APPLICATION NOTES
REGULATOR PROTECTION:
The MSK 5176 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.
HEAT SINK SELECTION:
To select a heat sink for the MSK 5176, 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
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 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 5176-3.3 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 = 1.3°C/W and R
θ
cs = 0.15°C/W for most thermal greases
Power Dissipation = (5V - 3.3V) x (1A)
= 1.7 Watts
Solve for R
θ
sa:
R
θ
sa =
125°C - 25°C
- 1.3°C/W - 0.15°C/W
1.7W
= 57.37°C/W
In this example, a heat sink with a thermal resistance of no
more than 57°C/W must be used to maintain a junction tem-
perature of no more than 125°C.
OUTPUT CAPACITOR:
The output voltage ripple of the MSK 5176 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 33µF
is recommended for optimum performance. This capacitor need
not be an expensive low ESR type: aluminum electrolytics are
adequate. In fact, extremely low ESR capacitors may contrib-
ute to instability. Tantalum capacitors are recommended for
systems where fast load transient response is important. Tran-
sient load response can also be improved by placing a capaci-
tor directly across the load.
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.
ENABLE PIN:
The MSK 5176 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 approximately 10µA of quiescent cur-
rent. If the enable function is not used, simply connect the
enable pin to the input.
DEVICE/CASE CONNECTION:
The MSK 5176 series are highly thermally conductive devices
and the thermal path from the package heat sink to the internal
junctions is very short. Since the case is electrically isolated
from the internal circuitry, the package can be directly con-
nected to a heat sink.
3
Rev. A 2/06
MSK [ M.S. KENNEDY CORPORATION ]
