STBP120 datasheet(12/36 Pages) STMICROELECTRONICS

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Application information

STBP120

12/36

Doc ID 15492 Rev 4

Application information

4.1

Calculating the power dissipation

The maximum power dissipation of the STBP120 internal power MOSFET can be calculated

using following formula:

PD = I

2 x R

DS(on)(max),

Where I is current flowing through the MOSFET and RDS(on)(max) is maximum value of

MOSFET resistance.

Example:

Rload = 5 Ω, VIN = 5 V, RDS(on)(max) = 150 mΩ

I = VIN / (RDS(on)(max) + Rload) = 5 / (5 + 0.150) = 0.97 A

PD = 0.97

2 x 0.15 = 0.14 W

The power dissipation of reverse diode (in powering peripherals mode) can be estimated as

PD = (VOUT - VIN) x I ≈ 0.7 x I.

4.2

Calculating the junction temperature

The maximum junction temperature for given power dissipation, ambient temperature and

thermal resistance junction - to - ambient can be calculated as

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

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 thermal resistance junction - to - ambient, depending on shape, dimension and

design of PCB. Two examples of PCB with appropriate thermal resistance are listed in

Table 3. The junction temperature may not exceed 125 °C (see Table 4), due to TOFF

(thermal shutdown threshold temperature).

Maximum allowed MOSFET current for ambient temperature TA = 85 °C and various RthJA

values are listed in Figure 5.

Example: For conditions listed in previous example, well designed PCB (RthJA = 82 °C/W)

and TA = 85 °C, the maximum junction temperature is

85 + 1.15 x 0.14 x 82 = 98.2 °C.

4.3

PCB layout recommendations

●

This device is intended as a protection device to the application from overvoltage.

It must be ensured that the clearances between PCB tracks satisfy the high voltage

design rules.

●

Input capacitor, C1, should be located as close as possible to the STBP120 device.

It should be a Low-ESR ceramic capacitor. Also the protective resistors RFLT, REN

(if used) should be located close to the STBP120.

●

For good thermal performance, it is recommended to connect the STBP120 exposed

thermal pads with the PCB ground plane. In most designs, this requires thermal vias

between the copper pads on PCB and the ground plane.

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