1 Introduction to Parallel LDOs Using Ballast Resistors

Parallel LDOs have become popular in recent years, with numerous LDOs in Texas Instruments' portfolio being paralleled in real world designs across many applications. Texas Instruments modernizes the design and analysis of parallel LDOs by using ballast resistors (see [1] and [2] in the References section). TI also developed a downloadable software tool to design the ballast resistor (R_B) for TI's LDOs against a set of user-defined system requirements (see [3] in the References section). Parallel LDOs offer numerous advantages over single LDOs, such as:

1. Increased load current
2. Improved PSRR for a given load current
3. Improved thermal spreading
4. Reduced headroom requirements (dropout)
5. Reduced volume over other converters: C_OUT typically drives the maximum system height
6. Reduced noise

For a comprehensive discussion on these advantages, see [1-3] in the References section. To date, no comprehensive white paper exists comparing and contrasting noise performance of the three common parallel LDO techniques. This white paper is intended to fill this gap and show the noise performance advantage of parallel LDOs using ballast resistors over the competing parallel LDO techniques.