AN2620 datasheet(7/20 Pages) STMICROELECTRONICS

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AN2620

Thermal considerations

Doc ID 13883 Rev 3

7/20

Thermal considerations

The dissipated power of the device is related to three different sources:

●

Switch losses due to the non-negligible RDS(on). These are equal to:

Equation 9

Equation 10

where, D is the duty cycle of the application. Note that the duty cycle is theoretically

given by the ratio between Vout and Vin, but in practical terms is quite higher than this

value to compensate the losses of the overall application. Due to this reason, the switch

losses related to the RDS(on) increase compared to the ideal case.

●

Switch losses due to its turn-on and off. These are given by the following relationship:

Equation 11

where Ton and Toff are the overlap times of the voltage across the power switch and the

current flowing into it during the turn-on and turn-off phases. TSW is the equivalent switching

time (typ. 30 ns).

●

Quiescent current losses

Equation 12

where IQ is the quiescent current.

The junction temperature of device is:

Equation 13

where TA is the ambient temperature and RthJ-A is the thermal resistance junction-to-

ambient.

ONP

DS on

()P I

out

⋅⋅

ONN

DS on

()N I

out

–

()

⋅⋅

⋅

Rth

–

TOT

⋅

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