LP2994M datasheet(8/15 Pages) NSC

8 / 15 page

Pin Descriptions

AVIN and PVIN

AVIN and PVIN are the input supply pins for the LP2994.

AVIN is used to supply all the internal control circuitry. PVIN,

however, is used exclusively to provide the rail voltage for

the output stage used to create V

TT. These pins have the

capability to work off separate supplies depending on the

application. Higher voltages on PVIN will increase the maxi-

mum continuous output current because of output RDSON

limitations at voltages close to V

TT. The disadvantage of high

values of PVIN is that the internal power loss will also

increase, thermally limiting the design. For SSTL-2 applica-

tions, a good compromise would be to connect the AVIN and

PVIN directly together at 2.5V. This eliminates the need for

bypassing the two supply pins separately. The only limitation

on input voltage selection is that PVIN must be equal to or

lower than AVIN.

DDQ

DDQ is the input used to create the internal reference

voltage for regulating V

TT. The reference voltage is gener-

ated from a resistor divider of two internal 50k

Ω resistors.

This guarantees that V

TT will track VDDQ / 2 precisely. The

optimal implementation of V

DDQ is as a remote sense. This

can be achieved by connecting V

DDQ directly to the 2.5V rail

at the DIMM instead of AVIN and PVIN. This ensures that the

reference voltage tracks the DDR memory rails precisely

without a large voltage drop from the power lines. For

SSTL-2 applications V

DDQ will be a 2.5V signal, which will

create a 1.25V termination voltage at V

TT (See Electrical

Characteristics Table for exact values of V

TT over tempera-

ture).

SENSE

The purpose of the sense pin is to provide improved remote

load regulation. In most motherboard applications the termi-

nation resistors will connect to V

TT in a long plane. If the

output voltage was regulated only at the output of the

LP2994 then the long trace will cause a significant IR drop

resulting in a termination voltage lower at one end of the bus

than the other. The V

SENSE pin can be used to improve this

performance, by connecting it to the middle of the bus. This

will provide a better distribution across the entire termination

bus. If remote load regulation is not used then the V

SENSE

pin must still be connected to V

TT. Care should be taken

when a long V

SENSE trace is implemented in close proximity

to the memory. Noise pickup in the V

SENSE trace can cause

problems with precise regulation of V

TT. A small 0.1uF ce-

ramic capacitor placed next to the V

SENSE pin can help filter

any high frequency signals and preventing errors.

Shutdown

The LP2994 contains an active low shutdown pin that can be

used to tri-state VTT. During shutdown V

TT should not be

exposed to voltages that exceed PVIN. With the shutdown

pin asserted low the quiescent current of the LP2994 will

drop, however, V

DDQ will always maintain its constant im-

pedance of 100k

Ω for generating the internal reference.

Therefore to calculate the total power loss in shutdown both

currents need to be considered. For more information refer

to the Thermal Dissipation section. The shutdown pin also

has an internal pull-up current, therefore to turn the part on

the shutdown pin can either be connected to AVIN or left

open.

TT is the regulated output that is used to terminate the bus

resistors. It is capable of sinking and sourcing current while

regulating the output precisely to V

DDQ / 2. The LP2994 is

designed to handle peak transient currents of up to +/- 3A

with excellent load regulation. The maximum continuous

current is a function of AVIN and PVIN and several curves

can be seen in the Typical Performance Characteristics sec-

tion. If a transient is expected to last above the maximum

continuous current rating for a significant amount of time,

then the bulk output capacitor should be sized large enough

to prevent an excessive voltage drop. Despite the fact that

the LP2994 is designed to handle large transient output

currents it is not capable of handling these for long durations

under all conditions. The reason for this is that the SO-8

package is not able to thermally dissipate an infinite amount

of heat as a result of internal power loss. If large currents are

required for longer durations, then care should be taken to

ensure that the maximum junction temperature is not ex-

ceeded. Proper thermal de-rating should always be used

(Please refer to the

Thermal Dissipation section).

Component Selection

Input Capacitor

The LP2994 does not require a capacitor for input stability,

but it is recommended for improved performance during

large load transients to prevent the input rail from dropping.

The input capacitor should be located as close as possible to

the PVIN pin. Several recommendations exist dependent on

the application required. A typical value recommended for AL

electrolytic capacitors is 47uF. Ceramic capacitors can also

be used, a value in the range of 10uF with X5R dielectric or

better would be an ideal choice. The input capacitance can

be reduced if the LP2994 is placed close to the bulk capaci-

tance from the output of the 2.5V DC-DC converter. If the two

supply rails (AVIN and PVIN) are separated then the 47uF

capacitor should be placed as close to possible to the PVIN

rail. An additional 0.1uF ceramic capacitor can be placed on

the AVIN rail to prevent excessive noise from coupling into

the device.

Output Capacitor

The LP2994 has been designed to be insensitive of output

capacitor size or ESR (Equivalent Series Resistance). This

allows the flexibility to use any capacitor desired. The choice

for output capacitor will be determined solely on the applica-

tion and the requirements for load transient response of VTT.

As a general recommendation, the output capacitor should

be sized above 100uF with a low ESR for SSTL applications

with DDR-SDRAM. The value of ESR should be determined

by the maximum current spikes expected and the extent at

which the output voltage is allowed to droop. Several capaci-

tor options are available on the market and a few of these

are highlighted below:

AL - It should be noted that many aluminum electrolytics only

specify impedance at a frequency of 120Hz, which indicates

they have poor high frequency performance. Only aluminum

electrolytics that have an impedance specified at a higher

frequency (approximately 100kHz) should be used for the

LP2994. To improve the ESR several AL electrolytics can be

combined in parallel for an overall reduction. An important

note to be aware of is the extent at which the ESR will

change over temperature. Aluminum electrolytic capacitors

can have their ESR rapidly increase at cold temperatures.

Ceramic - Ceramic capacitors typically have a low capaci-

tance, in the range of 10 to 100uF range, but they have

excellent AC performance for bypassing noise because of

very low ESR (typically less than 10mOhm). However, some

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