Linear Regulators Part 2

Linear Regulator Types (LDO, Standard, and Quasi-LDO) 

There are three basic types of linear regulator designs which will be covered:

     1. Standard (NPN Darlington) Regulator

     2.  Low Dropout or LDO Regulator

     3.  Quasi LDO Regulator

• The  single  most  important  difference  between  these  three  types  is  the  dropout voltage,  which  is  defined  as  the  minimum  voltage  drop  required  across  the regulator to maintain output voltage regulation.  A critical point to be considered is  that  the  linear  regulator  that  operates  with  the  smallest  voltage  across  it dissipates the least internal power and has the highest efficiency.  The LDO requires the least voltage across it, while the Standard regulator requires the most. 

• The  second  important  difference  between  the  regulator  types  is  the  ground  pin current  required  by  the  regulator  when  driving  rated  load  current.    The  Standard regulator has the lowest ground pin current, while the LDO generally has the highest. Increased  ground  pin  current  is  undesirable  since  it  is  "wasted"  current,  in  that  it must be supplied by the source but does not power the load.

Applications Where Linear Regulators Are Preferable ?

There are many applications in which linear regulators or LDOs provide superior solutions to switching supplies, including:

1. Simple/low cost solutions: Linear regulator or LDO solutions are simple and easy to use, especially for low power applications with low output current where thermal stress is not critical. No external power inductor is required.

2. Low noise/low ripple applications:For noise-sensitive applications, such as communication and radio devices,minimizing  the  supply  noise  is  very  critical.  Linear regulators have very low output voltage ripple because there are no elements switching on and off frequently and linear regulators can have very high bandwidth. So there is little EMI problem.

3. Fast transient applications: The linear regulator feed-back loop is usually internal, so no external compensation is required. Typically, linear regulators have wider control loop bandwidth and faster transient response than that of SMPS.

4. Low  dropout  applications:  For  applications  where output voltage is close to the input voltage, LDOs may be more efficient than an SMPS. An SMPS usually has lower light load efficiency because of its AC switching losses. In battery powered applications in which light load efficiency is also critical, an LDO can provide a better solution than an SMPS. 

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