參數(shù)資料
型號： MIC2595R-2BM TR
廠商： Micrel Inc
文件頁數(shù)： 22/29頁
文件大?。?/td> 4205K
描述： IC CTRLR HOT SWAP NEG HV 14-SOIC
標準包裝： 2,500
類型： 熱交換控制器
應用： 通用
內部開關： 無
電源電壓： -19 V ~ -80 V
工作溫度： -40°C ~ 85°C
安裝類型： 表面貼裝
封裝/外殼： 14-SOIC（0.154"，3.90mm 寬）
供應商設備封裝： 14-SOIC
包裝： 帶卷 (TR)
其它名稱： MIC2595R-2BMTR
MIC2595R-2BMTR-ND
Micrel
MIC2589/MIC2595
 
 
December 2005 
22
M9999-120505
 (408) 955-1690
 
A 0.5W sense resistor is a good choice in this
application.
Power MOSFET Selection
Selecting the proper external MOSFET for use with
theMIC2589/MIC2595 involves three straightforward
tasks:
"   Choice of a MOSFET that meets minimum voltage
requirements.
"   Selection of a device to handle the maximum
continuous current (steady-state thermal issues).
"   Verify the selected parts ability to withstand any
peak currents (transient thermal issues).
Power MOSFET Operating Voltage Requirements
The first voltage requirement for the MOSFET is that
the drain-source breakdown voltage of the MOSFET
must be greater than V
IN(MAX)
 = V
DD
  V
EE
(min).
The second breakdown voltage criterion that must be
met    is    the    gate-source    voltage.    For    the
MIC2589/MIC2595, the gate of the external MOSFET
is driven up to a maximum of 11V above VEE. This
means that the external MOSFET must be chosen to
have a gate-source breakdown voltage of 12V or
more; 20V is recommended. Most power MOSFETs
with a 20V gate-source voltage rating have a 30V
drain-source breakdown rating or higher. For many
48V telecom applications, transient voltage spikes can
approach,   and   sometimes   exceed,   100V.   The
absolute   maximum   input   voltage   rating   of   the
MIC2589/MIC2595 is 100V; therefore, a drain-source
breakdown voltage of 100V is suggested for the
external MOSFET. Additionally, an external input
voltage    clamp    is    strongly    recommended    for
applications that do not utilize conditioned power
supplies.
Power 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 datasheet 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 datasheet will almost always give a value of ON
resistance for a given MOSFET at a gate-source
voltage of 4.5V and 10V. For MIC2589/MIC2595
applications, choose the gate-source ON resistance at
10V and 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 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
 H 10m&[1 + (110  25)(0.005)] H14.3m&
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 TO-263 power MOSFET
flows almost entirely out of the drain tab. If
the drain tab can be soldered 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
substantially.   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 candidate MOSFET for
an application (assuming the above tips
show it to be a likely fit) is an empirical one.
Check the MOSFETs 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.
Power MOSFET Transient Thermal Issues
If the prospective MOSFET has been shown to
withstand the environmental voltage stresses and the
worst-case    steady-state    power    dissipation    is
addressed, the remaining task is to verify if the
MOSFET is capable of handling extreme overcurrent
load   faults,   such   as   a   short   circuit,   without
overheating. A power MOSFET can handle a much
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