AN2115 datasheet(22/33 Pages) STMICROELECTRONICS

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Application information

AN2115

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Doc ID 11165 Rev 5

6.2.3

Gate charge losses

The gate charge losses derive from switching the gate capacitance of the internal

MOSFETs. The gate capacitances (CH for the high side MOSFETs and CL for the low side

MOSFETs) are charged and discharged with the input voltage at the switching frequency.

Equation 21

These losses are also directly proportional to the switching frequency and input voltage but

are usually negligible compared with the conduction and switching losses.

6.2.4

Thermal consideration

Depending on the application conditions (input voltage, switching frequency, output current)

and ambient temperature, the heat produced by device losses could increase the junction

temperature to over its absolute maximum rating. The following equation can estimate the

junction temperature of the device:

Equation 22

where TA is the ambient temperature of the application, RTH_JA is the thermal resistance

junction to ambient of the package and Ptot is the overall power dissipated by the device.

RTH_JA depends to some degree on the application board but it can considered

approximately equal to 180 °C/W. Ptot given by:

Equation 23

Figure 13.

Thermal performance results: Vin = 3.7 V, Vout = 1.8 V, Iout = 800 mA

The figure above shows the thermal performance of the device mounted on the application

board, which is described in the following paragraph.

GATE

CHANGE

–

()

•

–

tot

•

tot

MOS

SWITCHING

GATE

CHARGE

–

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