NCP5208
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DETAILED OPERATING DESCRIPTIONS
General
The NCP5208 is a linear regulator with both sink and
source current capabilities used for active termination of
fast switching logic, DDR−I/II SDRAM terminations and
active buses termination of SSTL−2/3 logic. This device
can be operated from a single supply voltage as low as 1.7
V. For DDR−I applications, the device is capable of
sourcing and sinking current up to 1.5 A with output
voltage regulated to within ±3%. The separate voltage
remote feedback pin ensures superior load and line
regulation with fast tracking capability.
Protective features include Soft−Start, Source/Sink
Current Limits and internal Thermal Shutdown.
Additionally, an open−drain VTT OK output signal (POK)
is provided for system monitoring. The shutdown pin (SD)
can be used to tri−state the regulator output for Suspend To
RAM (STR) state. This device is available in a low profile,
space saving SOIC−8 package.
Supply Voltage Inputs
For added flexibility, separate input pins are provided for
each required supply input. AVIN is the device operating
voltage, VDDQ is used to generate the internal reference
for VTT output voltage control and PVIN is the power rail
for the linear regulator. The device will regulate the output
voltage, VTT, with respect to the internal voltage reference
generated from VDDQ input and track the VDDQ changes
closely. The separate PVIN pin allows the designer to
isolate the high current PVIN line changes from coupling
into the noise sensitive AVIN and VDDQ inputs. Since the
AVIN supplies the control to the output power MOSFETs,
PVIN should always be lower than or equal to AVIN.
Generation of Internal Reference Voltage
The prime function of a termination regulator is to
provide a termination voltage, VTT at its output, which can
track the mid−point of the logic voltage level closely, i.e.
½(VDDQ). The VTT voltage is used to terminate the bus
resistors. The NCP5208 generates a precise reference
voltage internally with a built in dead−band to avoid upper
and lower MOSFET shoot through.
Remote Voltage Feedback Sensing
The NCP5208 has a separate feedback pin to monitor the
output voltage at the remote point. With this capability, the
output voltage can be controlled precisely at the output
capacitor so that any noise and fluctuations along the power
path can be eliminated.
Termination Voltage Output Regulation
The NCP5208 includes a simple linear series regulator
with a pair of control error amplifiers, which takes care of
the current source and sink operations separately. The error
amplifiers control a pair of MOSFETs to maintain the
output voltage equal to the internal reference voltage for
both current sink and source conditions. In order to avoid
the MOSFETs turning on at the same time, a dead−band is
implemented internally for safe operation.
Regulator Shutdown Function
The operation of the NCP5208 can be suspended by
pulling the Shutdown (SD) pin to ground. When the device
is stopped, the regulator output will be tri−stated for
Suspend To RAM (STR) state in PC applications. The
shutdown pin is internally pulled high by a small current
source, if this feature is not used, this pin can be left open.
VTT Power OK for System Monitoring
NCP5208 provides an additional VTT power OK signal
for system monitoring. The VTT Power OK (POK) pin
goes low when the VTT voltage is in regulation and has
settled within the allowed window. For memory
termination applications, the system can check this pin to
ensure the termination voltage quality before accessing the
memory. The POK output is connected to a open−drain
switching FET and the designer is free to pull this pin to any
logic voltage level externally. When the VTT output is in
regulation, the internal FET is turned on and pulls the
power OK pin to ground.
Over−current Protection for Sink and Source Operation
In order to provide protection for the internal power
MOSFETs, bi−directional current limit protection circuits
are implemented. Current limit levels are internally set at
2.1 A typical for current source and 2.0 A typical for current
sink at 2.5 V operation.
Thermal Shutdown with Hysteresis
To guarantee safe operation, NCP5208 provides on−chip
thermal shutdown protection. When the chip junction
temperature exceeds 135_C typical, the part will
shutdown. When the junction temperature falls back to
105_C typical, the device resumes normal operation.
