MIC2593 datasheet(22/26 Pages) STMICROELECTRONICS

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MIC2593

Micrel

M9999-042204

April 2004

must be greater than V

IN(MAX) for the slot in question. For

instance, the 5V input may reasonably be expected to see

high-frequency transients as high as 6.5V. Therefore, the

drain-source breakdown voltage of the MOSFET must be at

least 7V.

The second breakdown voltage criteria which must be met is

a bit subtler than simple drain-source breakdown voltage, but

is not hard to meet. Low-voltage MOSFETs generally have

low breakdown voltage ratings from gate to source as well. In

MIC2593 applications, the gates of the external MOSFETs

are driven from the +12V input to the MIC2593 controller.

That supply may well be at 12V + (5% x 12V) = 12.6V. At the

same time, if the output of the MOSFET (its source) is

suddenly shorted to ground, the gate-source voltage will go

to (12.6V – 0V) = 12.6V. This means that the external

MOSFETs must be chosen to have a gate-source breakdown

voltage in excess of 13V; after 12V absolute maximum, the

next commonly available voltage class has a 20V maximum

gate-source voltage. At the present time, most power

MOSFETs with a 20V gate-source voltage rating have a 30V

drain-source breakdown rating or higher. As a general tip,

look to surface mount devices with a drain-source rating of

30V as a starting point.

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 manufacturer’s 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 dissi-

pated 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 7V to 8V of enhance-

ment (11.5V nominal V

GATE minus the 3.5V to 4.5V gate

threshold of the MOSFET). Call this value R

ON. 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 R

ON with

increasing die temperature. A good approximation for this

value is 0.5% increase in R

ON per °C rise in junction tempera-

ture 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 10mΩ at TJ = 25°C and the actual junction

temperature ends up at 110

°C, a good first cut at the operat-

ing value for R

ON would be:

ON ≅ 10mΩ[1 + (110 – 25)(0.005)] ≅ 14.3mΩ

Next, approximate the steady-state power dissipation (I2R)

using I

LOAD(CONT,max) and the approximated RON.

D ≅ [ILOAD(CONT, MAX)]

× R

≅ (8.93A)2 × 14.3mΩ ≅ 1.14W

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:

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 soldered

down to one square inch or more, the copper trace

will act as the heat sink for the part. This copper

trace 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 substan-

tially. 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 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 py-

rometer on the package, will then give a reason-

able idea of the device’s junction temperature.

MOSFET Transient Thermal Issues

Having chosen a MOSFET that will, a) withstand both the

applied voltage stresses, and b) handle the worst-case continu-

ous I2R power dissipation that it will endure; verifying the

MOSFET’s ability to handle short-term overload power dissipa-

tion without overheating is the lone item to be determined. A

MOSFET can handle a much higher pulsed power without

damage than its continuous dissipation ratings would imply.

The reason for this is that 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.”

Almost all power MOSFET data sheets give a Transient

Thermal Impedance Curve. For example, take the case

where t

FLT for the 5V supply has been set to 50ms,

LOAD(CONT, MAX) is 5.0A, the slow-trip threshold is 50mV

nominal, and the fast-trip threshold is 100mV. If the output is

connected to a 0.60

Ω load, the output current from the

MOSFET for the slot in question will be regulated to 5.0A for

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