MIC5191BML datasheet(9/13 Pages) MICREL

9 / 13 page

April 20049

M9999-042804

MIC5191

Micrel

The V

IN(min) to VOUT ratio and current will determine the

maximum R

DSON required. For example, for a 1.8V (±5%) to

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

calculated as follows (using V

IN(min):

1 71V 1 5V

DSON

OUT

DSON

−

()

−

()

Ω

For performance reasons, we do not want to run the N-

Channel in dropout. This will seriously affect transient re-

sponse and PSRR (power supply ripple rejection). For this

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

42m

Ω for our example application.

Size is another important consideration. Most importantly,

the design must be able to handle the amount of power being

dissipated.

The amount of power dissipated can be calculated as follows

(using V

IN(max)):

D = (VIN – VOUT) × IOUT

D = (1.89V – 1.5V) × 5A

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 we’re going to assume a

maximum of 65

°C ambient temperature, though different

MOSFETs have different maximum operating junction

temperatures. Most MOSFETs are rated to 150

°C, while

others are rated as high as 175

°C. In this case, we’re

going to limit our maximum junction temperature to 125

°C.

The MIC5191 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

°C junction temperature with

1.95W of power dissipation at an ambient temperature of

°C. Our thermal resistance calculates as follows:

T max

T ambient

125 C

65 C

1.95W

C W

= ()

−

()

°− °

=°

So our package must have a thermal resistance less than

°C /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 (D2pack)>1.4W

Table 1. Power Dissipation and

Package Recommendation

In our example, our power dissipation is greater than

1.4W, so we’ll choose a TO-263 (D2Pack) N-Channel

MOSFET.

JA is calculated as follows.

JA = θJC + θCS + θSA

Where

JC is the junction to case resistance, θCS is the

case-to-sink resistance and the

SA is the sink-to-ambi-

ent air resistance.

In the D2 package we’ve selected, the

JC is 2°C/W. The

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

can be approximated to 0.2

°C/W. This allows us to

calculate the minimum

SA:

SA= θJA– θCS – θJC

SA= 31°C/W – 0.2°C/W – 2°C/W

SA= 28.8°C/W

Referring to

Application Hint 17, Designing PCB Heat

Sinks, the minimum amount of copper area for a D2pack

at 28.8

°C/W is 2750mm2 (or 0.426in2 ). 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).

PC Board Heat Sink

Thermal Resistance vs. Area

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