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FUNCTIONAL DESCRIPTION
INTRODUCTION
FUNCTIONAL DESCRIPTION
INTRODUCTION
In modern microprocessor/memory applications, address
commands and control lines require system level termination
to a voltage (V TT ) equal to 1/2 the memory supply voltage
(V DDQ ). Having the termination voltage at midpoint, the power
supply insures symmetry for switching times. Also, a
reference voltage (V REF ) that is free of any noise or voltage
variations is needed for the DDR SDRAM input receiver,
V REF is also equal to 1/2 V DDQ . Varying the V REF voltage will
effect the setup and hold time of the memory. To comply with
DDR requirements and to obtain best performance, V TT and
V REF need to be tightly regulated to track 1/2 V DDQ across
voltage, temperature, and noise margins. V TT should track
any variations in the DC V REF value (V TT = V REF +/- 40mV),
(See Figure 4 ) for a DDR system level diagram.
The 34712 supplies the V TT and a buffered V REF output.
To ensure compliance with DDR specifications, the V DDQ line
is applied to the VREFIN pin and divided by 2 internally
through a precision resistor divider. This internal voltage is
then used as the reference voltage for the V TT output. The
same internal voltage is also buffered to give the V REF
voltage at the VREFOUT pin for the application to use without
continuous current. It provides protection against output
over-current, over-voltage, under-voltage, and over-
temperature conditions. It also protects the system from short
circuit events. It incorporates a power-good output signal to
alert the host when a fault occurs.
For boards that support the Suspend-To-RAM (S3) and
the Suspend-To-Disk (S5) states, the 34712 offers the STBY
and the SD pins respectively. Pulling any of these pins low,
puts the IC in the corresponding state.
By integrating the control/supervisory circuitry along with
the Power MOSFET switches for the buck converter into a
space-efficient package, the 34712 offers a complete, small-
size, cost-effective, and simple solution to satisfy the needs
of DDR memory applications.
Besides DDR memory termination, the 34712 can be used
to supply termination for other active buses and graphics card
memory. It can be used in Netcom/Telecom applications like
servers. It can also be used in desktop motherboards, game
consoles, set top boxes, and high end high definition TVs.
the need for an external resistor divider. The 34712 provides
the tight voltage regulation and power sequencing/tracking
V D DQ
V TT
V DD Q
required along with handling the DDR peak transient current
requirements. Buffering the V REF output helps its immunity
against noise and load changes.
The 34712 utilizes a voltage mode synchronous buck
switching converter topology with integrated low R DS(ON)
(50 m ? ) N-channel power MOSFETs to provide a V TT voltage
with an accuracy of less than ± 2.0%. It has a programmable
R S
BUS
R T
V REF
switching frequency that allows for flexibility and optimization
over the operating conditions and can operate at up to
1.0 MHz to significantly reduce the external components size
DDR Memory Controller
DDR Memory Input Receiver
and cost. The 34712 can sink and source up to 3.0 A of
Figure 4. DDR System Level Diagram
FUNCTIONAL PIN DESCRIPTION
REFERENCE VOLTAGE INPUT (VREFIN)
The 34712 will track 1/2 the voltage applied at this pin.
REFERENCE VOLTAGE OUTPUT (VREFOUT)
This is a buffered reference voltage output that is equal to
1/2 V REFIN . It has a 10 mA current drive capability. This
output is used as the V REF voltage rail and should be filtered
against any noise. Connect a 0.1 μF, 6.0 V low ESR ceramic
filter capacitor between this pin and the GND pin and
FREQUENCY ADJUSTMENT INPUT (FREQ)
The buck converter switching frequency can be adjusted
by connecting this pin to an external resistor divider between
VDDI and GND pins. The default switching frequency (FREQ
pin connected to ground, GND) is set at 1.0 MHz.
SIGNAL GROUND (GND)
Analog ground of the IC. Internal analog signals are
referenced to this pin voltage.
between this pin and V DDQ rail. V REFOUT is also used as the
reference voltage for the buck converter error amplifier.
34712
Analog Integrated Circuit Device Data ?
10
Freescale Semiconductor
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