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Application Notes

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  OPTI-LOOP Architecture Reduces Output Capacitance and Improves Transient Response &mdash AN76
  PDF, 271 Kb, File published: May 1, 1999
  Loop compensation is an uncomfortable subject for many engineers. Experienced power supply designers know that optimum loop compensation is necessary to get the best performance from their power supplies. This application note discusses power supply loop compensation utilizing the features provided by the OPTILOOPTM architecture. Loop compensation basics are presented and simple equations are given for frequency response approximations. Typical transient response requirements for the system supply and CPU supply, used in notebook computers, are discussed. Output voltage transient response waveforms and Bode plots are shown for both optimized and nonoptimized control loops as well as for circuits with optimized loops using different output capacitors. Although this publication focuses on circuits using the LTC1628, LTC1735 and LTC1736, the information applies to all regulators equipped with OPTI-LOOP architecture.
  Extract from the document

  Application Note 76
  May 1999
  OPTI-LOOP Architecture Reduces Output Capacitance and
  Improves Transient Response
  John Seago
  INTRODUCTION WHAT IS LOOP COMPENSATION? Removing output capacitors saves money and board
  space. Linear Technology’s OPTI-LOOPTM architecture
  allows you to use the output capacitors of your choice and
  compensate the control loop for optimum transient
  response and loop stability. Figure 1 shows the dramatic
  improvement possible with the OPTI-LOOP architecture.
  With the improvement shown in Figure 1, less capacitance
  is required or less expensive capacitors can be used. Load
  dynamics and supply tolerances will predict the minimum
  output capacitance required. The quality of loop compensation determines how close you can get to this minimum
  capacitance requirement in a real world system. Loop compensation is the adjustment of the control loop
  frequency response to assure loop stability and optimize
  the transient response of the power supply. The frequency
  response is determined by the gain and phase of the loop’s
  reaction to load current changes at all frequencies. A Bode
  plot is used to show the gain and phase of the frequency
  response. OUTPUT VOLTAGE (50mV/DIV) One of the least understood areas of power supply design
  is control loop compensation. Because of this, some …

Design Notes

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  Active Voltage Positioning Reduces Output Capacitors &mdash Design Solutions 10
  PDF, 130 Kb, File published: Nov 1, 1999
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  Design Solutions 10
  November 1999
  Active Voltage Positioning Reduces Output Capacitors
  by Robert Sheehan
  INTRODUCTION
  Power supply performance, especially transient response,
  is key to meeting today’s demands for low voltage, high
  current microprocessor power. In an effort to minimize
  the voltage deviation during a load step, a technique that
  has recently been coined “active voltage positioning” is
  generating substantial interest and gaining popularity in
  the portable computer market. The benefits include lower
  peak-to-peak output voltage deviation for a given load
  step, without having to increase the output filter capacitance. Alternatively, the output filter capacitance can be
  reduced while maintaining the same peak-to-peak transient response.
  BASIC PRINCIPLE
  The term “active voltage positioning” (AVP) refers to
  setting the power supply output voltage at a point that is
  dependent on the load current. At minimum load, the
  output voltage is set to a slightly higher than nominal level.
  At full load, the output voltage is set to a slightly lower than
  nominal level. Effectively, the DC load regulation is
  degraded, but the load transient voltage deviation will be …

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  LTC1735 Provides Low Cost, Efficient Mobile CPU Power &mdash DN199
  PDF, 78 Kb, File published: Mar 1, 1999
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  advertisement A Low Cost, Efficient Mobile CPU Power – Design Note 199
  Randy G. Flatness
  The LTC ®1735 is the newest member of Linear Technology’s third generation of synchronous step-down
  controllers. This controller uses the same constant
  frequency, current mode architecture and Burst
  ModeВ® operation as the industry standard LTC1435/
  LTC1437 controllers but with improved features. With
  OPTI-LOOPВ® compensation, new protection circuitry,
  tighter load regulation and stronger MOSFET drivers, the
  LTC1735 is ideal for the current and future generations
  of mobile CPUs. A companion part, the LTC1736, has
  all the features of the LTC1735 plus 5-bit VID voltage
  programming according to Intel mobile processor
  specifications.
  The LTC1735 is pin compatible with the previous generation LTC1435/LTC1435A controllers with only minor
  external component value changes. New protection
  features include internal foldback current limiting,
  output overvoltage crowbar and optional short-circuit
  shutdown. The 0.8V reference supports the low output voltages and 1% accuracy that will be demanded
  by future microprocessors. A constant operating
  frequency (synchronizable up to 500kHz) is set by an
  external capacitor, COSC, allowing maximum flexibility
  in optimizing efп¬Ѓciency. …

