LP2996LQ Datasheet, PDF (15/26 Pages) Texas Instruments – LP2996-N DDR Termination Regulator

LP2996-N
www.ti.com
SNOSA40J – NOVEMBER 2002 – REVISED MARCH 2013
OUTPUT CAPACITOR SELECTION
For applications utilizing the LP2996-N to terminate SSTL-2 I/O signals the typical application circuit shown in
Figure 30 can be implemented.
SD
VDDQ = 2.5V
VDD = 2.5V
+
PF
LP2996
SD
VDDQ
VREF
AVIN
VSENSE
PVIN
VTT
GND
VREF = 1.25V
+
0.01PF
VTT = 1.25V
+
220PF
Figure 32. Typical SSTL-2 Application Circuit
This circuit permits termination in a minimum amount of board space and component count. Capacitor selection
can be varied depending on the number of lines terminated and the maximum load transient. However, with
motherboards and other applications where V_TT is distributed across a long plane it is advisable to use multiple
bulk capacitors and addition to high frequency decoupling. Figure 31 shown below depicts an example circuit
where 2 bulk output capacitors could be situated at both ends of the V_TT plane for optimal placement. Large
aluminum electrolytic capacitors are used for their low ESR and low cost.
SD
VDDQ = 2.5V
VDD = 2.5V
+
47PF
LP2996
SD
VDDQ
VREF
AVIN
VSENSE
PVIN
VTT
GND
VREF = 1.25V
+
0.01PF
+
330PF
VTT = 1.25V
+
330PF
Figure 33. Typical SSTL-2 Application Circuit for Motherboards
In most PC applications an extensive amount of decoupling is required because of the long interconnects
encountered with the DDR-SDRAM DIMMs mounted on modules. As a result bulk aluminum electrolytic
capacitors in the range of 1000uF are typically used.
PCB Layout Considerations
1. The input capacitor for the power rail should be placed as close as possible to the PVIN pin.
2. V_SENSE should be connected to the V_TT termination bus at the point where regulation is required. For
motherboard applications an ideal location would be at the center of the termination bus.
3. V_DDQ can be connected remotely to the V_DDQ rail input at either the DIMM or the Chipset. This provides the
most accurate point for creating the reference voltage.
4. For improved thermal performance excessive top side copper should be used to dissipate heat from the
package. Numerous vias from the ground connection to the internal ground plane will help. Additionally these
can be located underneath the package if manufacturing standards permit.
5. Care should be taken when routing the V_SENSE trace to avoid noise pickup from switching I/O signals. A
0.1uF ceramic capacitor located close to the _SENSE can also be used to filter any unwanted high frequency
signal. This can be an issue especially if long _SENSE traces are used.
6. V_REF should be bypassed with a 0.01 µF or 0.1 µF ceramic capacitor for improved performance. This
capacitor should be located as close as possible to the V_REF pin.
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