Datasheet

LP2998/LP2998-Q1
SNVS521J –DECEMBER 2007–REVISED DECEMBER 2013 www.ti.com
Pin Descriptions
AVIN AND PVIN AVIN and PVIN are the input supply pins for the LP2998. 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, under the condition that AVIN is always greater than or equal to PVIN. For
SSTL-18 applications, it is recommended to connect PVIN to the 1.8V rail used for the memory core and AVIN to a rail
within its operating range of 2.2V to 5.5V (typically a 2.5V supply). PVIN should always be used with either a 1.8V or
2.5V rail. This prevents the thermal limit from tripping because of excessive internal power dissipation. If the junction
temperature exceeds the thermal shutdown threshold, the part will enter a shutdown state identical to the manual
shutdown where V
TT
is tri-stated and V
REF
remains active. A lower rail, such as 1.5V can be used but it will reduce the
maximum output current. Therefore it is not recommended for most termination schemes.
VDDQ VDDQ 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 ensures that V
TT
will precisely track VDDQ / 2. The optimal
implementation of VDDQ is as a remote sense. This can be achieved by connecting VDDQ directly to the 1.8V rail at
the DIMM instead of PVIN. This ensures that the reference voltage precisely tracks the DDR memory rails without a
large voltage drop from the power lines. For SSTL-18 applications, VDDQ will be a 1.8V signal, which will create a 0.9V
termination voltage at V
TT
(See Electrical Characteristics Table for exact values of V
TT
over temperature).
The purpose of the sense pin is to provide improved remote load regulation. In most motherboard applications, the
V
SENSE
termination resistors will connect to V
TT
in a long plane. If the output voltage was regulated only at the output of the
LP2998, 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 ceramic capacitor placed next to the V
SENSE
pin can help filter any high frequency signals and prevent errors.
SHUTDOWN The LP2998 contains an active low shutdown pin that can be used for suspend to RAM functionality. In this condition,
the V
TT
output will tri-state while the V
REF
output remains active providing a constant reference signal for the memory
and chipset. During shutdown, V
TT
should not be exposed to voltages that exceed PVIN. With the shutdown pin
asserted low the quiescent current of the LP2998 will drop. However, VDDQ 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
REF
V
REF
provides the buffered output of the internal reference voltage VDDQ / 2. This output should be used to provide the
reference voltage for the Northbridge chipset and memory. Since the inputs typically have an extremely high
impedance, there should be little current drawn from V
REF
. For improved performance, an output bypass capacitor can
be placed, close to the pin, to help with noise. A ceramic capacitor in the range of 0.1 µF to 0.01 µF is recommended.
This output remains active during the shutdown state and thermal shutdown events for the suspend to RAM
functionality.
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 VDDQ / 2. The LP2998 is designed to handle continuous currents of up to +/- 1.5A
with excellent load regulation. 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. If the LP2998 is to operate in elevated temperatures for long durations, care should be taken to ensure that the
maximum operating junction temperature is not exceeded. Proper thermal de-rating should always be used (Please
refer to the Thermal Dissipation section). If the junction temperature exceeds the thermal shutdown threshold, V
TT
will
tri-state until the part returns below the temperature hysteresis trip-point.
Component Selections
INPUT CAPACITOR
The LP2998 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 and is dependent on the application required.
A typical value recommended for AL electrolytic capacitors is 22 µF. Ceramic capacitors can also be used. A
value in the range of 10 µF with X5R or better would be an ideal choice. The input capacitance can be reduced if
the LP2998 is placed close to the bulk capacitance from the output of the 1.8V DC-DC converter. For the AVIN
pin, a small 0.1uF ceramic capacitor is sufficient to prevent excessive noise from coupling into the device.
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