MIC2590B-5BTQ TR【PDF 19/23 页】IC PCI HOT PLUG CTLR DUAL 48TQFP

Micrel, Inc.

MIC2590B

September 2008

19

M9999-091808

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 MIC2590B applications, the gates of

the external MOSFETs are driven from the +12V input to

the IC. 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 permissible gate-source voltage of 20V

maximum. This is a very suitable class of device. 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 Maximum On-State Resistance

The MOSFETs in the +3.3V and +5V MAIN power paths

will have a finite voltage drop, which must be taken into

account during component selection. A suitable

MOSFETs datasheet will almost always give a value of

4.5V, and another value at a gate-source voltage of 10V.

divide by two to get the on resistance of the device with 7

Volts of enhancement (keep this in mind; well use it in the

following Thermal Issues sections). The resulting value is

conservative, but close enough. Call this value R

ON

. Since

a heavily enhanced MOSFET acts as an ohmic (resistive)

device, almost all that is required to calculate the voltage

drop across the MOSFET is to multiply the maximum

current times the MOSFETs R

ON

. 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, the Vishay

(Siliconix) Si4430DY, which is a commonly used part in

this type of application, has a specified R

DS(ON)

of 8.0m&

max. at V

G-S

= 4.5V, and R

DS(ON)

of 4.7m& max. at V

G-S

=10V. Then R

ON

is calculated as:

(

)

6.35m&

2

8.0m&

4.7m&

R

ON

=

+

=

at 25癈 T

J

. If the actual junction temperature is estimated

to be 110癈, a reasonable approximation of R

ON

for the

Si4430DY at temperature is:

(

)

( )

9.05m&

C

0.5%

85

1

6.35m&

C

0.5%

25

100

1

6.35m&

E

?/DIV>

+

=

?/DIV>

+

Note that this is not a closed-form equation; if more

precision were required, several iterations of the

calculation might be necessary. This is demonstrated in

the section MOSFET Transient Thermal Issues.

For the given case, if Si4430DY is operated at an I

DRAIN

of

7.6A, the voltage drop across the part will be

approximately (7.6A)(9.05m&) = 69mV.

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?).

Now it gets easy: steady-state power dissipation is found

by calculating I

2

R. As noted in MOSFET Maximum On-

State Resistance, above, the one further concern is the

MOSFETs increase in R

ON

with increasing die

temperature. Again, use the Si4430DY MOSFET as an

example, and assume that the actual junction temperature

ends up at 110癈. Then R

ON

at temperature is again

approximately 9.05m&. Again, allow a maximum I

DRAIN

of

7.6A:

( )

0.523W

9.05m&

7.6A

R

I

n

Dissipatio

Power

2

ON

2

DRAIN

E

?/DIV>

=

?/DIV>

E

The next 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. Formally put, the steady-state electrical

model of power dissipated at the MOSFET junction is

analogous to a current source, and anything in the path of

that power being dissipated as heat into the environment

is analogous to a resistor. Its therefore necessary to

verify that the thermal resistance from the junction to the

ambient is equal to or lower than that value of thermal

resistance (often referred to as R

?JA)

) for which the

operation of the part is guaranteed. As an applications

issue, surface mount MOSFETs are often less than

ideally specified in this regardits become common

practice simply to state that the thermal data for the part is

specified under the conditions Surface mounted on FR-4

board, td10seconds, or something equally mystifying. So

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 of copper the

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

copper must be on the same layer of the board as

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