Datasheet Linear Technology LTC3805-5

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Design Notes

• Download
  Negative Buck Converter with Short-Circuit Protection and Shutdown &mdash DN1022
  PDF, 109 Kb, File published: Sep 8, 2011
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  Negative Buck Converter with Short-Circuit Protection and Shutdown
  Design Note 1022
  Victor Khasiev
  Introduction
  Negative buck converters are increasingly used to “step
  down” (by absolute value) negative voltages. The main
  reason behind the increasing demand is the standardization
  of switching transformers, which are typically produced
  with one or two secondary windings. For example, if a
  system employs a transformer with two secondary windings to produce ±12V, and the design also requires –3.3V,
  engineers tend to lean toward solutions, such as a negative
  buck, that don’t require changing the main transformer.
  Circuit Description and Performance
  Figure 1 shows a negative buck converter that generates
  –3.3V at 3A from a –12V rail. The power train includes
  inductor L1, diode D1 and MOSFET Q1. The LTC3805-5
  controller IC includes a complete set of essential functions
  including short-circuit protection (the current level can
  be precisely set), converter enable/disable, and programmable switching frequency. An internal shunt regulator allows biasing the IC directly
  from the input rail. Despite the simplicity of the topology,
  which makes it an attractive choice for many designers,
  there are two important design considerations in a negative
  buck converter: sensing the output voltage and remote …

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  Buck Converter Eases the Task of Designing Auxiliary Low Voltage Negative Rails &mdash DN458
  PDF, 76 Kb, File published: Jan 8, 2009
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  Buck Converter Eases the Task of Designing Auxiliary
  Low Voltage Negative Rails
  Design Note 458
  Victor Khasiev
  Introduction
  Many system designers need an easy way to produce
  a negative 3.3V power supply. In systems that already
  have a transformer, one option is to swap out the existing
  transformer with one that has an additional secondary
  winding. The problem with this solution is that many
  systems now use transformers that are standard, offthe-shelf components, and most designers want to
  avoid replacing a standard, qualified transformer with a
  custom version. An easier alternative is to produce the
  low negative voltage rail by stepping down an existing
  negative rail. For example, if the system already employs
  an off-the-shelf transformer with two secondary windings
  to produce ±12V, and a –3.3V rail is needed, a negative
  buck converter can produce the –3.3V output from the
  –12V rail.
  Leave the Transformer Alone: –3.3VOUT from –12VIN
  Figure 1 shows a negative buck converter that generates
  –3.3V at 3A from a –12V rail. The power train (indicated
  4.99k SHUTDOWN …

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Manufacturer's Classification

• Power Management > Switching Regulator > SEPIC Regulators | Flyback, Forward and Isolated Controllers
• Power Management > Switching Regulator > Step-Up (Boost) Regulators > External Power Switch Boost
• Space & Harsh Environment > Extended Temperature Plastic (H & MP) > Extended Temperature (H & MP) Switching Regulators