TPS51200
www.ti.com
SLUS812–FEBRUARY 2008
Because VTT accuracy has a direct impact on the memory signal integrity, it is imperative to understand the
tolerance requirement on VTT. Based on JEDEC VTT specifications for DDR and DDR2 (JEDEC standard: DDR
JESD8-9B May 2002; DDR2 JESD8-15A Sept 2003).
VTTREF – 40 mV < VTT < VTTREF + 40 mV, for both dc and ac conditions
The specification itself indicates that VTT must keep track of VTTREF for proper signal conditioning.
The TPS51200 ensures the regulator output voltage to be:
VTTREF –25 mV < VTT < VTTREF + 25mV, for both DC and AC conditions and –2 A < IVTT < 2 A
The regulator output voltage is measured at the regulator side, not the load side. The tolerance is applicable to
DDR, DDR2, DDR3 and Low Power DDR3/DDR4 applications (see Table 1 for detailed information). To meet the
stability requirement, a minimum output capacitance of 20 µF is needed. Considering the actual tolerance on the
MLCC capacitors, three 10-µF ceramic capacitors are sufficient to meet the above requirement.
Table 1. DDR, DDR2, DDR3 and LP DDR3 Termination Technology and Their Differences
Low Power
DDR
DDR2
DR3
DDR3
FSB Data Rates
Termination
200, 266, 333 and 400 MHz 400, 533, 677 and 800 MHz
800, 1066, 1330 and 1600 MHz
Same as DDR3
On-die termination for data group. On-die termination for data group.
VTT termination for address,
command and control signals
Motherboard termination to
VTT for all signals
VTT termination for address,
command and control signals
Same as DDR3
Same as DDR3
Not as demanding
Not as demanding
Only 34 signals (address,
command, control) tied to
VTT
Only 34 signals (address,
command, control) tied to
VTT
Max source/sink transient
currents of up to 2.6A to
2.9A
•
•
Termination
Current Demand
•
ODT handles data signals
•
ODT handles data signals
Less than 1A of burst current
Less than 1A of burst current
2.5V Core and I/O 1.25V
VTT
1.2V Core and
I/O 0.6V VTT
Voltage Level
1.8V Core and I/O 0.9V VTT
1.5V Core and I/O 0.75V VTT
The TPS51200 is designed as a Gm driven LDO. The voltage droop between the reference input and the output
regulator is determined by the transconductance and output current of the device. The typical Gm is 250 S at 2 A
and changes with respect to the load in order to conserve the quiescent current (that is, the Gm is very low at no
load condition). The Gm LDO regulator is a single pole system. Its unity gain bandwidth for the voltage loop is
only determined by the output capacitance, as a result of the bandwidth nature of the Gm (see Equation 1) .
Gm
F
=
UGBW
2´ p´ C
OUT
(1)
where
•
•
•
FUGBW is the unity gain bandwidth
Gm is transconductance
COUT is the output capacitance
There are two limitations to this type of regulator when it comes to the output bulk capacitor requirement. In order
to maintain stablility, the zero location contributed by the ESR of the output capacitors should be greater than the
-3-dB point of the current loop. This constraint means that higher ESR capacitors should not be used in the
design. In addition, the impedance characteristics of the ceramic capacitor should be well understood in order to
prevent the gain peaking effect around the Gm –3-dB point because of the large ESL, the output capacitor and
parasitic inductance of the VO trace.
12
Copyright © 2008, Texas Instruments Incorporated
Product Folder Link(s): TPS51200
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