LP2996LQ datasheet(13/26 Pages) TI

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VTT

LP2996

PVIN

VDDQ

VREF

AVIN

VREF= 0.9V

VSENSE

GND

VDDQ = 1.8V

AVIN = 3.3V or 5.5V

VTT = 0.9V

PVIN = 3.3V

CIN

COUT

CREF

VTT

LP2996

PVIN

VDDQ

VREF

AVIN

VREF = 0.9V

VSENSE

GND

VDDQ = 1.8V

AVIN = 2.2V to 5.5V

VTT = 0.9V

PVIN = 1.8V

CIN

COUT

CREF

LP2996-N

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SNOSA40J – NOVEMBER 2002 – REVISED MARCH 2013

DDR-II APPLICATIONS

With the separate VDDQ pin and an internal resistor divider it is possible to use the LP2996-N in applications

utilizing DDR-II memory. Figure 25 and Figure 26 show several implementations of recommended circuits with

output curves displayed in the Typical Performance Characteristics. Figure 25 shows the recommended circuit

configuration for DDR-II applications. The output stage is connected to the 1.8V rail and the AVIN pin can be

connected to either a 3.3V or 5V rail. For new designs, the LP2997 or LP2998 is recommended for DDR-II

applications.

Figure 27. Recommended DDR-II Termination

If it is not desirable to use the 1.8V rail it is possible to connect the output stage to a 3.3V rail. Care should be

taken to not exceed the maximum junction temperature as the thermal dissipation increases with lower VTT

output voltages. For this reason it is not recommended to power PVIN off a rail higher than the nominal 3.3V.

The advantage of this configuration is that it has the ability to source and sink a higher maximum continuous

current.

Figure 28. DDR-II Termination with higher voltage rails

LEVEL SHIFTING

If standards other than SSTL-2 are required, such as SSTL-3, it may be necessary to use a different scaling

factor than 0.5 times VDDQ for regulating the output voltage. Several options are available to scale the output to

any voltage required. One method is to level shift the output by using feedback resistors from VTT to the VSENSE

pin. This has been illustrated in Figure 29 and Figure 30. Figure 29 shows how to use two resistors to level shift

VTT above the internal reference voltage of VDDQ/2. To calculate the exact voltage at VTT the following equation

can be used.

VTT = VDDQ/2 ( 1 + R1/R2)

(11)

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