TDE1890 datasheet(7/12 Pages) STMICROELECTRONICS

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WORST CONDITION POWER DISSIPATION IN

THE ON-STATE

In IPS applications the maximum average power

dissipation occurs when the device stays for a

long time in the ON state. In such a situation the

internal temperature depends on delivered cur-

rent (and related power), thermal characteristics

of the package and ambient temperature.

At ambient temperature close to upper limit

(+85

°C) and in the worst operating conditions, it is

possible that the chip temperature could increase

so much to make the thermal shutdown proce-

dure untimely intervene.

Our aim is to find the maximum current the IPS

can withstand in the ON state without thermal

shutdown intervention, related to ambient tem-

perature. To this end, we should consider the fol-

lowing points:

1) The ON resistance RDSON of the output

NDMOS (the real switch) of the device in-

creases with its temperature.

Experimental results show that silicon resistiv-

ity increases with temperature at a constant

rate, rising of 60% from 25

°C to 125°C.

The relationship between RDSON and tem-

perature is therefore:

R DSON = R DSON0

( 1 + k ) ( T j − 25 )

where:

Tj is the silicon temperature in

°C

RDSON0 is RDSON at Tj=25

°C

k is the constant rate (k

= 4.711 ⋅ 10 −3)

(see fig. 4).

In the ON state the power dissipated in the

device is due to three contributes:

a) power lost in the switch:

P out = I out

⋅ R DSON (Iout is the output cur-

rent);

b) power due to quiescent current in the ON

state Iq, sunk by the device in addition to

Iout:P q

= I q ⋅ V s (Vs is the supply voltage);

c) an external LED could be used to visualize

the switch state (OUTPUT STATUS pin).

Such a LED is driven by an internal current

source (delivering Ios) and therefore, if Vos is

the voltage drop across the LED, the dissi-

pated power is: P os = I os

⋅ ( V s − V os ).

Thus the total ON state power consumption is

given by:

P on

= P out + P q + P os

(1)

In the right side of equation 1, the second and

the third element are constant, while the first

one increases with temperature because

RDSON increases as well.

3) The chip temperature must not exceed

ΘLim

in order do not lose the control of the device.

The heat dissipation path is represented by

the thermal resistance of the system device-

ambient (Rth). In steady state conditions, this

parameter relates the power dissipated Pon to

the silicon temperature Tj and the ambient

temperature Tamb:

T j

− T amb = P on ⋅ R th

(2)

From this relationship, the maximum power

Pon which can be dissipated without exceed-

ing

ΘLim at a given ambient temperature

Tamb is:

P on

ΘLim− T amb

R th

Replacing the expression (1) in this equation

and solving for Iout, we can find the maximum

current versus ambient temperature relation-

ship:

I outx

√

ΘLim − T amb

R th

− P q − P os

R DSONx

where RDSONxis RDSON at Tj=

ΘLim. Of

course, Ioutx values are top limited by the

maximum operative current Ioutx (2A nominal).

From the expression (2) we can also find the

maximum ambient temperature Tamb at which

a given power Pon can be dissipated:

T amb

=ΘLim

− P on ⋅ Rth =

=ΘLim −

( I out 2 ⋅ R DSONx + P q + P os ) ⋅ R th

In particular, this relation is useful to find the

maximum ambient temperature Tambx

which Ioutx can be delivered:

T ambx

=ΘLim −( I outx 2 ⋅ R DSONx +

+ P q + P os ) ⋅ R th

(4)

Referring to application circuit in fig. 6, let us con-

sider the worst case:

- The supply voltage is at maximum value of in-

dustrial bus (30V instead of the 24V nominal

value). This means also that Ioutx rises of 25%

(2.5A instead of 2A).

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