. Transient

load response can also be improved by placing a 33uF or larger

capacitor directly across the load.

CASE CONNECTIONS:

The case of the MSK5021 is connected to pin seven of the package

but isolated from the internal circuitry allowing direct attachment of

the heat sink to the case. It may be necessary in some applications

to ground the case to limit noise or eliminate oscillations on the out-

put. Pin seven can be left as a no connect if the designer chooses.

LOAD REGULATION:

For best results, the ground pin should be connected directly to

the load (see next note). 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 80mV/ft at a load current of 10 amps.

LOAD CONNECTIONS:

In voltage regulator applications where very large load currents

are present, the load connection is very important. The path con-

necting 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 same holds true for the connection from the low end of the

load to ground. For best load regulation, the low end of the load

must be connected directly to pin 3 of the MSK5021 and not to a

ground plane inches away from the hybrid.

ENABLE/DISABLE PIN:

The MSK5021 voltage regulator is equipped with a TTL compatible

ENABLE pin. A TTL high level on this pin activates 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 10µA

of quiescent current. This pin can be pulled up to VIN if the enable

function is not desired.

FAULT PIN CONNECTIONS:

Pin 6 of the MSK5021 is the Fault pin. When the output voltage

drops 6% or more below its nominal value, the voltage level on the

Fault pin drops to a logic low (typically less than 0.1 volts). This pin

can be used to drive a light emitting diode or other external circuitry

as long as the current is limited to less than 10.0mA (see typical

connection diagram). The Fault pin is an open collector output so

the high state output voltage will be equal to the pull up voltage

since no current flows under these conditions.

CPUMP:

For all applications, the user must connect a 1.0uF capacitor from pin

4 directly to ground. This capacitor is part of the circuit which drives

the gate of the internal MOSFETS. Approximately three times the

voltage seen on the input will appear across this capacitor. Careful

attention must be paid to capacitor voltage rating since voltages

larger than the power supply are present.

HEAT SINK SELECTION:

To select a heat sink for the MSK5021, the following formula for

convective heat flow must 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 175°C. The

equation may now be rearranged to solve for the required heat sink

to ambient thermal resistance (Rθsa).

EXAMPLE;

An MSK5021 is configured for Vin = +7V and Vout = +3.3V. Iout is a

continuous 10A DC level. The ambient temperature is +25°C. The

maximum desired junction temperature is 150°C. Rθjc = 0.6°C/W

and Rθcs = 0.15°C/W typically.

Power Dissipation = (7V - 3.3V) x (10A)

Solve for Rθsa:

= 37 Watts

Rθsa = 150°C - 25°C - 0.6°C/W - 0.15°C/W

37W

= 2.63°C/W

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

than 2.63°C/W must be used to maintain a junction temperature of

no more than 150°C.

POWER DISSIPATION:

The output pass transistors in the MSK5021 are rated to dissipate

nearly 200 watts. The limiting factor of this device is effective dis-

sipation of heat generated under such conditions. Careful consid-

eration must be paid to heat dissipation and junction temperature

when applying this device.

CURRENT LIMIT CONNECTIONS:

To implement current limiting, a sense resistor (Rsc) must be placed

from pin 5 to pins 8 and 9 as shown in the typical connection dia-

gram. When the voltage drop across the sense resistor reaches

35mV, the internal control loop limits the output current only enough

to maintain 35mV across the sense resistor. The device is not

disabled. The following formula may be used to find the correct

value of sense resistance:

RSC=35mV/ILIM

If current limit is not required simply connect the Vsc pins directly

to the input voltage along with the sense pin. Refer to the typical

connection diagram for an illustration.

3

8548-153 Rev. J 5/15

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