MIC5190BMM TR【PDF 9/13 页】IC REG CTRLR SGL POS ADJ 10-MSOP

MIC5190

Micrel

The V

IN

(min) to V

OUT

ratio and current will determine the

maximum R

DSON

required. For example, for a 1.8V (?%) to

1.5V conversion at 5A of load current, dropout voltage can be

calculated as follows (using V

IN

(min)):

R

V V

I

R

1 71V 1 5V

5A

R

m

DSON

IN

OUT

DSON

=

(

)

=

(

)

=

.

42 &

Running the N-Channel in dropout will seriously affect tran-

sient response and PSRR (power supply ripple rejection). For

this reason, we want to select a MOSFET that has lower than

42m& for our example application.

Size is another important consideration. Most importantly,

The amount of power dissipated can be calculated as follows

(using V

IN

(max)):

P

D

= (V

IN

V

OUT

) ?I

OUT

P

D

= (1.89V 1.5V) ?5A

P

D

= 1.95W

Now that we know the amount of power we will be dissipating,

we will need to know the maximum ambient air temperature.

For our case were going to assume a maximum of 65癈

ambient temperature. Different MOSFETs have different

maximum operating junction temperatures. Most MOSFETs

are rated to 150癈, while others are rated as high as 175癈.

In this case, were going to limit our maximum junction

temperature to 125癈. The MIC5190 has no internal thermal

protection for the MOSFET so it is important that the design

provides margin for the maximum junction temperature. Our

design will maintain better than 125癈 junction temperature

with 1.95W of power dissipation at an ambient temperature of

65癈. Our thermal resistance calculates as follows:

So our package must have a thermal resistance less than

31癈 /W. Table 1. shows a good approximation of power

dissipation and package recommendation.

Package

Power Dissipation

TSOP-6

<850mW

TSSOP-8

<950mW

TSSOP-8

<1W

PowerPAK"1212-8

<1.1W

SO-8

<1.125W

PowerPAK" SO-8 D-Pack

<1.4W

TO-220/TO-263 (D

2

Pack)

>1.4W

Table 1. Power Dissipation and

Package Recommendation

In our example, our power dissipation is greater than

1.4W, so well choose a TO-263 (D

2

Pack) N-Channel

MOSFET. ?/DIV>

JA

is calculated as follows.

?/DIV>

JA

= ?/DIV>

JC

+ ?/DIV>

CS

+ ?/DIV>

SA

Where ?/DIV>

JC

is the junction-to-case resistance, ?/DIV>

CS

is the

case-to-sink resistance and the ?/DIV>

SA

is the sink-to-ambient

air resistance.

In the D

2

package weve selected, the ?/DIV>

JC

is 2癈/W. The

?/DIV>

CS

, assuming we are using the PCB as the heat sink, can

be approximated to 0.2癈/W. This allows us to calculate

the minimum ?/DIV>

SA

:

?/DIV>

SA

= ?/DIV>

JA

?/DIV>

CS

?/DIV>

JC

?/DIV>

SA

= 31癈/W 0.2癈/W 2癈/W

?/DIV>

SA

= 28.8癈/W

Referring to Application Hint 17, Designing PCB Heat

Sinks, the minimum amount of copper area for a D

2

Pack

at 28.8癈/W is 2750mm

2

(or 0.426in

2

). The solid line

denotes convection heating only (2 oz. copper) and the

dotted line shows thermal resistance with 250LFM air-

flow. The copper area can be significantly reduced by

increasing airflow or by adding external heat sinks.

Figure 8. PC Board Heat Sink

Another important characteristic is the amount of gate

capacitance. Large gate capacitance can reduce tran-

sient performance by reducing the ability of the MIC5190

to slew the gate. It is recommended that the MOSFET

used has an input capacitance <10nF (C

ISS

).

?/DIV>

JA

J

D

JA

T max T ambient

P

125 C 65 C

1.95W

C W

=

( )

(

)

=

?nbsp ?/DIV>

= ?/DIV>

31 /

PC Board Heat Sink

Thermal Resistance vs. Area

December 2005

9

M9999-120105

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