AN1882

Application information

Doc ID 10209 Rev 4

19/30

Equation 18

ESR of the input and output capacitors are usually negligible, particularly when using

ceramic caps (very low ESR). Anyway, in case of high ESR values for these caps, their

conduction losses are:

Equation 19

where

ΔI is the current ripple flowing through the choke and D is the duty cycle of the

application. The conduction losses are particularly important at high current cause they

depends on its squared value.

5.2.2

Switching losses

The switching losses are due to the turn on and off of the internal high side MOSFET.

Equation 20

These are approximately in the range of 15 ns to 20 ns.This loss is important at high

frequency.

5.2.3

Gate charge losses

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

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.

P

MOS

I

O

2

R

P

D

() R

N

1D

–

() R

L

+

⋅

+

⋅

()

⋅

=

P

CIN COUT

,

I

O

2

D1

D

–

()

⋅

() ESR

CIN

I

2

Δ

12

-------- ESR

COUT

⋅

+

⋅⋅

=

P

SWITCHING

V

IN

I

O

F

SW

T

ON

T

OFF

+

()

2

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

⋅⋅

⋅

=

P

GATECHARGE

V

IN

C

H

C

L

+

() F

SW

⋅⋅

=

-