Micrel
MIC2589/MIC2595
A 0.5W sense resistor is a good choice in this
application.
voltage of 4.5V and 10V. For MIC2589/MIC2595
applications, choose the gate-source ON resistance at
10V and call this value RON. Since a heavily enhanced
MOSFET acts as an ohmic (resistive) device, almost
all that’s required to determine steady-state power
dissipation is to calculate I2R. The one addendum to
this is that MOSFETs have a slight increase in RON
Power MOSFET Selection
Selecting the proper external MOSFET for use with
theMIC2589/MIC2595 involves three straightforward
tasks:
with
increasing
die
temperature.
A
good
• Choice of a MOSFET that meets minimum voltage
approximation for this value is 0.5% increase in RON
per °C rise in junction temperature above the point at
which RON was initially specified by the manufacturer.
For instance, if the selected MOSFET has a
calculated RON of 10mꢀ at TJ = 25°C, and the actual
junction temperature ends up at 110°C, a good first
cut at the operating value for RON would be:
requirements.
• Selection of a device to handle the maximum
continuous current (steady-state thermal issues).
• Verify the selected part’s ability to withstand any
peak currents (transient thermal issues).
Power MOSFET Operating Voltage Requirements
RON ≈ 10mꢀ[1 + (110 – 25)(0.005)] ≈14.3mꢀ
The first voltage requirement for the MOSFET is that
the drain-source breakdown voltage of the MOSFET
must be greater than VIN(MAX) = VDD – VEE(min).
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 °C/W) as that with
which the MOSFET’s performance was specified by
the manufacturer. Here are a few practical tips:
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.
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
supply’s fan, or the outlet of a processor’s
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 MOSFET’s temperature in the
actual layout of the expected final circuit, at
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:
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 device’s junction
temperature.
• The value of ILOAD(CONT, MAX.) for the output in
question (see Sense Resistor Selection).
• The manufacturer’s datasheet for the candidate
MOSFET.
• The maximum ambient temperature in which the
Power MOSFET Transient Thermal Issues
device will be required to operate.
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
• 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
load faults, such as
overheating. A power MOSFET can handle a much
a
short circuit, without
22
M9999-120505
(408) 955-1690
December 2005
MICREL [ MICREL SEMICONDUCTOR ]
