參數(shù)資料
型號： MIC2585-2MBTS TR
廠商： Micrel Inc
文件頁數(shù)： 24/28頁
文件大小： 273K
描述： IC CTRLR HOT SWAP DUAL 24-TSSOP
標準包裝： 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
MIC2584/2585
Micrel
MIC2584/2585
24
March 2005
MOSFET Steady-State Thermal Issues
The selection of a MOSFET to meet the maximum continuous
current is a fairly straightforward exercise. First, arm yourself
with the following data:
"  The value of I
LOAD(CONT, MAX.)
 for the output in
question (see "Sense Resistor Selection").
"  The manufacturers data sheet for the candidate
MOSFET.
"  The maximum ambient temperature in which the
device will be required to operate.
"  Any knowledge you can get about the heat
sinking available to the device (e.g., can heat be
dissipated into the ground plane or power plane,
if using a surface-mount part? Is any airflow
available?).
The data sheet will almost always give a value of on resis-
tance given for the MOSFET at a gate-source voltage of 4.5V,
and another value at a gate-source voltage of 10V. As a first
approximation, add the two values together and divide by two
to get the on-resistance of the part with 8V of enhancement.
Call this value R
ON
. Since a heavily enhanced MOSFET acts
as an ohmic (resistive) device, almost all thats required to
determine steady-state power dissipation is to calculate I
2
R.
The one addendum to this is that MOSFETs have a slight
increase in R
ON
 with increasing die temperature. A good
approximation for this value is 0.5% increase in R
ON
 per 癈
rise in junction temperature above the point at which R
ON
 was
initially specified by the manufacturer. For instance, if the
selected MOSFET has a calculated R
ON
  of 10m& at a
T
J
= 25癈, and the actual junction temperature ends up
at 110癈, a good first cut at the operating value for R
ON
would be:
R
ON
 E 10m&[1 + (110 - 25)(0.005)] E 14.3m&
The final step is to make sure that the heat sinking available
to the MOSFET is capable of dissipating at least as much
power (rated in 癈/W) as that with which the MOSFETs
performance was specified by the manufacturer. Here are a
few practical tips:
1. The heat from a surface-mount device such as
an SO-8 MOSFET flows almost entirely out of
the drain leads. If the drain leads can be sol-
dered down to one square inch or more, the
copper will act as the heat sink for the part. This
copper must be on the same layer of the board
as the MOSFET drain.
2. Airflow works. Even a few LFM (linear feet per
minute) of air will cool a MOSFET down sub-
stantially. If you can, position the MOSFET(s)
near the inlet of a power supplys fan, or the
outlet of a processors cooling fan.
3. The best test of a surface-mount MOSFET for
an application (assuming the above tips show it
to be a likely fit) is an empirical one. Check the
MOSFET's temperature in the actual layout of
the expected final circuit, at full operating
current. The use of a thermocouple on the drain
leads, or infrared pyrometer on the package, will
then give a reasonable idea of the devices
junction temperature.
MOSFET Transient Thermal Issues
Having chosen a MOSFET that will withstand the imposed
voltage stresses, and the worse case continuous I
2
R power
dissipation which it will see, it remains only to verify the
MOSFETs ability to handle short-term overload power dissi-
pation without overheating. A MOSFET can handle a much
C3
0.05礔
SENSE1
VCC1
ON
CPOR
FB1
GATE1
GND
/POR
Undervoltage (Output) = 11.0V
/POR Delay = 25ms
START-UP Delay = 6ms
*Recommended for MOSFETs with gate-source
breakdown of 20V or less for catastrophic output
short circuit protection. (IRF7822 V
GS
(MAX) = 12V)
Channel 2 and additional pins omitted for clarity.
Q1
IRF7822
(SO-8)
R3
10&
*D2
1N5240B
10V
R5
13.3k&
1%
R1
33k&
R2
33k&
MIC2584
C1
1礔
C2
0.01礔
C
LOAD1
220礔
R4
100k&
1%
V
OUT
12V@6A
V
IN
12V
11
9
7
16
15
12
6
14
R
SENSE1
0.006&
5%
1
2
3
4
DOWNSTREAM
SIGNAL
D1
(18V)
Figure 12. Zener Clamped MOSFET Gate
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