MIC5165 datasheet(13/24 Pages) MICREL

13 / 24 page

Micrel, Inc.

MIC5165

June 2010

M9999-061510-B

calculation for the high-side MOSFET is as follows:

PD = (VDDQ − VTT) × I_SOURCE

Where I_source is the average source current.

Power dissipation for the low-side MOSFET is as

follows:

PD = VTT × I_SINK

where I_sink is the average sink current.

typical

3.5A

peak

DDR3

circuit,

power

considerations for MOSFET selection would occur as

follows:

PD = (VDDQ −VTT) × I_SOURCE

PD = (1.5V −0.75V) × 1.75A

PD = 1.3125 W

This typical DDR3 application would require the high-

side and low-side N-Channel MOSFETs to be able to

handle

1.3125

Watts

each.

higher

current

applications, multiple N-Channel MOSFETs may be

placed in parallel to spread the power dissipation. These

MOSFETs

will

current,

distributing

power

dissipation across each device.

The maximum MOSFET die (junction) temperature limits

maximum power dissipation. The ability of the device to

dissipate heat away from the junction is specified by the

junction-to-ambient (θJA) thermal resistance. This is the

sum of junction-to-case (θJC) thermal resistance, case-

to-sink (θCS) thermal resistance and sink-to-ambient

(θSA) thermal resistance:

θJA = θJC + θCS + θSA

In our example of a 3.5A peak DDR3 termination circuit,

we have selected a D-pack N-Channel MOSFET that

has a maximum junction temperature of 125°C. The

device has a junction-to-case thermal resistance of

1.5°C/W. Our application has a maximum ambient

temperature of 60°C.

The required junction-to-ambient thermal resistance can

be calculated as follows:

−

Where TJ is the maximum junction temperature, TA is the

maximum ambient temperature and PD is the power

dissipation.

In our example:

−

3125

125

θJA

This shows that our total thermal resistance must be

better than 49.52°C/W. Since the total thermal

resistance is a combination of all the individual thermal

resistances, the amount of heat sink required can be

calculated as follows:

θSA = θJA − (θJC + θCS)

In our example:

θSA = θJA − (θJC + θCS)

)

(

θSA

In most cases, case-to-sink thermal resistance can be

assumed to be about 0.5°C/W.

The DDR3 termination circuit for our example, using two

D-pack N-Channel MOSFETs (one high-side and one

low-side) will require enough copper area to spread the

heat away from the MOSFET. In this example to

dissipate 1.3W from TO-252 package a 2 oz copper of

0.4 in

2 on component side is required. In some cases,

airflow may also help to reduce thermal resistance. For

different MOSFET package refer to manufacturer Data

Sheet for copper area requirements.

MIC5165 Datasheet(PDF) 13 Page - Micrel Semiconductor

MIC5165 Datasheet(HTML) 13 Page - Micrel Semiconductor

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Datasheet Download

Link URL