NCP51198, NCV51198
www.onsemi.com
7
APPLICATIONS INFORMATION
General
The NCP/NCV51198 is a bus termination, linear
regulator designed to meet the JEDEC requirements for
DDR−I, DDR−II and DDR−III memory termination. The
NCP/NCV51198 is capable of sourcing and sinking current
while accurately tracking and regulating the V
TT
output
voltage equal to (V
DDQ
/ 2). The output stage has been
designed to maintain excellent load regulation and
preventing shoot−through. The NCP/NCV51198 uses two
distinct power rails to separate the analog circuitry from the
power output stage and decrease internal power dissipation.
Supply Voltage Inputs
For added flexibility, separate input pins (V
CC
and PV
CC
)
are provided for each required supply input. V
CC
is used to
supply all the internal control circuitry and PV
CC
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 with the condition that V
CC
is always
greater than or equal to PV
CC
, and should always be used
with either a 1.8 V or 2.5 V rail. 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. Lower voltage
rails, such as 1.5 V can be used but will reduce the maximum
available output current.
Generation of Internal Voltage Reference
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 50 kW
resistors. This guarantees that V
TT
will precisely track
(V
DDQ
/ 2). The optimal implementation of the V
DDQ
input
pin is as a remote sense. This can be achieved by connecting
V
DDQ
directly to the 1.8 V rail at the DIMM memory
module instead of connecting it to PV
CC
. This ensures that
the reference voltage precisely tracks the DDR memory
power rail without introducing a large voltage drop due to
power traces. For DDR−II applications the V
DDQ
input will
be 1.8 V, which will create a (V
DDQ
/ 2) = 0.9 V termination
voltage at the V
TT
output.
V
REF
provides a buffered output of the internal reference
voltage (V
DDQ
/ 2). For improved performance, an output
bypass capacitor can be placed, close to the pin, to help
reduce any potential stray noise. A ceramic capacitor in the
range of 0.01 mF to 0.1 mF is recommended. The V
REF
output
remains active during the shutdown state and thermal
shutdown events for the suspend to RAM functionality.
Remote Voltage Feedback Sensing
The purpose of the V
TTS
sense pin is to provide improved
remote load regulation. In most motherboard applications,
the termination resistors will connect to V
TT
in a long plane.
If the output voltage was regulated only at the output of the
NCP/NCV51198, then any long traces will generate a
significant IR drop resulting in a sagging termination
voltage at one end of the bus than the other. The V
TTS
pin can
be used to improve performance by connecting it to the
middle of the bus. This will provide better power
distribution across the entire termination bus. If remote load
regulation is not used, then the V
TTS
pin must still be
connected to V
TT
. Care should be taken when a long V
TTS
trace is implemented in close proximity to the memory.
Noise pickup in the V
TTS
trace can cause problems with
precise regulation of V
TT
. A small 0.1 mF ceramic capacitor
placed next to the V
TTS
pin can help filter out any high
frequency noise and thereby keeping the V
TT
power rail in
spec.
Regulator Shutdown Function
The NCP/NCV51198 contains an active low enable pin
(/SS) that can be used for suspend to RAM functionality. In
this condition the V
TT
output will tri−state, with the V
REF
output remaining active in order to provide a constant
reference signal for the memory and chipset. During
shutdown, V
TT
should not be exposed to voltages that
exceed PV
CC
.
With the enable pin asserted low the quiescent current of
the NCP/NCV51198 will drop, however the V
DDQ
input pin
will always draw a constant current due to the integrated
100 kW impedance used for generating the internal
reference. Therefore, to calculate the total power loss in
shutdown, both currents need to be considered. The enable
pin also has an internal pull−up current. Therefore, to turn
the part on, the enable pin can either be connected to V
CC
or
left open.
Termination Voltage Output Regulation
V
TT
is the regulated output that is used to terminate the
bus resistors. It is capable of sourcing and sinking current
while regulating the output precisely to V
DDQ
/ 2. The
NCP/NCV51198 is designed to handle continuous currents
of up to ±1.5 A 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.
Thermal Shutdown with Hysteresis
If the NCP/NCV51198 is to operate in elevated
temperatures for long durations, care should be taken to
ensure that the maximum operating junction temperature is
not exceeded. To guarantee safe operation, the
NCP/NCV51198 provides on−chip thermal shutdown
protection. When the chip junction temperature exceeds
165°C (typical) the part will shutdown. When the junction
temperature falls back to 155°C (typical) the device resumes
normal operation. If the junction temperature exceeds the
thermal shutdown threshold, V
TT
will tri−state until the part
returns below the temperature hysteresis trip−point.
