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APPLICATION NOTES
HEAT SINK SELECTION:
To select a heat sink for the MSK 5100, the following formula
for convective heat flow may be used.
Governing Equation:
Tj = Pd x (R
θ
jc + R
θ
cs + R
θ
sa) + Ta
WHERE:
Tj = Junction Temperature
Pd = Total Power Dissipation
R
θ
jc = Junction to Case Thermal Resistance
R
θ
cs = Case to Heat Sink Thermal Resistance
R
θ
sa = Heat Sink to Ambient Thermal Resistance
Ta = Ambient Temperature
REGULATOR PROTECTION:
The MSK 5100 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 5100 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 response
can also be improved by placing a capacitor directly across the
load.
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
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 5100 series requires a mini-
mum of 10mA of load current to stay in regulation.
ENABLE PIN:
The MSK 5100 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.
FLAG OUTPUT PIN:
All of the fixed output voltage versions of the MSK 5100 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,
overtemperature shutdown and output current limit. Extremely
high level input voltage transients will also cause the flag out-
put pin to activate.
MSK5100-00 OUTPUT ADJUSTMENT:
The MSK 5100-00 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.
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 maximum allowable junction temperature is 125°C. The
equation may now be rearranged to solve for the required heat
sink to ambient thermal resistance (R
θ
sa).
EXAMPLE:
An MSK 5100-3.3 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.
R
θ
jc = 2°C/W and R
θ
cs = 0.5°C/W typically.
Power Dissipation = (5V - 3.3V) x (1A)
Solve for R
θ
sa:
R
θ
sa = 125°C - 25°C - 2°C/W - 0.5°C/W
1.7W
= 56.32°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 temperature
of no more than 125°C.
= 1.7 Watts
DEVICE SOLDERING/CASE CONNECTION:
The MSK 5100 series are highly thermally conductive devices
and the thermal path from the package heat sink to the internal
junctions is very short. Standard surface mount techniques
should be used when soldering the device into a circuit board.
A hole can be cut in the printed circuit board to allow the heat
sink of the package to be thermally bonded to an external heat
sink for very high power applications. The external heat sink
needs to be connected to ground because the heat sink and lid
of the MSK 5100 are also electrically connected to ground.
The user is urged to keep this in mind when designing the printed
circuit board for the MSK 5100. There should be no printed
circuit traces making contact with the case or lid of the device
except for ground. The ground plane can be used to pull heat
away from the device.
Rev. D 10/00
3