AND8079 Datasheet, PDF (2/20 Pages) ON Semiconductor – A Low Cost DDR Memory Power Supply Using the NCP1571 Synchronous Buck Converter and a LM358 Based Linear Voltage Regulator

AND8079/D
Many industry experts have predicted that DDR memory
will soon become the standard for desktop computers, with
notebooks shortly behind. Next generation DDR-II
generation systems are likely to have a lower V_dd voltage of
1.8 V with a V_tt and V_REF voltage equal to 900 mV. This
lower voltage will be required to satisfy the consumer’s
requirement for more memory without a large increase in
required power.
Supply Voltage (V_dd)
The V_dd 2.5 V power supply is created with the NCP1571
low voltage synchronous buck controller. The NCP1571
controller contains the required circuitry for a synchronous
N-channel MOSFET buck regulator. The V^2t control
method is used to achieve a fast 200 ns transient response
and an output regulation of ±1.0%. The IC operates at a fixed
internal frequency of 200 kHz. In addition, the NCP1571
provides the following features: undervoltage lockout
protection, programmable soft start, power good signal with
delay and overvoltage protection. Note the NCP1570 and
NCP1571 are functionally and pin for pin equivalent. The
NCP1571’s under voltage lockout operation (UVLO)
feature has been modified for applications that require a
parallel standby power supply in addition to the main power
supplied by the buck converter.
Termination Supply Voltage (V_tt) and Reference
Voltage (V_REF)
The V_tt supply voltage is equal to one half of the Vdd
voltage, or approximately 1.25 V. Operational amplifiers
U_2A and U_2B function as voltage followers to create the V_tt
voltage. The input to U_2B is created by the resistive voltage
divider formed by R₅ and R₆ and divides the 2.5 V V_dd
supply by two to form the V_REF reference voltage. Also, U_2B
provides filtering to remove any of the high frequency
switching noise that is results from the synchronous buck
converter. The V_tt output of the circuit formed by U_2A and
transistors Q₄ and Q₅ tracks the voltage at the non-inverting
terminal by virtue of the voltage follower circuit
configuration. Thus, the output of voltage of the V_tt supply
is referenced to 50% of the 2.5 V V_dd supply, rather than an
absolute 1.25 V reference.
The sink and source ability of the V_tt supply is provided
by MOSFETs Q₄ and Q₅ which are used to extend the
current capability of the operational amplifier circuit. When
the V_tt supply is in the current sinking mode of operation, Q₄
is “OFF” and Q₅ is “ON”. The output of U_2A will be at a
negative voltage (i.e. –5.0 V) to control the V_gs of the
P-channel MOSFET (Q₅) in order to maintain the V_tt
voltage of 1.25 V. In a similar manner, when the V_tt supply
is in the current sourcing mode of operation, Q₄ is “ON” and
Q₅ is “OFF”. The output of U_2A will reach a positive voltage
(i.e. + 4.5 V) to control the V_gs of the N-channel MOSFET
(Q₅) in order to maintain the V_tt voltage of 1.25 V. Resistor
R₇ is used to isolate the output of U_2B from V_tt and the bulk
capacitor C₂₀.
The slew rate of the operational amplifier and the ability
of the bulk capacitors to hold the voltage at 1.25 V under the
load conditions control the transient response of the V_tt
control loop. Note that the bulk capacitors maintain the V_tt
voltage at approximately 1.25 V; therefore, the operational
amplifier is only required to slew its output a relatively small
amount; therefore, the relatively slow slew rate of the
LM358 operational amplifiers is not a limiting factor in the
design.
Standby Power Operation
The demonstration PCB has the provision of providing a
low power standby mode of operation to the DDR memory
system. This mode could be used to provide a 2.5 V low
current standby voltage to the memory ICs when the main
5.0 V input power is not available. A MC33375 (U₃)
300 mA low dropout voltage regulator (LDO) was chosen
for the design to provide the 2.5 V standby power. The
MC33375 has an ON/OFF enable pin and is available in a
SOT-223 package. The performance of the standby
regulator was not verified.
Q₁, a N-Channel MOSFET, serves as a diode to prevent
current flow back to the main 5.0 volt input power supply
during the standby mode. The MOSFET was chosen instead
of a Schottky diode in order to minimize the voltage drop
and power consumption of the diode.
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