L7986TA datasheet(28/43 Pages) STMICROELECTRONICS

28 / 43 page

Application information

L7986TA

28/43

Doc ID 022098 Rev 3

where D is the duty cycle of the application and the maximum RDSon overtemperature is 220

Ω. Note that the duty cycle is theoretically given by the ratio between VOUT and VIN, but

actually it is quite higher in order to compensate the losses of the regulator. So the

conduction losses increase compared with the ideal case.

switching losses due to power MOSFET turn-on and turn-off; these can be

calculated as:

Equation 36

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 15.

TSW is the equivalent switching time. For this device the typical value for the equivalent

switching time is 40 ns.

quiescent current losses, calculated as:

Equation 37

where IQ is the quiescent current (IQ=2.4 mA).

The junction temperature TJ can be calculated as:

Equation 38

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 the following

paragraph is about 40 °C/W for the HSOP package.

OUT

RISE

FALL

()

----------------------------------------------

Fsw

⋅⋅

⋅

OUT

⋅⋅

⋅

Rth

TOT

⋅

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