Internal Reference Voltage Droop under External Analog Loading
External analog load currents extract direct current and charge pulses from internal bandgap reference nodes, inducing static droop and transient conversion errors.
Voltage Droop
Transient Response
Transient supply deviation occurs when a rapid increase in load current causes a temporary decrease in output voltage from a regulator. This reduction, termed voltage droop, is caused by the finite output impedance and response time of the power source. This event can lead to logic errors in digital circuits if the supply voltage falls below the minimum threshold.
Current Demand
Sudden current demands from modern processors switching at high frequencies exceed the bandwidth of standard feedback loops. When the load transitions from an idle state to full operation, the regulator cannot adjust its output instantly to match this change. In this interval, the energy must be supplied by the decoupling capacitors placed near the load.
If the capacitors are depleted before the regulator responds, a significant voltage droop will occur.
Capacitor Array
Decoupling capacitors placed in parallel across the supply lines store charge that is released quickly during load transients. This array must include a mix of low-capacitance ceramic capacitors for high frequency response and larger bulk capacitors to sustain the voltage over longer intervals. Each capacitor’s equivalent series resistance and inductance must be kept low to minimize the initial impedance of the distribution network.
A carefully designed capacitor array is the primary defense against excessive voltage droop.
Board Regulation
Active voltage positioning is a technique used in high-current regulators to manage this transient behavior. By allowing the output voltage to decrease slightly as the load current increases, the regulator reduces the peak-to-peak voltage variation. This design reduces the number of decoupling capacitors required on the board, saving space and cost.
Thermal considerations also benefit from this approach because the average operating voltage of the processor is lower under heavy workloads. This steady-state offset is calibrated to ensure that the voltage remains within the safe operating limits of the processor under all load conditions.