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LT1085 LT1085

Application Notes

• Download
  Step Down Switching Regulators &mdash AN35
  PDF, 6.2 Mb, File published: Aug 1, 1989
  Discusses the LT1074, an easily applied step-down regulator IC. Basic concepts and circuits are described along with more sophisticated applications. Six appended sections cover LT1074 circuitry detail, inductor and discrete component selection, current measuring techniques, efficiency considerations and other topics.
  Extract from the document

  Application Note 35
  August 1989
  Step-Down Switching Regulators
  Jim Williams
  lost in this voltage-to-current-to-magnetic п¬Ѓeld-to-current-to-charge-to-voltage conversion. In practice, the
  circuit elements have losses, but step-down efficiency is
  still higher than with inherently dissipative (e.g., voltage
  divider) approaches. Figure 2 feedback controls the basic
  circuit to regulate output voltage. In this case switch ontime (e.g., inductor charge time) is varied to maintain the
  output against changes in input or loading.
  REGULATED
  OUTPUT IN PULSE
  WIDTH
  MODULATOR Figure 1 is a conceptual voltage step-down or “buck”
  circuit. When the switch closes the input voltage appears
  at the inductor. Current flowing through the inductor-capacitor combination builds over time. When the switch
  IN OUT AN35 F01 Figure 1. Conceptual Voltage Step-Down (“Buck”) Circuit opens current flow ceases and the magnetic field around
  the inductor collapses. Faraday teaches that the voltage
  induced by the collapsing magnetic п¬Ѓeld is opposite to the
  originally applied voltage. As such, the inductor’s left side
  heads negative and is clamped by the diode. The capacitors accumulated charge has no discharge path, and a DC
  potential appears at the output. This DC potential is lower
  than the input because the inductor limits current during …

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  5V to 3.3V Converters for Microprocessor Systems &mdash AN58
  PDF, 310 Kb, File published: Sep 1, 1993
  Many popular microprocessors operate from 3.3V supplies, yet they are used in systems where the predominate source of power is 5V. AN58 presents a collection of both linear and switching regulator solutions for conversion of 5V to 3.3V at currents ranging from 100mA to 20A. Applications information and a comparison of various bypass capacitor types is included. Most of the designs can be easily modified for other intermediate voltages such as 3.45V, 3.7V, and 4.1V.
  Extract from the document

  Application Note 58
  September 1993
  5V to 3.3V Converters for Microprocessor Systems
  Robert Dobkin, Mitchell Lee, Dennis O'Neill and Milt Wilcox
  Introduction Linear Regulators The new generation of high performance microprocessors are built on dense, low breakdown voltage processes
  in order to accommodate increased transistor counts.
  These new processors require high current power at 3.3V,
  developed from the 5V input used to power the rest of the
  system. Special techniques are required to ensure proper
  operation of the microprocessor and good heat dissipation within the computer system. Table 1 shows the range of components available for linear
  regulation of 3.3V with a 5V input. With only 1.7V of
  headroom, low dropout is essential. Low dropout regulators are available delivering currents from 125mA to 7.5A,
  allowing almost any microprocessor to be powered with a
  local 3.3V generation circuit. The first four devices (LT1020,
  LT1120, LT1121 and LT1129) are PNP micropower low
  dropout regulators. Since PNP transistors are much larger
  than monolithic NPNs, higher current regulators use an
  NPN pass device. The LT1117, LT1086, LT1083 through
  LT1085, and LT1087 all use NPN pass devices. The NPN
  structure requires about 1.2V headroom compared to the
  400mV to 500mV dropout typical of PNP regulators, and
  ground current of 5mA or 10mA, independent of output
  current. Because of this constant quiescent current, the …

Design Notes

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  Powering 3.3V Digital Systems &mdash DN33
  PDF, 77 Kb, File published: Apr 1, 1990
  Extract from the document

