VTT

LP2998

PVIN

VDDQ

VREF

AVIN

VREF = 1.25V

VSENSE

GND

+

+

+

VDDQ = 2.5V

AVIN = 3.3V or 5V

VTT = 1.25V

SD

SD

PVIN = 3.3V

CIN

COUT

CREF

VTT

LP2998

PVIN

VDDQ

VREF

AVIN

VREF = 1.25V

VSENSE

GND

+

+

+

VDDQ = 2.5V

AVIN = 2.2V to 5.5V

VTT = 1.25V

SD

SD

PVIN = 1.8V

CIN

COUT

CREF

VTT

LP2998

PVIN

VDDQ

VREF

AVIN

VREF = 1.25V

VSENSE

GND

+

+

+

VDDQ = 2.5V

VDD = 2.5V

VTT = 1.25V

SD

SD

CIN

COUT

CREF

LP2998, LP2998-Q1

www.ti.com

SNVS521K – DECEMBER 2007 – REVISED AUGUST 2014

Typical Application (continued)

Figure 23. Recommended SSTL-2 Implementation

If power dissipation or efficiency is a major concern then the LP2998 has the ability to operate on split power

rails. The output stage (PVIN) can be operated on a lower rail such as 1.8 V and the analog circuitry (AVIN) can

be connected to a higher rail such as 2.5 V, 3.3 V, or 5 V. This allows the internal power dissipation to be

lowered when sourcing current from VTT. The disadvantage of this circuit is that the maximum continuous current

is reduced because of the lower rail voltage, although it is adequate for all motherboard SSTL-2 applications.

Increasing the output capacitance can also help if periods of large load transients will be encountered.

Figure 24. Lower Power Dissipation SSTL-2 Implementation

The third option for SSTL-2 applications in the situation that a 1.8 V rail is not available and it is not desirable to

use 2.5 V, is to connect the LP2998 power rail to 3.3 V. In this situation AVIN will be limited to operation on the

3.3 V or 5 V rail as PVIN can never exceed AVIN. This configuration has the ability to provide the maximum

continuous output current at the downside of higher thermal dissipation. Care should be taken to prevent the

LP2998 from experiencing large current levels which cause the junction temperature to exceed the maximum.

Because of this risk it is not recommended to supply the output stage with a voltage higher than a nominal 3.3 V

rail.

Figure 25. SSTL-2 Implementation with Higher Voltage Rails

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Product Folder Links: LP2998 LP2998-Q1