ADP3208 ON Semiconductor, ADP3208 Datasheet - Page 15

• Manufacturer Part Number: ADP3208
• Description: 7-bit, Programmable, Dual-phase, Mobile, Cpu, Synchronous Buck Controller
• Manufacturer: ON Semiconductor

THEORY OF OPERATION
The ADP3208 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 ADP3208 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 ADP3208 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), ADP3208 switches to single-phase mode to maximize the
power conversion efficiency.
In addition to changing the number of phases, the ADP3208 is
also capable of dynamically changing the control method. In
dual-phase operation, the ADP3208 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
ADP3208 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 between the VRPM and
RPM pins. 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 ADP3208 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 ADP3208 dynamically changes the
number of active phases and transitions the operation mode
based on system signals and operating conditions.
CORE
output voltage
ADP3208