ST1S40 datasheet(17/29 Pages) STMICROELECTRONICS

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ST1S40

Application information

6.4

Thermal dissipation

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

junction temperature goes above 150 °C. The three different sources of losses within the

device are:

conduction losses due to the ON resistance of high side switch (RHS) and low side

switch (RLS); these are equal to:

Equation 22

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 is actually slightly higher to compensate the losses of

the regulator.

switching losses due to high side Power MOSFET turn ON and OFF; these can be

calculated as:

Equation 23

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 7. 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:

Equation 24

where IQ is the quiescent current (IQ = 2.5 mA maximum).

The junction temperature TJ can be calculated as:

Equation 25

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 VFQFPN and HSOP packages and about 55 °C/W for

the SO8-BW package.

PCOND

RHS IOUT

2 DR

LS IOUT

–





PSW

VIN IOUT

TRISE TFALL



------------------------------------------- Fsw





VIN IOUT TSW FSW





VIN IQ



TA RthJA PTOT



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