ADP3208C ON Semiconductor, ADP3208C Datasheet - Page 19

• Manufacturer Part Number: ADP3208C
• Description: 7-bit,programmable,dual- Phase,mobile,cpu,synchronous Buck Controller
• Manufacturer: ON Semiconductor

THEORY OF OPERATION
The ADP3208C combines multimode pulse-width-modulated
(PWM) control and ramp-pulse-modulated (RPM) control with
multiphase logic outputs for use in single- and dual-phase
synchronous buck CPU core supply power converters. The
internal 7-bit VID DAC conforms to the Intel IMVP-6+
specifications.
Multiphase operation is important for producing the high
currents and low voltages demanded by today’s microprocessors.
Handling high currents in a single-phase converter would put
too high of a thermal stress on system components such as the
inductors and MOSFETs.
The multimode control of the ADP3208C is a stable, high
performance architecture that includes
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NUMBER OF PHASES
The number of operational phases can be set by the user. Tying
the SP pin to the VCC pin forces the chip into single-phase
operation. Otherwise, dual-phase operation is automatically
selected, and the chip switches between single- and dual-phase
modes as the load changes to optimize power conversion efficiency.
In dual-phase configuration, SP is low and the timing
relationship between the two phases is determined by internal
circuitry that monitors the PWM outputs. Because each phase
is monitored independently, operation approaching 100% duty
cycle is possible. In addition, more than one output can be
active at a time, permitting overlapping phases.
Current and thermal balance between phases
High speed response at the lowest possible switching
frequency and minimal count of output decoupling capacitors
Minimized thermal switching losses due to lower frequency
operation
High accuracy load line regulation
High current output by supporting 2-phase operation
Reduced output ripple due to multiphase ripple cancellation
High power conversion efficiency with heavy and light loads
Increased immunity from noise introduced by PC board
layout constraints
Ease of use due to independent component selection
Flexibility in design by allowing optimization for either low
cost or high performance
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OPERATION MODES
The number of phases can be static (see the Number of Phases
section) or dynamically controlled by system signals to
optimize the power conversion efficiency with heavy and light
loads.
If SP is set low (user-selected dual-phase mode) during a VID
transient or with a heavy load condition (indicated by DPRSLP
being low and PSI being high), the ADP3208C runs in 2-phase,
interleaved PWM mode to achieve minimal V
ripple and the best transient performance possible. If the load
becomes light (indicated by PSI being low or DPRSLP being
high), ADP3208C switches to single-phase mode to maximize
the power conversion efficiency.
In addition to changing the number of phases, the ADP3208C is
also capable of dynamically changing the control method. In
dual-phase operation, the ADP3208C runs in PWM mode,
where the switching frequency is controlled by the master clock.
In single-phase operation (commanded by the PSI low state),
the ADP3208C runs in RPM mode, where the switching
frequency is controlled by the ripple voltage appearing on the
COMP pin. In RPM mode, the DRVH1 pin is driven high each
time the COMP pin voltage rises to a voltage limit set by the
VID voltage and an external resistor connected from the RPM
to GND. If the device is in single-phase mode and the system
signal DPRSLP is asserted high during the deeper sleep mode of
CPU operation, the ADP3208C continues running in RPM
mode but offers the option of turning off the low-side
(synchronous rectifier) MOSFET when the inductor current
drops to 0. Turning off the low-side MOSFETs at the zero
current crossing prevents reversed inductor current build up
and breaks synchronous operation of high- and low-side
switches. Due to the asynchronous operation, the switching
frequency becomes slower as the load current decreases,
resulting in good power conversion efficiency with very light
loads.
Table 4 summarizes how the ADP3208C dynamically changes
the number of active phases and transitions the operation mode
based on system signals and operating conditions.
ADP3208C
CORE
output voltage