LP2994_14 Datasheet, PDF (9/19 Pages) Texas Instruments – DDR Termination Regulator

Obsolete Device
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, how-
ever, is used exclusively to provide the rail voltage for the
output stage used to create V_TT. These pins have the capa-
bility to work off separate supplies depending on the applica-
tion. Higher voltages on PVIN will increase the maximum
continuous output current because of output RDSON limita-
tions at voltages close to V_TT. The disadvantage of high
values of PVIN is that the internal power loss will also in-
crease, 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.
V_DDQ
V_DDQ is the input used to create the internal reference voltage
for regulating V_TT. The reference voltage is generated from a
resistor divider of two internal 50kΩ resistors. This guaran-
tees that V_TT will track V_DDQ / 2 precisely. The optimal imple-
mentation 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 in-
stead 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 termina-
tion voltage at V_TT (See Electrical Characteristics Table for
exact values of V_TT over temperature).
V_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 con-
nected to V_TT. Care should be taken when a long V_SENSE trace
is implemented in close proximity to the memory. Noise pick-
up in the V_SENSE trace can cause problems with precise
regulation of V_TT. A small 0.1uF ceramic 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 ex-
posed 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 impedance
of 100kΩ 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.
V_TT
V_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 de-
signed 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 section. 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 ca-
pable 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 max-
imum junction temperature is not exceeded. Proper thermal
de-rating should always be used (Please refer to the Thermal
Dissipation section).
Component Selections
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 capacitor 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 fre-
quency (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
www.national.com
8