APPLICATION NOTES
REGULATOR PROTECTION:
The MSK 5251 series are high performance linear regulators for high
current, low voltage applications requiring fast transient response.
The devices are fully protected from damage due to fault conditions,
offering constant current limiting and thermal shutdown. The thermal
shutdown junction temperature is typically 140°C and is 100% tested
to verify thermal shutdown occurs above 130°C.
THERMAL SHUTDOWN:
The MSK 5251 series of devices is equipped with a thermal shut-
down circuit that will turn off the device when the junction tempera-
ture reaches approximately 140°C. It is important for the user to be
aware that high temperature operation will limit the current capabil-
ity of the device due to this thermal shutdown protection. In cases of
maximum input voltage and high case temperature, the output cur-
rent available may be less than 3 Amps. See curve below for clarifi-
cation.
INPUT SUPPLY VOLTAGE:
The input voltage must be maintained at a minimum of 3.0 volts for
proper operation for devices with output voltage below 1.0 volt.
With an output voltage of 1.0 volt or higher, the input voltage must
be a minimum of 2.1 volts above the output.
MINIMIZING POWER DISSIPATION:
To maximize the performance and reduce power dissipation of the
MSK 5251 series devices, Vin should be maintained as close to
dropout as possible. See Input Supply Voltage requirements. A series
resistor can be used to lower Vin close to the dropout specification,
lowering the input to output voltage differential. In turn, this will
decrease the power that the device is required to dissipate. Knowing
peak current requirements and worst case voltages, a resistor can be
selected that will drop a portion of the excess voltage and help to
distribute the heating. The circuit below illustrates this method.
PACKAGE CONNECTIONS:
The MSK 5251 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 soldering techniques should be
used when mounting the device. Some applications may require
additional heat sinking of the device.
HEAT SINK SELECTION:
The maximum resistor value can be calculated from the following:
R1 max =
Vin min - (Vout max + Vdrop)
Iout peak + Quiescent Current
To select a heat sink for the MSK 5251, the following for-
mula for convective heat flow may be used:
First, the power dissipation must be calculated as follows:
Power Dissipation = (Vin - Vout) x Iout + Vin x Quiescent Current
Next, the user must select a maximum junction tempera-
ture. The equation may now be arranged to solve for the
required heat sink to ambient thermal resistance (Rθsa).
EXAMPLE:
An MSK 5251-1.3 is configured for Vin=+3.4V and Vout=+1.3V.
Iout is a continuous 5Amp DC level. Under these conditions the
maximum quiescent current would be 120mA. The ambient tempera-
ture is +25°C and the maximum junction temperature is 125°C.
Rθjc = 2.0°C/W and Rθcs = 0.5°C/W typically.
Power Dissipation = (3.4V - 1.3V) x (5A + 3.4 x 120mA)
Solve for Rθsa:
= 10.9 Watts
Where: Vin min = Minimum input voltage
Vout max = Maximum output voltage across the
full temperature range
Vdrop = Worst case dropout voltage (Typically
2.1 Volts)
Iout peak = Maximum load current
Quiescent Current = Max. quiescent current at Iout peak
INPUT CAPACITOR:
If the device is to be located more than 4 inches from the bulk supply
capacitance, a minimum 1uF capacitor should be placed as close to
the input pin as possible for proper bypassing. A smaller value ca-
pacitor such as 0.01uF should be placed in parallel with the larger
value capacitor. Larger input capacitor values will help to improve
ripple rejection.
OUTPUT CAPACITOR:
The MSK 5251 series devices require a minimum of external compo-
nents to maintain stability. A minimum of output capacitance is nec-
essary for stable operation. Due to the wide bandwidth design, the
device will operate with a wide range of capacitance and ESR val-
ues. For most applications, a 10uF ceramic capacitor will suffice.
Ideally, this should be an X7R ceramic capacitor or a tantalum ca-
pacitor due to their thermal performance. There is no upper limit to
the amount of output capacitance that may be used.
3
Rθsa = 125°C - 25°C - 2.0°C/W - 0.5°C/W
1.7W
= 6.67°C/W
In this example, a heat sink with a thermal resistance of no
more than 6.6°C/W must be used to maintain a junction
temperature of no more than 125°C.
Rev. D
2/07
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MSK [ M.S. KENNEDY CORPORATION ]
