MSK5142-32 Datasheet, PDF (4/9 Pages) M.S. Kennedy Corporation – Fast Transient Response

APPLICATION NOTES CONT'D
MINIMIZING POWER DISSIPATION:
To maximize the performance and reduce power dissipation
of the MSK5142 series devices, VIN should be maintained
as close to dropout or at VIN minimum when possible. See
Input Supply Voltage requirements. A series resistor can be
used to lower VIN close to the dropout specification, lower-
ing 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 volt-
ages, 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.
HEAT SINK SELECTION
To select a heat sink for the MSK5142, the following for-
mula for convective heat flow may be used.
Governing Equation:
TJ = PD X (RθJC + RθCS + RθSA) + TA
Where
TJ
PD
RθJC
RθCS
RθSA
TA
= Junction Temperature
= Total Power Dissipation
= Junction to Case Thermal Resistance
= Case to Heat Sink Thermal Resistance
= Heat Sink to Ambient Thermal Resistance
= Ambient Temperature
Power Dissipation=(VIN-VOUT) x IOUT
The maximum resistor value can be calculated from the
following:
R1 max = VIN min - (VOUT max + VDROP)
IOUT peak + GND Pin Current
Where:
VIN min=Minimum input voltage
VOUT max=Maximum output voltage across the full
temperature range
VDROP=Worst case dropout voltage (Typically 430mV)
IOUT peak=Maximum load current
GND Pin Current=Max. GND Pin Current at IOUT peak
Next, the user must select a maximum junction tempera-
ture. The absolute maximum allowable junction temperature
is 150°C. The equation may now be rearranged to solve for
the required heat sink to ambient thermal resistance (RθSA).
Example:
An MSK5142 is connected for VIN=+5V and
VOUT=+3.3V. IOUT is a continuous 2A DC level. The am-
bient temperature is +25°C. The maximum desired junc-
tion temperature is +125°C.
RθJC=5.6°C/W and RθCS=0.15°C/W for most thermal
greases
Power Dissipation=(5V-3.3V) x (2A)
=3.4 Watts
Solve for RθSA:
RθSA=
125°C - 25°C
3.4W
- 3.8°C/W - 0.15°C/W
= 25.5°C/W
In this example, a heat sink with a thermal resistance of no
more than 25.5°C/W must be used to maintain a maximum
junction temperature of no more than 125°C.
4
8548-105 Rev. C 8/14