L5987 View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
'L5987' PDF : 40 Pages View PDF
L5987
6.5
Application information
Thermal considerations
The thermal design is important to prevent the thermal shutdown of the device if junction
temperature goes above 150 °C. The three different sources of losses within the device are:
a) Conduction losses due to the not negligible RDS(on) of the power switch; these are
equal to:
Equation 31
PON = RDSON IOUT2 D
Where D is the duty cycle of the application and the maximum RDS(on) overtemperature is
220 m. Note that the duty cycle is theoretically given by the ratio between VOUT and VIN,
but actually it is quite higher to compensate the losses of the regulator. So the conduction
losses increase compared with the ideal case.
b) Switching losses due to Power MOSFET turn ON and OFF; these can be
calculated as:
Equation 32
PSW = VIN IOUT ---T----R----I-S----E----+2-----T----F---A----L---L--- Fsw = VIN IOUT TSW FSW
Where TRISE and TFALL are the overlap times of the voltage across the power switch (VDS)
and the current flowing into it during turn ON and turn OFF phases, as shown in Figure 17.
TSW is the equivalent switching time. For this device the typical value for the equivalent
switching time is 50 ns.
c) Quiescent current losses, calculated as:
Equation 33
PQ = VIN IQ
where IQ is the quiescent current (IQ = 2.4 mA).
The junction temperature TJ can be calculated as:
Equation 34
TJ = TA + RthJA PTOT
Where TA is the ambient temperature and PTOT is the sum of the power losses just seen.
RthJA is the equivalent thermal resistance junction to ambient of the device; it can be
calculated as the parallel of many paths of heat conduction from the junction to the ambient.
For this device the path through the exposed pad is the one conducting the largest amount
of heat. The RthJA measured on the demonstration board described in Section 6.6: Layout
considerations is about 60 °C/W for the VFQFPN package and about 40 °C/W for the HSOP
package.
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