Q.
How close to drop out voltage can one operate an LDO in order to squeeze the last ounce out of my circuit's efficiency?
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A.
A low-dropout regulator, LDOs can operate with a very small input to output voltage differential. The advantage of operating the LDO close to the dropout voltage is higher efficiency and lower heat dissipation. Dropout voltage is defined as the minimum input-to-output voltage difference for a regulated output. Low power LDO can operate with dropout voltages as low as 0.1 volts. The load current and pass element resistance determines the point at which the LDO enters drop out. For instance, if the effective resistance of the pass element is 1 ohm, and the load current is 1 amp, the LDO will enter dropout at Vout + 1V. If the load current is only 10mA, the LDO will enter dropout at Vout + 10mV. When considering PSRR the peak to peak amplitude of the noise on the input of the LDO must be considered. For example, if the LDO is operating at Vout + 10mV, any noise at the input of the LDO that is greater than about 2mVpp will start to push the LDO from small signal to large signal operation. The PSRR in large signal operation is much worse than small signal operation.Load regulation behavior is similar to PSRR in that an increase in load current from 10mA to 20mA will force the LDO further into dropout and cause the voltage to drop 10mV.