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  2-Step Voltage Regulation Improves Performance and Decreases CPU Temperature in Portable Computers &mdash DN209
  PDF, 79 Kb, File published: Aug 1, 1999
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  advertisement 2-Step Voltage Regulation Improves Performance and
  Decreases CPU Temperature in Portable Computers
  Design Note 209
  John Seago
  2-step regulation allows CPU power supply optimization
  in portable applications. Higher CPU clock frequencies
  mean lower core voltages, higher supply currents and more
  CPU power dissipation. Regulating CPU power from 5V
  reduces regulator switching losses permitting a higher
  switching frequency. Higher frequency operation results
  in smaller inductor size, fewer output capacitors and
  better transient response. OPTI-LOOPВ® compensation
  and a 3.5V to 36V input range make the LTC В®1736 a
  good choice for either 1-step or 2-step configurations. CPU core voltages are currently in the 1.5V region.
  Using the 1-step approach, regulating 24V down to
  1.5V forces the regulator to regulate narrow “slivers”
  of input current to meet the transient requirements of
  high speed CPUs. A 24V wall adapter forces a 6.25%
  duty cycle when supplying a CPU voltage of 1.5V which
  means that the top MOSFET conducts for 0.2Ојs each
  cycle, at 300kHz.
  Using 2-step regulation, CPU voltages are regulated
  from the 5V system supply. Decreasing the input …

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  Microprocessor Core Supply Voltage Set by I2C Bus Without VID Lines &mdash DN279
  PDF, 75 Kb, File published: Mar 1, 2002
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  advertisement Microprocessor Core Supply Voltage Set by I2C Bus
  without VID Lines – Design Note 279
  Mark Gurries
  Introduction
  Many modern CPUs run at two different clock speeds,
  where each speed requires a different core operating
  voltage to assure optimum performance. These voltages
  are documented in the manufacturer’s VID (voltage
  identification) section of the CPU specification. Some
  new DC/DC converters (the LTCВ®1909, for example) have
  built-in VID control that supports dual programmable
  output voltages, but many existing converters do not.
  The LTC1699 is a precision 2-state resistive divider that
  uses a simple SMBus interface to allow VID control on
  non-VID-enabled DC/DC converters. No dedicated VID
  lines are required.
  How it Works
  DC/DC converters maintain consistent output voltage
  by comparing the output voltage, through an accurate
  voltage divider, against an internal reference and adjusting the output to compensate for differences. The
  LTC1699 is a 2-state resistive divider that replaces the
  п¬Ѓxed voltage divider in the feedback circuit, thus allowing the circuit to support two different voltage outputs.
  It is speciп¬Ѓcally designed to work with DC/DC PWM …

Articles

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  Third-Generation DC/DC Controllers Reduce Size and Cost &mdash LT Journal
  PDF, 140 Kb, File published: Feb 1, 1999
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  LINEAR TECHNOLOGY
  VOLUME IX NUMBER 1 FEBRUARY 1999 IN THIS ISSUE…
  COVER ARTICLE
  Third-Generation DC/DC Controller
  Reduces Size and Cost . 1
  Randy G. Flatness Issue Highlights . 2
  LTCВ® in the News . 2
  DESIGN FEATURES
  New Universal Continuous-Time Filter
  with Extended Frequency Range . 7
  Max W. Hauser SOT-23 Switching Regulators
  Deliver Low Noise Outputs
  in a Small Footprint . 11
  Steve Pietkiewicz Versatile New Switching Regulator
  Fits in SO-8 . 14
  Craig Varga 16-Bit Parallel DAC Has 1LSB
  Linearity, Ultralow Glitch and
  Accurate 4-Quadrant Resistors . 18
  Patrick Copley Fast Rate Li-Ion Battery Charger
  . 24
  Goran Perica DESIGN IDEAS
  No RSENSE Controller Delivers 12V and
  100W at 97% Efficiency . 26 …

