MSK5021 Datasheet, PDF (3/6 Pages) M.S. Kennedy Corporation – HIGH CURRENT, SUPER LOW DROPOUT ADJUSTABLE VOLTAGE REGULATOR

APPLICATION NOTES
MINIMIZING OUTPUT RIPPLE:
The output voltage ripple of the MSK 5021 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 MSK 5021 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 elimi-
nate 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 be-
tween 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 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 same holds true for the connection
from the low end of the load to ground. For best load regula-
tion, the low end of the load must be connected directly to pin
3 of the MSK 5021 and not to a ground plane inches away
from the hybrid.
ENABLE/DISABLE PIN:
The MSK 5021 voltage regulator is equipped with a TTL com-
patible 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 MSK 5021 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. Ap-
proximately three times the voltage seen on the input will ap-
pear 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 MSK 5021, 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 5021 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)
= 37 Watts
Solve for Rθsa:
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 tem-
perature of no more than 150°C.
POWER DISSIPATION:
The output pass transistors in the MSK 5021 are rated to dissi-
pate nearly 200 watts. The limiting factor of this device is
effective dissipation of heat generated under such conditions.
For example, to dissipate 200 watts, calculations show that
the MSK 5021 would have to be bolted to the underbelly of a
submarine submerged in the Arctic Ocean! Careful consider-
ation 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 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.
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Rev. F 1/05