AST1S31HF datasheet(22/33 Pages) STMICROELECTRONICS

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Application information

AST1S31HF

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DocID026968 Rev 2

7.4

Thermal dissipation

The thermal design is important to prevent the 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 the high-side switch (RHS) and low-

side switch (RLS); these are equal to:

Equation 33

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 it is actually slightly higher to compensate the losses of

the regulator.

Switching losses due to the high-side power MOSFET turn-on and turn-off; these

can be calculated as:

Equation 34

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 the turn-on and turn-off phases, as shown

in Figure 9. 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 35

where IQ is the quiescent current (IQ = 1.2 mA maximum).

The junction temperature TJ can be calculated as:

Equation 36

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 (see Figure 12 on page 26) is

about 50 °C/W.

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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