Voltage Droop

Voltage Droop, Output: defines the loss in output voltage from a device as it tries to drive a [capacitive] load. The value of the Decoupling Capacitor that should be used with an IC depends on the load the IC has to drive. If device A has to drive two separate inputs at 3.3 volts then the current demand depends on the load of both inputs, taking into consideration the rise time of the signal. The input capacitance for a given device is parameter Ci in the device data sheet. If the receiver [B] has a load of 12pF [Ci] and the output driver [A] has a rise time of 1nS then the current required is: I = dV / dt or I = 12pF*(3.3v)/1nS. The current required = 39.6mA. If we keep the voltage droop to 3.0 volts, or a reduction of 300mV. The required by-pass capacitor then equals: C = I * dt / dV. C = 39.6mA * 1nS/300mV = 22pF

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