APPLICATION NOTES
MINIMIZING OUTPUT RIPPLE:
The output voltage ripple of the MSK 5010 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 (10uF to 20uF) capacitor directly across the load.
HEAT SINK SELECTION:
To select a heat sink for the MSK 5010, the following formula
for convective heat flow must 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
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 (R
θ
sa).
EXAMPLE;
An MSK 5010-3.3 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 tempera-
ture is 150°C. R
θ
jc = 0.8°C/W and R
θ
cs = 0.15°C/W typi-
cally.
Power Dissipation = (7V - 3.3V) x (10A)
= 37 Watts
Solve for R
θ
sa:
R
θ
sa = 150°C - 25°C - 0.8°C/W - 0.15°C/W
37W
In this example, a heat sink with a thermal resistance of no
more than 2.43°C/W must be used to maintain a junction tem-
perature of no more than 150°C. The Thermalloy Corporation
makes a heat sink with a thermal resistance of 2.2°C/W that
would work well for this application.
(See Thermalloy part number 7023)
CASE CONNECTIONS:
The case of the MSK 5010 is electrically isolated from the
internal circuitry so that a direct connection can be made to
the heat sink for most efficient heat dissipation. However, it
may be necessary in some applications to connect the case
to ground. Grounding the case will help eliminate oscillations
and produce a clean, noise free output.
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 8mV/ft at a load current of
1 amp.
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 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 con-
nected directly to pin 2 of the MSK 5010 and not to a
ground plane inches away from the hybrid.
POWER DISSIPATION:
The output pass transistor in the MSK 5010 is rated to
dissipate nearly 100 watts. The limiting factor of this device
is effective dissipation of heat generated under such condi-
tions. For example, to dissipate 100 watts, calculations
show that the MSK 5010 would have to be bolted to the
underbelly of a submarine submerged in the Arctic Ocean!
Careful consideration must be paid to heat dissipation and
junction temperature when applying this device.
ENABLE/SHUTDOWN PIN:
The MSK 5010 series of voltage regulators are 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 a maximum of 5µA of quiescent
current.
CHARGE CAPACITOR:
For all applications, the user must connect a 1.0uF capacitor
from pin 5 directly to ground. This capacitor is part of the
circuit which drives the gate of the internal MOSFET.
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.
3
Rev. H 10/02
MSK [ M.S. KENNEDY CORPORATION ]
