Thermal dissipation calculation

AN2482

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2

Thermal dissipation calculation

2.1

Normal forward polarity of all inputs case

Referring to the PCLT diagram defined in Figure 1., the dissipated power for both

configurations can be calculated as follows:

datasheet):

and when the voltage drops between PCLT input and output or LED_output.

But this normal forward input polarity is not the worst case of PCLT power dissipation. See

the next section.

2.2

Reverse polarity on a single input case

corresponds to a connection mistake or a reverse biasing that is generated by the

demagnetization of a monitored inductive solenoid.

The involved input can withstand a high reverse current up to 20 mA. Its corresponding

opto-coupler output is then OFF and is protected by the clamping input diode. The other

input remains operational, and some power is dissipated in their clamping protections.

The losses in reverse input polarity configuration (see Figure 6.) are:

()

()

LED

the

of

drop

voltage

forward

the

is

input

coupler

opto

the

of

drop

voltage

forward

the

is

sensor

input

the

of

drop

voltage

the

is

2.9V

V

if

4

.

0

6

.

0

2

0.75V

V

if

1

.

0

3

.

0

6

.

0

2

MOD

_

_

MOD

_

_

_

where V

V

V

V

where V

V

V

V

where V

I

R

V

V

V

I

V

I

V

I

V

I

V

P

I

V

I

V

I

V

I

V

P

LED

LED

IN

IN_LED

OUT

OUT

IN

IN_OUT

SENSOR

IN

IN

SENSOR

CC

IN

IN

IN

IN

LED

IN

DD

MOD

C

C

MODE

CMOS

IN

IN

IN

OUT

IN

IN

LED

IN

C

C

MODE

OPTO

−

=

−

=

⋅

−

−

=

>

⋅

⋅

+

⋅

⋅

+

⋅

+

⋅

=

<

⋅

⋅

+

⋅

⋅

+

⋅

⋅

+

⋅

=

2.0mA

I

8.8mA

I

C_MAX

IN_MAX

=

=

mW

P

mW

P

MODE

CMOS

MODE

OPTO

440

430

_

_

=

=

mW

Pdis

P

P

P

P

Pdis

PCLT

RIN

dis

RIN

dis

RC

dis

ed

TOTdeliver

PCLT

566

_

_

2

_

1

_

_

=

−

−

−

=