AN4501 datasheet(32/48 Pages) STMICROELECTRONICS

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Selection of the switching MOSFET

AN4501

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DocID026394 Rev 1

Selection of the switching MOSFET

The MOSFET M1 is the main switching element of the boost converter. The most important

parameters for its selection are:

•

the breakdown voltage

•

the peak and the RMS currents

•

the RDSON

•

the thermal resistance RTH-J-AMB

•

the turn-on and the turn-off time

Note:

The gate driver of the L99LD01 controls M1 with a typ. voltage, VG1, of ~ 10V, provided that

the supply voltage is high enough (10 V + Dropout of the VCC2 internal regulator ~ 10.2 V). If

VIN is below ~10.2 V, VCC2 will be ~VIN – 0.2 V and so does VG1. Therefore, a logic level

MOSFET is required to keep the converter’s performance at VIN below ~ 10.2 V.

8.1

Breakdown voltage

During tOFF the drain-source voltage of the MOSFET M1 is equal to VOUT + VF,DIODE. Some

margin must be added in case of ringing at the switching node.

8.2

MOSFET peak current

During tON, the MOSFET M1 is turned on and the inductor current flows into M1 (see

Figure 4 and Figure 7). Therefore, the MOSFET maximum peak current is equal to the

inductor maximum peak current:

8.3

MOSFET power dissipation

The MOSFET’s power dissipation mainly comes from the the conduction losses and the

switching losses.

M1 PEAK MAX

L PEAK M AX

OUT

MAX

–

-------------------------

OU T

MAX

–

()

×η

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