MIC5162 datasheet(9/24 Pages) MICREL

9 / 24 page

Micrel, Inc.

MIC5162

March 2010

M9999-033110

where I_SINK is the average sink current.

typical

peak

SSTL_2

circuit,

power

considerations for MOSFET selection would occur as

follows.

PD = (VDDQ −VTT) × I_SOURCE

PD = (2.5V −1.25V) × 1.6A

PD = 2W

This typical SSTL_2 application would require both high-

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

handle 2 Watts each. In applications where there is

excessive

power

dissipation,

multiple

N-Channel

MOSFETs may be placed in parallel. These MOSFETs

will share 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 the example of a 3A peak SSTL_2 termination circuit,

a D-pack N-Channel MOSFET that has a maximum

junction temperature of 125°C has been selected. The

device has a junction-to-case thermal resistance of

1.5°C/Watt. The application has a maximum ambient

temperature of 60°C. The required junction-to-ambient

thermal resistance can be calculated as follows:

T −

Where TJ is the maximum junction temperature, TA is the

maximum ambient temperature and PD is the power

dissipation.

In this example:

T −

125

θJA

This shows that our total thermal resistance must be

better than 32.5°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 this example:

(

)

θSA

θ SA

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

assumed to be about 0.5°C/W.

The SSTL termination circuit for this example, using 2 D-

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

the low side) will require at least a 30.5°C/W heat sink

per MOSFET. This may be accomplished with an

external heat sink or even just the copper area that the

MOSFET is soldered to. In some cases, airflow may also

be required to reduce thermal resistance.

MOSFET Gate Threshold

N-Channel MOSFETs require an enhancement voltage

greater than its source voltage. Typical N-Channel

MOSFETs have a gate-source threshold (VGS) of 1.8V

and higher. Since the source of the high side N-Channel

is connected to VTT, the MIC5162 VCC pin requires a

voltage greater than the VGS voltage. For example, the

SSTL_2 termination circuit has a VTT voltage of 1.25V.

For an N-Channel that has a VGS rating of 2.5V, the VCC

voltage can be as low as 3.75V, but not less than 3.0V.

With an N-Channel that has a 4.5V VGS, the minimum

VCC required is 5.75V. Although these N-Channels are

driven below their full enhancement threshold, it is

recommended that the VCC voltage has enough margin

to be able to fully enhance the MOSFETs for large signal

transient response. In addition, low gate thresholds

MOSFETs

are

recommended

reduce

the

VCC

requirements.

MIC5162 Datasheet(PDF) 9 Page - Micrel Semiconductor

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