AN458 APPLICATION NOTE

6/19

POWER DISSIPATION

It can be considered as the addition of three values:

Ptot = Psat + Pq + Psw where:

Psat: Saturation losses of the power transistor plus the sensing resistor power dissipation.

V32 = dropout voltage between input (pin 3) and output (pin 2).

For worst case (for I2 = 3A switch current) the V32 = 2V

Pq: Losses due to the stand-by current and to the power driving current.

In fig. 6 and fig. 7 are showed these two typical values of quiescent current. For the we worst case we can con-

sidered:

I’3q (0% d.c.) =13mA

I"3q (100% d.c.) = 17mA

Psw: Power transistor switching losses:

The fig. 8 shows the total power dissipation of the device.

dissipation for this device is 2W at Tamb of 70° and is this value that limits the output current value.

EFFICIENCY

The system efficiency is expressed by the following formula.

other system losses. The expression of the efficiency becomes threfore the following.

The three terms concerning the device power losses have already been discussed in the previous paragraph.

We examine now the last two terms concerning the external components losses.

Psat

V32 I

o

To n

T

-----------

V32 I

o

V

o

V

i

------

⋅⋅

=

⋅⋅

=

Pq

Vi I

′3q Vi I′′3q

t

on

T

-------

Vi I

′3q Vo I′′3q

⋅

+

⋅

==

⋅

+

⋅

=

Psw

Vi Io

tr

tf

+

2T

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

⋅

⋅

=

η%

P

O

P

I

------- 100

=

η

P

O

P

O

P

sa t

P

q

P

Sw

P

D

P

L

++

+

+

+

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

=