AST1S31HF Datasheet, PDF (22/33 Pages) STMicroelectronics – Internal soft-start and enable

Application information
AST1S31HF
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:
a) Conduction losses due to the on-resistance of the high-side switch (R_HS) and low-
side switch (R_LS); these are equal to:
Equation 33
P_COND
=
R_HS

I_OU
2
T

D
+
R_LS

I_O
U
2
T

1
–
D
where D is the duty cycle of the application. Note that the duty cycle is theoretically given by
the ratio between V_OUT and V_IN, but it is actually slightly higher to compensate the losses of
the regulator.
b) Switching losses due to the high-side power MOSFET turn-on and turn-off; these
can be calculated as:
Equation 34
P_SW = V_IN  I_OUT  ^---^T----^R----^I-^S----^E----+₂-----^T----^F---^A----^L---^L---^  Fsw = V_IN  I_OUT  T_SW  F_SW
where T_RISE and T_FALL are the overlap times of the voltage across the high-side power
switch (V_DS) and the current flowing into it during the turn-on and turn-off phases, as shown
in Figure 9. T_SW is the equivalent switching time. For this device the typical value for the
equivalent switching time is 20 ns.
c) Quiescent current losses, calculated as:
Equation 35
P_Q = V_IN  I_Q
where I_Q is the quiescent current (I_Q = 1.2 mA maximum).
The junction temperature T_J can be calculated as:
Equation 36
T_J = T_A + Rth_JA  P_TOT
where T_A is the ambient temperature and P_TOT is the sum of the power losses just seen.
Rth_JA 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 Rth_JA measured on the demonstration board (see Figure 12 on page 26) is
about 50 °C/W.
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