WORST CONDITION POWER DISSIPATION IN
THE O N -S TA T E
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 temper ature 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).
2) In the ON state the power dissipated in the
device is due to three contribu tes:
a) power lost in the switch:
P out = I out 2 ⋅ R DSON (Iout is the output cur-
rent);
b) power due to quiescent current in the ON
state Iq, sunk by the device in addit ion 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 STAT US pin).
Such a LED is driven by an internal current
source (delivering Ios) and therefor e, if Vos is
the voltage drop across the LED, the dissi-
pated power is: P os = I os ⋅ ( V s − V os ).
Thus the t otal ON state power co nsumption 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 = ΘLi m − T amb
R th
Replacing the expression (1) in this equation
and solving for Iout, we can find t he maximum
current versus ambient temperature relation-
ship:
I outx = √
ΘLim − T amb
R th − P q − P os
R DSONx
where RDSONx is RDSON at Tj=ΘLim. Of
course, Ioutx values are top limited by the
maximum operative current Ioutx (2A n ominal).
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 ⋅ R th =
= 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 at
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 f ig. 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 ris es of 25%
(2.5A instead of 2A).
TDE1890 - TDE1891
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