5020 查看數據表（PDF） - M.S. Kennedy

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5020

HIGH CURRENT, SUPER LOW DROPOUT FIXED VOLTAGE REGULATORS

M.S. Kennedy 

APPLICATION NOTES

MINIMIZING OUTPUT RIPPLE:

The output voltage ripple of the MSK 5020 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 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 MSK 5020 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 output. 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

connecting the output of the regulator to the load must be ex-

tremely 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 MSK 5020 and not to a ground plane inches away from the

hybrid.

FILTER CAPACITOR:

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

from pin 4 directly to ground. This capacitor is part of the cir-

cuit which drives the gate of the internal MOSFETS. Approxi-

mately three times the voltage seen on the input will appear

across this capacitor. Careful attention must be paid to capaci-

tor voltage rating since voltages larger than the power supply

are present.

HEAT SINK SELECTION:

To select a heat sink for the MSK 5020, 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 MSK 5020-3.3 is configured for Vin = +7V and Vout =

+3.3V. Iout is a continuous 10A DC level. The ambient tem-

perature is +25°C. The maximum desired junction temperature

is 150°C. Rθjc = 0.5°C/W and Rθcs = 0.15°C/W typically.

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

= 37 Watts

Solve for Rθsa:

Rθsa = 150°C - 25°C - 0.5°C/W - 0.15°C/W

37W

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

than 2.73°C/W must be used to maintain a junction tempera-

ture of no more than 150°C.

ENABLE/DISABLE PIN:

The MSK 5020 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 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 MSK 5020 series 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.

POWER DISSIPATION:

The output pass transistors in the MSK 5020 are rated to dissi-

pate nearly 200 watts. The limiting factor of this device is ef-

fective dissipation of heat generated under such conditions. For

example, to dissipate 200 watts, calculations show that the MSK

5020 would have to be bolted to the underbelly of a submarine

submerged in the Artic Ocean! Careful consideration 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 8,9 and 10 as shown in the typical

connection diagram. When the voltage drop across the sense

resistor reaches 35mV, the internal control loop limits the out-

put 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 di-

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

typical connection diagram for an illustration.

3

Rev.A 8/98

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