STBP112 datasheet(28/38 Pages) STMICROELECTRONICS

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Application information

STBP112

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Doc ID 023357 Rev 3

Application information

8.1

Calculating the power dissipation

The worst case power dissipation of the STBP112 internal power MOSFET can be

calculated using the following formula:

Equation 1

PD = ILOAD

2 x R

DS(on)(max),

where ILOAD is the load current and RDS(on)(max) is the maximum value of MOSFET

resistance.

Example 1

VIN = 5 V, RLOAD = 5 Ω, RDS(on)(max) = 280 mΩ

ILOAD = VIN / (RDS(on)(max) + RLOAD) = 5 / (5 + 0.280) = 0.95 A

PD = 0.95

2 x 0.28 = 0.25 W

The power dissipation of the reverse diode in powering accessories mode can be estimated

as PD = (VOUT - VIN) x IREVERSE ≈ 0.7 x IREVERSE.

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

TJ = TA + 1.15 x PD x RthJA = TA + 1.15 x ILOAD

2 x R

DS(on)(max) x RthJA,

where TJ is junction temperature, TA is given ambient temperature, 1.15 is a derating factor,

and RthJA 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 TA = 85 °C and various RthJA

values are listed in Figure 9 on page 14.

Example 2

For conditions listed in the previous example, with a well designed PCB (ensuring

RthJA = 59 °C/W) and TA = 85 °C, the maximum junction temperature is:

Equation 3

TJ = 85 + 1.15 x 0.25 x 59 = 102 °C,

which is a safe value (below 125 °C).

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