The safe operating area (SOA) is a term used to define the input and output voltage range, and load current range within which any device is designed to operate reliably. Exceeding these limits will result in a catastrophic failure or will render the device temporarily inoperative, depending upon the device and its performance characteristics. Integrated circuit voltage regulators with internal current Limiting, thermal and short-circuit protection will merely shut down. External components, such as pass transistors on the other hand, may respond with catastrophic failure.
REGULATOR SOA CONSIDERATIONS
Although particular design equations depend upon the type of integrated circuit voltage regulator used and its application, there are several boundaries that apply to all regulator circuits for safe, reliable performance.
The limits on the input voltage are derived from three considerations:
The absolute maximum rated input voltage as referenced to the regulator’s ground. This is a safe operating area (SOA) destruct limit.
(VI – VO)min
The input-to-output differential voltage also referred to as the dropout voltage, at which the regulator ceases to function properly. This is a functional limit.
(VI – VO)max
The maximum input-to-output differential voltage. Usually, the regulator’s power dissipation is exceeded prior to the (VI – VO)max limit. This is an SOA level that can be limited by the allowable Power Dissipation (PDmax).
The maximum load current deliverable from the integrated circuit regulator. If internal current limiting is not provided, external protection should be provided. This is a functional limit that may be further limited by PD max.
The maximum power that can be dissipated within the regulator. Power dissipation is the product of the input-to-output differential voltage and the load current, and is normally specified at or below a given case temperature. This rating is usually based on a 150°C junction temperature limit. The power rating is an SOA limit unless the integrated circuit regulator provides an internal thermal protection.
Output Voltage of an Adjustable-Voltage Regulator
The minimum output voltage a regulator is capable of regulating. This is usually a factor of the regulator’s internal reference and is a functional limit.
The maximum output voltage a regulator is capable of regulating. This is largely dependent on the input voltage and is a functional limit.
External Pass Transistor
For applications requiring additional load current, integrated circuit voltage regulator capabilities may be boosted with the addition of an external pass transistor. When employed, the external pass transistor, in addition to the voltage regulator, must be protected against operation outside its safe operating area. Operation outside the safe operating area is catastrophic to most discrete transistors.
The maximum current the transistor is capable of sustaining. ICmax now becomes the maximum current the regulator circuit is capable of delivering to the load. Associated with ICmax is a collector-emitter voltage (VCE = VI-VO). If the product (VI- VOmax) ICmax exceeds the SOA then ICmax will have to be derated. This will then become a functional limit instead of a catastrophic limit. ICmax is related to power dissipation and junction or case temperature. ICmax must again be derated if the thermal or power ratings at which it is specified are exceeded. The resulting derated ICmax should continue to be considered as a catastrophic limit. Actual ICmax limits and derating information will appear on the individual transistor specification.
The maximum collector-emitter voltage that can be applied to the transistor in the off-state. Exceeding this limit can be catastrophic.
The maximum power that can be dissipated by the transistor. This is usually specified at a specific junction or case temperature. If the transistor is operated at higher temperatures, the maximum power must be derated in accordance with the operating rules specified in the transistor’s applicable specification. Prolonged operation above the transistor’s maximum power rating will result in degradation or destruction of the transistor.