Datasheet Linear Technology LTC3855

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Application Notes

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  PCB Layout Considerations for Non-Isolated Switching Power Supplies &mdash AN136
  PDF, 215 Kb, File published: Sep 20, 2012
  The best news when you power up a prototype supply board for the very first time is when it not only works, but also runs quiet and cool. Unfortunately, this does not always happen. A common problem of switching power supplies is “unstable” switching waveforms. Sometimes, waveform jittering is so pronounced that audible noise can be heard from the magnetic components. If the problem is related to the printed circuit board (PCB) layout, identifying the cause can be difficult. This is why proper PCB layout at the early stage of a switching supply design is very critical. Its importance cannot be overstated.
  Extract from the document

  Application Note 136
  June 2012
  PCB Layout Considerations for Non-Isolated Switching
  Power Supplies
  Henry J. Zhang
  Introduction
  The best news when you power up a prototype supply board
  for the very first time is when it not only works, but also
  runs quiet and cool. Unfortunately, this does not always
  happen. A common problem of switching power supplies
  is “unstable” switching waveforms. Sometimes, waveform
  jittering is so pronounced that audible noise can be heard
  from the magnetic components. If the problem is related
  to the printed circuit board (PCB) layout, identifying the
  cause can be difficult. This is why proper PCB layout at
  the early stage of a switching supply design is very critical.
  Its importance cannot be overstated.
  The power supply designer is the person who best understands the technical details and functional requirements of
  the supply within the final product. He or she should work
  closely with the PCB layout designer on the critical supply
  layout from the beginning. A good layout design optimizes
  supply efficiency, alleviates thermal stress, and most importantly, minimizes the noise and interactions among traces
  and components. To achieve these, it is important for the …

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  Modeling and Loop Compensation Design of Switching Mode Power Supplies &mdash AN149
  PDF, 1.4 Mb, File published: Mar 23, 2015
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  Application Note 149
  January 2015
  Modeling and Loop Compensation Design of
  Switching Mode Power Supplies
  Henry J. Zhang
  Introduction Identifying The Problem Today’s electronic systems are becoming more and more
  complex, with an increasing number of power rails and
  supplies. To achieve optimum power solution density,
  reliability and cost, often system designers need to design
  their own power solutions, instead of just using commercial power supply bricks. Designing and optimizing high
  performance switching mode power supplies is becoming
  a more frequent and challenging task. A well-designed switching mode power supply (SMPS)
  must be quiet, both electrically and acoustically. An undercompensated system may result in unstable operations.
  Typical symptoms of an unstable power supply include:
  audible noise from the magnetic components or ceramic
  capacitors, jittering in the switching waveforms, oscillation
  of output voltage, overheating of power FETs and so on. Power supply loop compensation design is usually
  viewed as a difficult task, especially for inexperienced
  supply designers. Practical compensation design typically
  involves numerous iterations on the value adjustment
  of the compensation components. This is not only time
  consuming, but is also inaccurate in a complicated
  system whose supply bandwidth and stability margin …

Design Notes

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  Dual Output Step-Down Controller Produces Accurate, Efficient and Reliable High Current Rails &mdash DN478
  PDF, 84 Kb, File published: Mar 23, 2010
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  Dual Output Step-Down Controller Produces 10% Accurate,
  Efficient and Reliable High Current Rails – Design Note 478
  Mike Shriver and Theo Phillips
  Introduction
  The LTC В®3855 makes it possible to generate high current rails with the accuracy and efficiency to satisfy the
  most demanding requirements of today’s leading edge
  network, telecommunications and server applications.
  This 2-phase, dual output synchronous buck controller
  includes strong gate drivers that support operation with
  per-phase currents above 20A. The accurate 0.6V В±0.75%
  reference and its integrated differential amplifier (diff
  amp) allow remote sensing of the output of critical rails.
  This controller has an output voltage range from 0.6V to
  12.5V when used without the diff amp and from 0.6V to
  3.3V with the diff amp.
  The LTC3855 uses the reliable peak current mode architecture to achieve a fast and accurate current limit and real
  time current sharing. Its current sense comparators are
  designed to sense the inductor current with either a sense
  resistor or with inductor DCR sensing. DCR sensing offers
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Articles

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  6mm × 6mm DC/DC Controller for High Current DCR Sensing Applications &mdash LT Journal
  PDF, 494 Kb, File published: Apr 1, 2010
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  6mm Г— 6mm DC/DC Controller for High Current
  DCR Sensing Applications
  Eric Gu, Theo Phillips, Mike Shriver and Kerry Holliday The LTC3855 is a versatile 2-phase synchronous buck controller IC with on-chip drivers,
  remote output voltage sensing and inductor temperature sensing. These features are
  ideal for high current applications where cycle-by-cycle current is measured across
  the inductor (DCR sensing). Either channel is suitable for inputs up to 38V and outputs
  up to 12.5V, further increasing the controller’s versatility. The LTC3855 is based on
  the popular LTC3850, described in the October 2007 issue of Linear Technology.
  MONITORING THE TEMPERATURE “lossless” method is less accurate than
  using a sense resistor, in large part because
  as the inductor heats up, its resistance
  increases with a temperature coefficient of resistivity (TCR) of 3930ppm/В°C.
  Therefore as the temperature rises, the
  current limit decreases. When DCR sensing When current is sensed at the inductor,
  either a sense resistor is placed in series
  with the inductor, or an R-C network
  across the inductor is used to infer the
  current information across the inductor’s DC resistance (DCR sensing). This is used, the current limit for the LTC3855
  is determined by the peak sense voltage as
  measured across the inductor’s DCR. The
  LTC3855 includes a temperature sensing
  scheme designed to compensate for the
  TCR of copper by effectively raising the …

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Manufacturer's Classification

• Power Management > Switching Regulator > Step-Down (Buck) Regulators > Multiple Output Buck | External Power Switch Buck Controllers