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  Active Voltage Positioning Saves Output Capacitors in Portable Computer Applications &mdash LT Journal
  PDF, 169 Kb, File published: Feb 1, 2000
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  DESIGN IDEAS Active Voltage Positioning Saves
  Output Capacitors in Portable by John Seago
  and Ajmal Godil
  Computer Applications
  Introduction
  pp. 16–20) can take advantage of
  active voltage positioning. Active voltage positioning is a technique that can be used to save cost
  and space by reducing the number of
  output capacitors required to meet a
  microprocessor’s power supply
  requirements. Total system cost and
  required PCB space are important
  aspects of today’s portable equipment
  designs, so decreasing the number of
  large, expensive output capacitors is
  worth some effort. Both the LTC1735/
  LTC1736 current mode switching
  regulator controllers (Linear Technology IX:1, February 1999, pp. 1, 3–5,
  35) and the LTC1702/LTC1703
  voltage mode controllers (Linear Technology IX:3, September 1999, Microprocessor Load Steps
  Microprocessors frequently change
  their load current requirement from
  almost no load to maximum load current and back again very quickly. The …

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  LTC1645/LTC1735 Circuit Solves PCI Power Problem &mdash LT Journal
  PDF, 232 Kb, File published: Feb 1, 2000
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  DESIGN IDEAS LTC1645/LTC1735 Circuit Solves
  PCI Power Problem
  by Ajmal Godil
  to plugging a circuit board into or
  removing it from a live backplane.
  When this is done, the supply bypass
  capacitors on the board can draw
  huge transient currents from the
  backplane power bus as they charge.
  The transient currents can cause permanent damage to the connector pins In some applications, it is necessary to select and hot swap the higher
  of two supplies and generate a regulated output voltage from the selected
  supply. If only one input supply is
  present, the circuit should select it
  and generate the same output voltage. The term “hot swapping” refers Q1-A
  1/2 Si4936 VIN1
  5V
  R1
  10k D1
  MBR0520 Q3-A
  1/2 Si4936 VIN2
  3.3V and cause glitches on the system
  supply, causing other boards in the
  system to reset. A circuit based on an …

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  A Third Generation Dual, Opposing-Phase Switching Regulator Controller &mdash LT Journal
  PDF, 172 Kb, File published: Jun 1, 1999
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  DESIGN FEATURES A Third Generation Dual, OpposingPhase Switching Regulator Controller
  by Steve Hobrecht Introduction
  The LTC1628 is the newest member
  of Linear Technology’s third
  generation of DC/DC controllers. (The
  latest single-output members include
  the LTC1735, LTC1735-1 and the 5bit VID controlled LTC1736.) These
  controllers use a similar constant
  frequency, current mode architecture
  and Burst Mode operation as the
  previous generation LTC1435–
  L TC1439 controllers but with
  improved features. A key new feature
  is control circuitry that drives the two
  top switches 180 degrees out of phase
  to minimize the input capacitance
  requirement and reduce conducted
  and radiated EMI. The resultant input ripple is effectively half the
  amplitude and double the frequency
  of nonphased controllers, significantly
  reducing the input capacitance
  requirement. OPTI-LOOPв„ў compensation, 5V and 3.3V standby
  regulators, new protection circuitry, …

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  SMBus Controls CPU Voltage Regulators without VID Pins &mdash LT Journal
  PDF, 148 Kb, File published: Sep 1, 2001
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  DESIGN IDEAS SMBus Controls CPU Voltage
  Regulators without VID Pins by Mark Gurries Introduction
  The LTC1699 allows a variety of nonVID equipped voltage regulators to be
  programmed to VID voltages via the
  SMBus. Many microprocessors
  require core voltage changes when
  switching from one clock frequency to
  another. These voltages are specified
  in 5-bit VID (voltage identification)
  tables. The LTC1699 is a precision,
  digitally programmed resistor divider
  that provides output voltage feedback to PWM controllers that have a
  reference voltage of 0.8V, such as the
  LTC1702, LTC1628, LTC1735 and
  the new LTC1778. It uses the SMBus
  to determine the two output voltages
  that can be selected by the SEL pin. A
  Power Good signal is provided to tell
  the system that CPU voltage is in
  regulation. An enhanced version of
  the part, the LTC1699EGN, expands
  the power sequencing to coordinate
  multiple DC/DC converters that manage other CPU system voltages such …

Model Line

Manufacturer's Classification

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

Other Names:

LTC1735ISTRPBF, LTC1735IS TRPBF