MIC2588-2BMTR Micrel, MIC2588-2BMTR Datasheet - Page 17

• Manufacturer Part Number: MIC2588-2BMTR
• Description: Semiconductors and Actives, ic, soic
• Manufacturer: Micrel

then add the rise in temperature due to the maximum power
dissipated during a transient overload caused by a short
circuit condtion. The equation to estimate the maximum
steady-state junction temperature is given by:
T
an overcurrent condition, at which the MOSFET will operate
and is estimated from the following equation based on the
highest ambient temperature of the system environment.
Let’s assume a maximum ambient of 60°C. The power dis-
sipation of the MOSFET is determined by the current through
the MOSFET and the on-resistance (I
mate at 17mΩ (specification given at T
example information and substituting into Equation 11,
Substituting the variables into Equation 10, T
by:
T
Since this is not a closed-form equation, getting a close ap-
poroximation may take one or two iterations. On the second
iteration, start with T
Doing so in this example yields;
T
September 2005
MIC2588/MIC2594
J
J
C
(steady-state) ≅ 66.06°C+[17mΩ+(73.36°C–25°C)×(0.005/°C)
T
T
T
(steady-state) ≅ T
(max) is the highest anticipated case temperaure, prior to
J
C
C
(steady-state) ≅ T
(max) = T
(max) = 60°C + [((3A)
= 66.06°C
≅ 73.62°C
A
(max) + P
≅ 66.06°C+[17mΩ+(66.06°C–25°C)(0.005/°C)
≅ 66.06°C + 7.30°C
≅ 73.36°C
C
(max)+[R
J
C
equal to the value calculated above.
(max) + ΔT
×(17mΩ)][(3A)
× (R
× (17mΩ)][(3A)
D
2
× (R
ON
× 17mΩ) × (40 – 0.4)°C/W]
ON
FIgure 7. Transient Thermal Impedance - SUM110N10-09
+(T
)][I
θ(J-A)
J
C
2
×(R
(max)–T
2
2
×(40–0.4)]°C/W
– R
J
R), which we will esti-
θ(J-A)
= 125°C). Using our
2
×(40–0.4)°C/W]
θ(J-C)
–R
C
J
)(0.005)
is determined
θ(J-C)
)
)]
(10)
(11)
17
Another iteration shows that the result (73.63°C) is converg-
ing quickly, so we’ll estimate the maximum T
74°C.
The use of the Transient Thermal Impedence Curves is
necessary to determine the increase in junction temperature
associated with a worst-case transient condition. From our
previous calculation of the maximum power dissipated during
a short circuit event for the MIC2588/MIC2594, we calculate
the transient junction temperature increase as:
Assume the MOSFET has been on for a long time – several
minutes or more – and delivering the steady-state load current
of 3A to the load when the load is short circuited. The control-
ler will regulate the GATE output voltage to limit the current
to the programmed value of 4.2A for approximately 400µs
before immediately shutting off the output. For this situation
and almost all hot swap applications, this can be considered a
single pulse event as there is no significant duty cycle. From
Figure 7, find the point on the X-axis (“Square-Wave Pulse
Duration”) for 1ms, allowing for a healthy margin of the 400µs
t
the Single Pulse curve. This point is the normalized transient
thermal impedence (Z
impedence is the product of R
in this example. Solving Equation 12,
Finally, add this result to the maximum steady state junction
temperature calculated previously to determine the estimated
maximum transient junction temperature of the MOSFET:
T
under the specified maximum junction temperature of 200°C
for the SUM110N10-09.
FLT
J
T
T
(max.transient) = 74°C + 36.3°C = 110.3°C, which is safely
J
, and read up the Y-axis scale to find the intersection of
J
(transient) = P
(transient) = (201.6W) × (0.4°C/W) × 0.45 = 36.3°C
D
(short) × R
θ(J-C)
), and the effective transient thermal
θ(J-C)
θ(J-C)
and the multiplier, 0.45
× Multiplier
J(steady-state)
M9999-083005
(12)
Micrel
at