MIC5162 Datasheet, PDF (9/11 Pages) Micrel Semiconductor – Dual Regulator Controller For High-Speed Bus Termination

MIC5162
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_CA) thermal resistance and sink-to-ambient (θ_SA) thermal
resistance;
θ_JA = θ_JC + θ_CS + θ_SA
In our example of a 3A peak SSTL_2 termination circuit, we
have selected a D-pack N-Channel MOSFET that has a
maximum junction temperature of 150°C. The device has a
junction-to-case thermal resistance of 1.5°C/Watt. Our appli-
cation has a maximum ambient temperature of 60°C. The
required junction-to-ambient thermal resistance can be cal-
culated as follows;
θ_JA
=
T_J − T_A
P_D
Where T_J is the maximum junction temperature, T_A is the
maximum ambient temperature and P_D is the power dissipa-
tion.
In our example;
θ_JA
=
T_J − T_A
P_D
θ_JA
=
150°C − 60°C
2W
θ_JA = 45°C/W
This shows that our total thermal resistance must be better
than 45°C/W. Since the total thermal resistance is a combi-
nation of all the individual thermal resistances, the amount of
heat sink required can be calculated as follows;
( ) θ_SA = θ_JA − θ_JC + θ_CA
In our example;
θ_SA = 45°C/W − (1.5°C/W + 0.5°C/W)
θ_SA = 43°C/W
In most cases, case-to-sink thermal resistance can be as-
sumed to be about 0.5°C/W.
The SSTL termination circuit for our example, using 2 D-pack
N-Channel MOSFETs (one high side and one on the low side)
will require at least a 43°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 resis-
tance.
Micrel, Inc.
MOSFET Gate Threshold
N-Channel MOSFETs require an enhancement voltage
greater than its source voltage. Typical N-Channel MOS-
FETs have a gate-source threshold (V_GS) of 1.8V and higher.
Since the source of the high side N-Channel is connected to
V_TT, the MIC5162 V_CC pin requires a voltage equal to or
greater than the V_GS voltage. For example, our SSTL_2
termination circuit has a V_TT voltage of 1.25V. For an N-
Channel that has a V_GS rating of 2.5V, the V_CC voltage can
be as low as 3.75V. With an N-Channel that has a 4.5V V_GS,
the minimum V_CC required is 5.75V. Although these N-
Channels are driven below their full enhancement threshold,
it is recommended that the V_CC voltage has enough margin
to be able to fully enhance the MOSFETs for large signal
transient response. In addition, low gate thresholds MOS-
FETs are recommended to reduce the V_CC requirements.
February 2005
9
M9999-092004