L6983 datasheet(51/63 Pages) STMICROELECTRONICS

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Thermal dissipation

The thermal design is important in order to prevents 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 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

(49)

where D is the duty cycle of the selected application and is given by the following formula:

D= VOUT+ RDSON_LS+DCRl ∙IOUT

VIN− RDSONHS−RDSON_LS ∙IOUT

(50)

In order to obtain a more accurate extimation it is necessary to keep into account that the amount of resistance of

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

RDSONLS, should be increased from the typical of a factor equal to 15%.

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

PSW=VIN∙IOUT∙ TRISE+TFALL

2 FSW=VIN∙IOUT∙TSW∙FSW

(51)

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 57. Switching losses.

TSW is the equivalent switching time. For this device the typical value for the equivalent switching time is 20 ns.

Quiescent current losses, calculated as the equation below:

PQ=VIN∙IQ,MAX

(52)

where IQ is the quiescent current and depends on the VBIAS connections. If VBIAS is connected to GND, the

maximum is equal to 3 mA. Otherwise if VBIAS is connected to VOUT the quiescent current is given by:

IQ,MAX=0.8mA+ 1ηL6983∙VBIASVIN∙2.3mA

(53)

The power losses are given by:

PLOSS=PCOND+PSW+PQ

(54)

The junction temperature TJ can be calculated as:

TJ=TA+RtℎJA∙PLOSS

(55)

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 junctions 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 30 °C/W.

L6983

Thermal dissipation

DS13116 - Rev 1

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