Datasheet
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 R
DSON
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 R
DSON
and tem-
perature is therefore:
R
DSON
=
R
DSON0
(
1
+
k
)
(
T
j
−
25
)
where:
T
j
is the silicon temperature in °C
R
DSON0
is R
DSON
at T
j
=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 contributes:
a) power lost in the switch:
P
out
=
I
out
2
⋅
R
DSON
(I
out
is the output cur-
rent);
b) power due to quiescent current in the ON
state Iq, sunk by the device in addition to
I
out
: P
q
=
I
q
⋅
V
s
(V
s
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 I
os
) and therefore, if V
os
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
R
DSON
increases as well.
3) The chip temperature must not exceed
Θ
Lim
in order do not lose the control of the device.
The heat dissipation path is represented by
the thermal resistance of the system device-
board-ambient (R
th
). In steady state condi-
tions, this parameter relates the power dissi-
pated P
on
to the silicon temperature T
j
and
the ambient temperature T
amb
:
T
j
−
T
amb
=
P
on
⋅
R
th
(2)
From this relationship, the maximum power P
on
which can be dissipated without exceeding
Θ
Lim at a given ambient temperature T
amb
is:
P
on
=
Θ
Lim
−
T
amb
R
th
Replacing the expression (1) in this equation
and solving for I
out
, we can find the maximum
current versus ambient temperature relation-
ship:
I
outx
=
√
Θ
Lim
−
T
amb
R
th
−
P
q
−
P
os
R
DSONx
where R
DSON
x is R
DSON
at T
j
=
Θ
Lim. Of
course, I
outx
values are top limited by the
maximum operative current I
outx
(500mA
nominal).
From the expression (2) we can also find the
maximum ambient temperature T
amb
at which
a given power P
on
can be dissipated:
T
amb
=
Θ
Lim
−
P
on
⋅
R th
=
=
Θ
Lim
−
(
I
out
2
⋅
R
DSONx
+
P
q
+
P
os
)
⋅
R
th
In particular, this relation is useful to find the
maximum ambient temperature T
ambx
at
which I
outx
can be delivered:
T
ambx
=
Θ
Lim
−
(
I
outx
2
⋅
R
DSONx
+
+
P
q
+
P
os
)
⋅
R
th
(4)
Referring to application circuit in fig. 5, 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 I
outx
rises of 25%
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