STBP110 Datasheet, PDF (30/40 Pages) STMicroelectronics – Overvoltage protection device

Application information
8
Application information
STBP110
8.1
Calculating the power dissipation
The worst case power dissipation of the STBP110 internal power MOSFET can be
calculated using the following formula:
Equation 1
P_D = I_LOAD2 x R_DS(on)(max),
where I_LOAD is the load current and R_DS(on)(max) is the maximum value of MOSFET
resistance.
Example 1
V_IN = 5 V, R_LOAD = 5 Ω, R_DS(on)(max) = 280 mΩ
I_LOAD = V_IN / (R_DS(on)(max) + R_LOAD) = 5 / (5 + 0.280) = 0.95 A
P_D = 0.95² x 0.28 = 0.25 W
The power dissipation of the reverse diode in powering accessories mode can be estimated
as P_D = (V_OUT - V_IN) x I_REVERSE ≈ 0.7 x I_REVERSE.
8.2
Calculating the junction temperature
The maximum junction temperature for given power dissipation, ambient temperature, and
thermal resistance junction-to-ambient can be calculated as:
Equation 2
T_J = T_A + 1.15 x P_D x R_thJA = T_A + 1.15 x I_LOAD2 x R_DS(on)(max) x R_thJA,
where T_J is junction temperature, T_A is given ambient temperature, 1.15 is a derating factor,
and R_thJA is a junction-to-ambient thermal resistance, depending on PCB design. The
junction temperature may not exceed 125 °C (see Table 4) to stay within the specified range.
Maximum allowed MOSFET current for ambient temperature T_A = 85 °C and various R_thJA
values are listed in Figure 9.
Example 2
For conditions listed in the previous example, with a well designed PCB (ensuring
R_thJA = 59 °C/W) and T_A = 85 °C, the maximum junction temperature is:
Equation 3
T_J = 85 + 1.15 x 0.25 x 59 = 102 °C,
which is a safe value (below 125 °C).
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