L7987 datasheet(29/37 Pages) STMICROELECTRONICS

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L7987

Application information

The resultant control loop and other transfer functions gain are shown in Figure 14.

Figure 14. Type III compensation - bode plot

5.5

Thermal considerations

The thermal design is important to prevent the thermal shutdown of the device if junction

temperature goes above 170 °C (typ.). The three different sources of losses within the

device are:



Conduction losses due to the non-negligible RDSON of the power switch; these are

equal to:

Equation 29

PHS,ON = RHS,ON • D • (IOUT)2

where D is the duty cycle of the application and the maximum RDSON in the full

temperature range is 380 m

. 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 OFF; these can be calculated

as:

Equation 30

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.

PHS SW



VIN IOUT

TRISE TFALL



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



fSW VIN IOUT TTR fSW







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