AN4501 datasheet(12/48 Pages) STMICROELECTRONICS

12 / 48 page

Guideline for the selection of external components

AN4501

12/48

DocID026394 Rev 1

The additional input voltage ripple caused by the ESR is:

which is indeed negligible compared to 70 mV. The sum of both contributions is below the

maximum target of 100 mV (

ΔVIN,MAX).

Step 6: Selection of the switching MOSFET

The minimum required breakdown voltage is the same as the breakdown voltage of the

diode.

A 60 V, 16 m

Ω max @ 25°C is considered.

Estimation of the switching losses

The turn-on and turn-off times of the considered MOSFET with is estimated to 20 ns.

The maximum switching losses are given by:

Calculation of the max. RMS current

The MOSFET’s maximum RMS current is estimated by:

Iterative calculation of the total power losses and of the maximum junction

temperature

Considering at maximum ambient temperature of 85°C, a thermal coefficient of the RDSON

of 0.006 K-1, 20 ns of rise and fall times and a thermal resistance of 25 K/W, an iterative

calculation leads to a junction temperature of 125°C and a RDSON of 25.6 mΩ with a

precision of 0.2 m

Ω . Refer to Section 8.3.2 for the calculation details.

This confirms that the MOSFET can operate in the worst case conditions, without exceeding

the maximum rating of the junction temperature (175°C in general).

ESR

CI N

L,PP,MAX

210

–

2.36

⋅

4.8mV

M1,SW

OUT

LM AX

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

M1,SWON

M1,SWOFF

()

M1,SW

4.81

450 10

⋅

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

20 10

–

⋅

20 10

–

⋅

()

1.13W

M1,RMS,MAX

OU T

MAX

–

------------------------- D

MAX

------

L,PP,VINMIN

L,MAX

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





 2







M1,RMS,MAX

1.0

0.791

–

------------------------ 0.791

------

1.55

4.81

-----------



 2





4.30A

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