  Powering 3.3V Digital Systems – Design Note 33
  Dennis O’Neill
  The new generation of high density digital devices
  requiring 3.3V power supplies impose some unique
  constraints on power supply designers. In nearly all
  cases the computers using these devices already have
  a 5V system supply. Deriving the 3.3V supply from the
  existing 5V rail permits system upgrades with a simple
  on-card solution. In many cases the 5V rail is the only
  supply available, mandating this approach. The п¬Ѓrst decision to be made is whether to use a switching regulator
  or a linear regulator? Switchers have a clear efficiency
  advantage when there is a large difference between the
  input and output voltage, but that advantage diminishes
  as the input voltage approaches the output voltage.
  Simple calculations show that the efficiency of a switcher
  is marginally better in this application. Assuming a
  nominal input voltage of 5.0V and an output voltage of
  3.3V, the efficiency of a linear regulator, (LT®1083 type
  Figure 1A), independent of output current, is simply
  3.3V/5.0V = 66%. For a switcher (Figure 1B) the efficiency is tougher to calculate. With only 5V available
  a PNP switch must be used, a MOSFET is not practical
  due to its gate drive requirements. The average inductor 5V IN + 10ОјF LT1083 OUT
  100 ADJ 3.3V + 10ОјF 165 current will be equal to the load current. The duty cycle …

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  Techniques for Deriving 3.3V from 5V Supplies &mdash DN74
  PDF, 73 Kb, File published: Sep 1, 1993
  Extract from the document

  advertisement Techniques for Deriving 3.3V from 5V Supplies – Design Note 74
  Mitchell Lee
  Microprocessor chip sets and logic families that operate from 3.3V supplies are gaining acceptance in both
  desktop and portable computers. Computing rates, and
  in most cases, energy consumed by these circuits show
  a strong improvement over 5V technology. The main
  power supply in most systems is still 5V, necessitating
  a local 5V to 3.3V regulator.
  Linear regulators are viable solutions at lower (IO ≤ 1A)
  currents, but they must have a low dropout voltage in
  order to maintain regulation with a worst-case input
  5 4.5V TO 5.5V IN OUT 1 + LT1129-3.3
  4 SHUTDOWN SENSE 2 GND 3.3V OUT
  700mA
  3.3ОјF
  SOLID TANTALUM of only 4.5V. Figure 1 shows a circuit that converts a
  4.5V minimum input to 3.3V with an output tolerance
  of only 3% (100mV). The LTВ®1129-3.3 can handle up to
  700mA in surface mount configurations, and includes
  both 16ОјA shutdown and 50ОјA standby currents for
  system sleep modes. Unlike other linear regulators, the
  LT1129-3.3 combines both low dropout and low voltage
  operation. Small input and output capacitors facilitate …

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  5V to 3.3V Regulator with Fail-Safe Switchover &mdash DN82
  PDF, 74 Kb, File published: May 1, 1994
  Extract from the document

  advertisement 5V to 3.3V Regulator with Fail-Safe Switchover – Design Note 82
  Mitchell Lee
  Newer microprocessors designed for replacing existing
  5V units operate from lower voltage supplies. In the past
  a processor swap was simply a matter of removing one
  IC and replacing it with an updated version. But now
  the upgrade path involves switching from a 5V chip to
  one that requires 3.xxV.
  One means of changing supply voltage from 5V to 3.xxV
  is to clip a jumper that bypasses a local 3.xxV regulator.
  This is not a good solution since it leaves too much to
  chance. Failure to remove the jumper can result in the
  instant destruction of the new microprocessor upon
  application of power. A means of automatically sensing
  the presence of a 3.xxV or 5V processor is necessary.
  Intel microprocessors include a special pin called
  “VOLDET” which can be used to determine whether
  or not a particular chip needs 3.xxV or 5V. Figure 1
  shows a simple circuit that takes advantage of this
  pin to automatically “jumper out” a 3.xxV regulator
  whenever a 5V processor is inserted into the socket. VOLDET is pulled low on 3.xx processors; it is buffered
  by transistor Q2 which grounds the gate of a bypassing
  switch (Q1). Q1 is turned off leaving the LTВ®1085 to …

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Manufacturer's Classification

• Power Management > LDO Linear Regulators > Positive Linear Regulators (LDO)