MIC5165 Datasheet, PDF (13/24 Pages) Micrel Semiconductor – Dual Regulator Controller for DDR3 GDDR3/4/5 Memory Termination

Micrel, Inc.
calculation for the high-side MOSFET is as follows:
P_D = (V_DDQ − V_TT) × I_SOURCE
Where I_source is the average source current.
Power dissipation for the low-side MOSFET is as
follows:
P_D = V_TT × I_SINK
where I_sink is the average sink current.
In a typical 3.5A peak DDR3 circuit, power
considerations for MOSFET selection would occur as
follows:
P_D = (V_DDQ −V_TT) × I_SOURCE
P_D = (1.5V −0.75V) × 1.75A
P_D = 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. In higher current
applications, multiple N-Channel MOSFETs may be
placed in parallel to spread the power dissipation. 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 (θ_JA) thermal resistance. This is the
sum of junction-to-case (θ_JC) thermal resistance, case-
to-sink (θ_CS) thermal resistance and sink-to-ambient
(θ_SA) thermal resistance:
θ_JA = θ_JC + θ_CS + θ_SA
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.
MIC5165
The required junction-to-ambient thermal resistance can
be calculated as follows:
θ _JA
= ^{TJ − TA}
P_D
Where T_J is the maximum junction temperature, T_A is the
maximum ambient temperature and P_D is the power
dissipation.
In our example:
θ _JA
= ^{TJ − TA}
P_D
125°C - 60°C
θ_JA = _1.3125W
°C
θ _JA
=
49.52
W
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:
θ_SA = θ_JA − (θ_JC + θ_CS)
In our example:
θ_SA = θ_JA − (θ_JC + θ_CS)
°C
°C
°C
θ_SA
=
49.52
W
-
(1.5 + 0.5
W
W
)
°C
θ_SA
= 47.52
W
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 in² 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.
June 2010
13
M9999-061510-B