L6981 datasheet(38/49 Pages) STMICROELECTRONICS

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Thermal dissipation

The thermal design is important in order to prevent thermal shutdown of the device if junction temperature goes

above 165°C. The three different sources of losses within the device are:

•

Conduction losses due to the ON resistance of the high-side switch (RDSON_HS) and low-side switch

(RDSON_LS); these are equal to:

PCOND=RDSON_HS∙IOUT2∙D+RDSON_LS∙IOUT2∙ 1−D

(43)

Where D is the duty cycle of the selected application and is given by:

D= VOUT+ RDSON_LS+DCRl ∙IOUT

VIN− RDSONHS−RDSON_LS ∙IOUT

(44)

In order to obtain a more accurate estimation it is necessary to keep in mind that the amount of resistance of the

internal power MOSFET increases with the temperature. For this reason, the value of RDSONHS and RDSONLS,

should be increased from the typical of a factor equal to 20%.

•

Switching losses due to high-side Power MOSFET turn ON and OFF; these can be calculated as:

PSW=VIN∙IOUT∙ TRISE+TFALL

2 FSW=VIN∙IOUT∙TSW∙FSW

(45)

Where TRISE and TFALL are the overlap times of the voltage across the high-side power switch (VDS) and

the current flowing into it during turn ON and turn OFF phases, as shown in Figure 45. TSW is the equivalent

switching time. For this device the typical value for the equivalent switching time is 30 ns.

•

Quiescent current losses, calculated as: PQ=VIN∙IQ,MAX

(46)

The quiescent current for constant current operation is equal to 3 [mA]:

The power losses are given by:

PLOSS=PCOND+PSW+PQ

(47)

The junction temperature TJ can be calculated as:

TJ=TA+RtℎJA∙PLOSS

(48)

Where TA is the ambient temperature. 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 section is about 65 °C/W.

L6981

Thermal dissipation

DS13554 - Rev 1

page 38/49

L6981 Datenblatt(PDF) 38 Page - STMicroelectronics