Typical Application Circuits

Several different application circuits have been shown in

Figure 4 through Figure 13 to illustrate some of the options

that are possible in configuring the LP2994. Graphs of the

individual circuit performance can be found in the Typical

Performance Characteristics section in the beginning of the

datasheet. These curves illustrate how the maximum output

current is affected by changes in AVIN and PVIN.

SSTL-2 Applications

For the majority of applications that implement the SSTL-2

termination scheme, it is recommended to connect all the

input rails to the 2.5V rail. This provides an optimal trade-off

between power dissipation and component count and selec-

tion. An example of this circuit can be seen in

Figure 4.

If power dissipation or efficiency is a major concern then the

LP2994 has the ability to operate on split power rails. The

output stage (PVIN) can be operated on a lower rail such as

1.8V and the analog circuitry (AVIN) can be connected to a

higher rail such as 2.5V, 3.3V or 5V. 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.

The third option for SSTL-2 applications in the situation that

a 1.8V rail is not available and it is not desirable to use 2.5V,

is to connect the LP2994 power rail to 3.3V. In this situation

AVIN will be limited to operation on the 3.3V or 5V 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 LP2994 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

3.3V rail.

20045904

FIGURE 4. Recommended SSTL-2 Implementation

20045905

FIGURE 5. Lower Power Dissipation SSTL-2 Implementation

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