As a rule, voltage regulators require input voltages higher than their outputs. The difference between input and output voltage is called the overhead. Input voltage must stay above the minimum value that maintains the overhead. Otherwise the output voltage drops out of regulation.
Low-dropout (LDO) regulators keep working when input voltages only allow very small overheads. Products using embedded electronics most likely have an LDO regulator powering the microprocessor for a simple reason. The amount of voltage available to power the primary system limits the size of the possible overhead. Most embedded applications get power from either high-voltage multiphase or low-voltage single-phase sources. The ac voltage is transformed and rectified to 12 or 24 Vdc. These values get stepped down to a lower dc voltage.
A step-down switching regulator typically performs the dc/dc conversion for maximum power conversion efficiency. Switching regulators display conversion efficiencies over 90% compared to the maximum 50% of linear regulators. This low dc voltage is the primary power, or system rail, for the embedded circuitry. The most common system rail values are 3.3 and 5 Vdc. Embedded microcontrollers, though, are powered by voltages ranging from 1.8 to 5 Vdc. The LDO regulator synthesizes the lower voltages from the system rail.
A 3.3-Vdc system rail is converted down to 2.5 Vdc to operate embedded microprocessors. This leaves only 800 mV of headroom. The system must work properly with this headroom over a wide temperature range from 40 to 125°C. Dropout voltage under normal 25°C conditions cannot exceed 300 mV to insure the system operates within limits at the temperature extremes. It is good design practice to choose an LDO with the smallest overhead.
Generally a small overhead offers the highest flexibilityand fault tolerance. Embedded microcontrollers also act as system supervisors by performing tasks such as power condition monitoring and power management. The latest versions of LDOs send relevant information about power conditions to the microcontroller. Similarly, the embedded microcontroller directs the LDO in response to changes in system status.
Designers should always consider load current as their first parameter. But several other LDO specifications and features are considered critical in embedded applications. These include dropout voltage, a power-good output to the microcontroller, and a shutdown feature to turn off the LDO regulator when power is no longer needed. Power-good indicators are particularly important. Most embedded systems must handle certain housekeeping functions in direct response to power-level fluctuations. Say, for example, an embedded application converts a 3.3-V system rail to 2.5 V. Further suppose the system rail voltage falls to 2.8 V. There may not be enough headroom to maintain regulation at 2.5V. The power-good output on the LDO regulator is asserted to tell the microcontroller about the power problem.
Another critical LDO feature is a shutdown command. This comes either from the embedded microcontroller or some other system-level intelligence. When an embedded microcontroller switches off, either voluntarily or as a fault response, the LDO regulator must shut down to preserve power or prevent damage.