DDR Termination Regulator
TJ2995
V
SENSE
The purpose of the sense pin is to provide improved remote load regulation. In most motherboard
applications the termination resistors will connect to V
TT
in a long plane. If the output voltage was
regulated only at the output of the TJ2995, then the long trace will cause a significant IR drop, resulting in
a termination voltage lower at one end of the bus than the other. The V
SENSE
pin can be used to improve
this performance, by connecting it to the middle of the bus. This will provide a better distribution across
the entire termination bus. If remote load regulation is not used, then the V
SENSE
pin must still be
connected to V
TT
.
V
REF
V
REF
provides the buffered output of the internal reference voltage V
DDQ
/ 2. This output should be
used to provide the reference voltage for the Northbridge chipset and memory. Since these inputs
are typically an extremely high impedance, there should be little current drawn from
V
REF
. For improved
performance, an output bypass capacitor can be used, located close to the pin, to help with noise. A
ceramic capacitor in the range of 0.1 µF to 0.01 µF is recommended. This output remains active
during the shutdown state and thermal shutdown events for the suspend to RAM functionality.
V
TT
V
TT
is the regulated output that is used to terminate the bus resistors. It is capable of sinking and
sourcing current while regulating the output precisely to V
DDQ
/ 2. The TJ2995 is designed to handle
peak transient currents of up to ± 3A with a fast transient response. If a transient is expected to last
above the maximum continuous current rating for a significant amount of time then the output capacitor
should be sized large enough to prevent an excessive voltage drop. Despite the fact that the TJ2995 is
designed to handle large transient output currents it is not capable of handling these for long durations,
under all conditions. The reason for this is the standard packages are not able to thermally dissipate the
heat as a result of the internal power loss. If large currents are required for longer durations, then care
should be taken to ensure that the maximum junction temperature is not exceeded. Proper thermal
derating should always be used (please refer to the Thermal Dissipation section).
THERMAL DISSIPATION
Since the TJ2995
is a linear regulator any current flow from V
TT
will result in internal power dissipation
generating heat. To prevent damaging the part from exceeding the maximum allowable junction
temperature, care should be taken to derate the part dependent on the maximum expected ambient
temperature and power dissipation. The maximum allowable internal temperature rise, T
Rmax
can be
calculated given the maximum ambient temperature, T
Amax
of the application and the maximum allowable
junction temperature, T
Jmax
.
T
Rmax
= T
Jmax
−
T
Amax
From this equation, the maximum allowable power dissipation, P
Dmax
of the part can be
calculated:
P
Dmax
= T
Rmax
/
θ
JA
Oct. 2009 - Rev. 1.1
6/9
HTC
HTC [ HTC KOREA TAEJIN TECHNOLOGY CO. ]
