G2993 Datasheet, PDF (7/10 Pages) Global Mixed-mode Technology Inc – DDR Termination Regulator

Global Mixed-mode Technology Inc.
G2993
VDDQ
A voltage divider of two 50kΩ is connected between
VDDQ and ground, to create the internal reference
voltage (VDDQ/2). This guarantees that V_TT will track
VDDQ/2 precisely. The optimal implementation of
VDDQ is as a remote sensing. This can be achieved
by connecting VDDQ directly to the 2.5V rail (SSTL-2
applications) at the DIMM instead of AVIN and PVIN.
This will ensure that the reference voltage tracks the
DDR memory rails precisely without a large voltage
drop from the power lines.
V_TT
V_TT is the regulated output that is used to terminate
the bus resistors of DDR-SDRAM. It can precisely
track the VDDQ/2 voltage with the sinking and sourc-
ing current capability. The G2993 is designed to de-
liver 1.5A continuous current and peak current up to
3A with a fast transient response @ 2.5V supply rail.
The maximum continuous current sourcing from V_TT is
a function of PVIN. Using a higher PVIN will increase
the source current from V_TT, but it also increase the
internal power dissipation and reduce the efficiency.
Although the G2993 can deliver the larger current,
care should be taken for the thermal dissipation when
larger current is required. The R_DS of MOS will in-
crease when the junction temperature increases. If the
heat is not dealt with well, the maximum output current
will be degraded. When the temperature exceeds the
junction temperature, the thermal shutdown protection
is activated. That will drive the V_TT output into tri-state
until the temperature returns below the hysteretic trig-
ger point.
Capacitors
The G2993 does not require the capacitors for input
stability, but it is recommended for improving the
performance during large load transition to prevent the
input power rail from dropping, especially for PVIN.
The input capacitor for PVIN should be as close as
possible. The typical recommended value is 50µF for
AL electrolytic capacitors, 10uF with X5R for the ce-
ramic capacitors. To prevent the excessive noise cou-
pling into this device, an additional 0.1µF ceramic ca-
pacitor can be placed on the AVIN power rail for the
better performance.
The output capacitor of the G2993 is suggested to use
the capacitors with low ESR. Using the capacitors with
low ESR (as ceramic, OS-CON, tantalum) will have
the better transition performance which is with smaller
voltage drop when the peak current occurring at the
transition. As a general recommendation the output
capacitor should be sized above 220µF with the low
ESR for SSTL applications with DDR-SDRAM.
Thermal Dissipation
When the current is sinking to or sourcing from V_TT,
the G2993 will generate internal power dissipation
resulting in the heat. Care should be taken to prevent
the device from damages caused by the junction tem-
perature exceeding the maximum rating. The maxi-
mum allowable internal temperature rise (T_RMAX) can
be calculated under the given maximum ambient
temperature (T_AMAX) of the application and the maxi-
mum allowable junction temperature (T_JMAX).
T_RMAX= T_JMAX - T_AMAX
From this equation, the maximum power dissipation
(P_DMAX) of the G2993 can be calculated:
P_DMAX = T_RMAX /θ_JA
θ_JA of the G2993 will be dependent on several vari-
ables: the packages used, the thickness and size of
the copper, the number of vias and the airflow. The
better θ_JA is not only protecting the device well, but
also increasing the maximum current capability at the
same ambient temperature.
Ver: 1.3
Jan 13, 2004
TEL: [phone]
http://www.gmt.com.tw
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