FAN5098 Two Phase Interleaved Synchronous Buck Converter

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FAN5098Two Phase Interleaved Synchronous Buck
Converter

• By
• Ed Torrente
• EE136

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APPLICATION

• Programmable synchronous multi-phase DC-DC
  controller IC.
• Can deliver 40A of output current when designed
  with proper components.
• Functions as a frequency PWM step down regulator
  with High Efficiency mode (E) at light load.

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FEATURES

• Programmable output form 800mV to 1.550V in 15mV
  steps using an integrated 5-bit DAC.
• Two interleaved synchronous phases with maximum
  performance
• 100ns response time
• Built-in current sharing between phases.

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FEATURES

• Programmable Active Droop (Voltage Positioning)
• Switching frequency can be programmed from 100kHz
  to 1MHz per phase
• Integrated high-current gate drivers

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FEATURES

• Integrated Power Good, OV, UV, Enable/Soft Start
  functions
• Drives N-channel MOSFETs
• Operation optimized for 12V
• High efficiency mode (E) at light load

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INTENDED MARKET

• This programmable step-down power supply is
  intended for the AMD Athlon and Hammer
  microprocessors.

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CIRCUIT DIAGRAM
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BLOCK DIAGRAM
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CIRCUIT ANALYSIS
Signal conditioning amplifier

• Consists of comparators feeding into signal
  conditioning amplifiers that provides the input
  to the digital control block.
• The signal conditioning section accepts inputs
  from a current sensor and a voltage sensor.

Comparators
Signal conditioning amplifier
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CIRCUIT ANALYSIS

• The voltage sensor amplifies the difference
  between the VFB signal and the reference voltage
  of the DAC and presents the output to each of the
  conditioning blocks.

Voltage sensor
VFB
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CIRCUIT ANALYSIS

• The current control path for each phase takes the
  difference between PGND and SW pins when the
  low-side MOSFET is on, reproducing the voltage
  across the MOSFET and thus the input current. It
  represents the resulting signal to the
  comparators, adding its signal to the voltage
  amplifier signal with a certain gain resulting in
  two signals being added.

SW
PGND
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CIRCUIT ANALYSIS

• The sum is then presented to the Signal
  conditioning blocks along with the oscillator
  ramp signal, which provides the main PWM control
  to the digital control block.

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CIRCUIT ANALYSIS

• The oscillator ramps are 180 out of phase from
  each other such that the two phases are on
  alternately.
• The digital control blocks takes the signal from
  the Signal conditioning amplifiers to provide the
  appropriate pulses to the HDRV and LDRV output
  pins for each phase.

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MEASUREMENTS(Light load efficiency)

• During light load, current will flow away from
  its output and towards the input.
• This reverse current flow is seen as a positive
  voltage on the low-side MOSFET.

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MEASUREMENTS(Normal Operation)

• High-side Gate during normal operation
• Note the 180 out of phase for fast transient
  response.

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DEVICE SELECTION

• Requires N-channel Enhancement mode FETs.
• RDS,(on)
• Drain-Source voltage rating15V

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DEVICE SELECTION(Gate Resistors)

• Use the gate resistors are mandatory for all
  MOSFETs and should be placed as close as possible
  to the MOSFETs.
• Gate resistors also limits power dissipation
  inside the IC which would result in switching
  frequency limitations.

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DEVICE SELECTION(Inductors)

• In choosing the inductor value there is a
  trade-off between the allowable ripple voltage
  and required transient response.
• Choosing a smaller inductor value will be best
  since it will produce greater ripple while
  producing better transient response.
• Typical values of inductors are 1.3µH at an
  oscillator frequency of 600kHz.

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DEVICE SELECTION(Output Filter Capacitors )

• Output bulk capacitor helps determine the output
  ripple and its transient response time.
• Most commonly used are electrolytics for their
  low cost and low ESR.
• The output capacitance should also include a
  number of small value ceramic capacitors
  preferably 0.1µF and 0.01µF.

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CONCLUSION

• Capable of producing up to 40A of current to
  supply the microprocessor.
• RDS,(on)RDS,(on) 20mO.
• Competitors include Texas Instrument, Maxim, and
  National Semiconductor.

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ACKNOWLEDGEMENTS

• Professor Zhou
• Mr. Cosimo Friolo
• Ms.Trina Noor
• Professor Ghadiri
• Fairchild Semiconductor