參數(shù)資料
型號(hào)： MIC2585-2MBTS TR
廠商： Micrel Inc
文件頁(yè)數(shù)： 25/28頁(yè)
文件大?。?/td> 273K
描述： IC CTRLR HOT SWAP DUAL 24-TSSOP
標(biāo)準(zhǔn)包裝： 2,500
類型： 熱交換控制器
應(yīng)用： 通用
內(nèi)部開關(guān)： 無
電源電壓： 1 V ~ 13.2 V
工作溫度： -40°C ~ 85°C
安裝類型： 表面貼裝
封裝/外殼： 24-TSSOP（0.173"，4.40mm 寬）
供應(yīng)商設(shè)備封裝： 24-TSSOP
包裝： 帶卷 (TR)
其它名稱： MIC2585-2MBTSTR
MIC2585-2MBTSTR-ND
March 2005
25
MIC2584/2585
MIC2584/2585
Micrel
higher pulsed power without damage than its continuous
dissipation ratings would imply. The reason for this is that, like
everything else, thermal devices (silicon die, lead frames,
etc.) have thermal inertia.
In terms related directly to the specification and use of power
MOSFETs, this is known as transient thermal impedance,
or Z
?J-A)
. Almost all power MOSFET data sheets give a
Transient Thermal Impedance Curve. For example, take the
following case: V
IN
 = 12V, t
OCSLOW
 has been set to 100msec,
I
LOAD(CONT. MAX)
 is 1.2A, the slow-trip threshold is 50mV
nominal, and the fast-trip threshold is 100mV. If the output is
accidentally connected to a 6& load, the output current from
the MOSFET will be regulated to 1.2A for 100ms (t
OCSLOW
)
before the part trips. During that time, the dissipation in the
MOSFET is given by:
P = E x I E
MOSFET
 = [12V-(1.2A)(6&)] = 4.8V
P
MOSFET
= (4.8V x 1.2A) = 5.76W for 100msec.
At first glance, it would appear that a really hefty MOSFET is
required to withstand this sort of fault condition. This is where
the transient thermal impedance curves become very useful.
Figure 13 shows the curve for the Vishay (Siliconix) Si4410DY,
a commonly used SO-8 power MOSFET.
Taking the simplest case first, well assume that once a fault
event such as the one in question occurs, it will be a long time,
10 minutes or more, before the fault is isolated and the
channel is reset. In such a case, we can approximate this as
a single pulse event, that is to say, theres no significant duty
cycle. Then, reading up from the X-axis at the point where
Square Wave Pulse Duration is equal to 0.1sec (=100msec),
we see that the Z
?J-A)
 of this MOSFET to a highly infrequent
event of this duration is only 8% of its continuous R
?J-A)
.
This particular part is specified as having an R
?J-A)
  of
50癈/W for intervals of 10 seconds or less. Thus:
Assume T
A
 = 55癈 maximum, 1 square inch of copper at the
drain leads, no airflow.
Recalling from our previous approximation hint, the part has
an R
ON
of (0.0335/2) = 17m& at 25癈.
Assume it has been carrying just about 1.2A for some time.
When performing this calculation, be sure to use the highest
anticipated ambient temperature (T
A(MAX)
) in which the
MOSFET will be operating as the starting temperature, and
find the operating junction temperature increase (T
J
) from
that point. Then, as shown next, the final junction temperature
is found by adding T
A(MAX)
 and T
J
. Since this is not a closed-
form equation, getting a close approximation may take one or
two iterations, But its not a hard calculation to perform, and
tends to converge quickly.
Then the starting (steady-state)T
J
 is:
T
J
E T
A(MAX)
 + T
J
E T
A(MAX)
 + [R
ON
 + (T
A(MAX)
 T
A
)(0.005/癈)(R
ON
)]
  x I
2
x R
?J-A)
T
J
E 55癈 + [17m& + (55癈-25癈)(0.005)(17m&)]
x (1.2A)
2
 x (50癈/W)
T
J
E (55癈 + (0.02815W)(50癈/W)
E 54.6癈
Iterate the calculation once to see if this value is within a few
percent of the expected final value. For this iteration we will
start with T
J
 equal to the already calculated value of 54.6癈:
T
J
E T
A
 + [17m& + (54.6癈-25癈)(0.005)(17m&)]
  x (1.2A)
2
 x (50癈/W)
T
J
 E ( 55癈 + (0.02832W)(50癈/W) E 56.42癈
So our original approximation of 56.4癈 was very close to the
correct value. We will use T
J
 = 56癈.
Finally, add (5.76W)(50癈/W)(0.08) = 23癈 to the steady-state
T
J
 to get T
J(TRANSIENT MAX.)
= 79癈. This is an acceptable
maximum junction temperature for this part.
2
1
0.1
0.01
10
4
10
3
10
2
10
1
1
10
30
0.2
0.1
0.05
0.02
Single Pulse
Duty Cycle = 0.5
1. Duty Cycle, D =
2. Per Unit Base = R
thJA
 = 50?/SPAN>C/W
3. T
JM
 
T
A
 = P
DM
Z
thJA
(t)
t
1
t
2
t
1
t
2
Notes:
4. Surface Mounted
P
DM
Normalized Thermal Transient Impedance, Junction-to-Ambient
Square Wave Pulse Duration (sec)
Figure 13. Transient Thermal Impedance
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