Current-Mode LDO with Active Dropout Optimization

Power supplies for portable end equipment need to be optimized for highest efficiency and low output voltage ripple e.g. for TFT display applications. In general, the output voltage ripple of switched mode power supplies can be further reduced by means of an additional linear regulator (LDO) which is used as post-regulator/filter for the output voltage. Since the LDO is not coupled to the DC/DC converter there is either a too high dropout voltage which results in a reduced over-all efficiency or the dropout voltage is not high enough so the ripple rejection of the post regulator is not sufficient since the pass element of the LDO is operated in the linear region. To overcome this problem, a current mode LDO in combination with a load current dependent dropout tracking circuit where the output current information comes from a replica of the pass transistor in the LDO is used. The dropout tracking circuit compares the voltages on the LDO terminals and sends a load current dependent control signal to the DC/DC converter. The goal of this configuration is a high power supply rejection ratio and therefore a low output voltage ripple at optimized efficiency. Depending on the type of the converter (hysteretic or linear regulated PWM) the dropout tracking circuit has to be realized as a hysteretic type with a comparator or linear type with an OP-AMP. A current mode LDO means that the PMOS passelement works like a voltage controlled current source. This concept is possible for all converter topologies like buck-, boost-converters, charge-pumps etc. The converter can be regulated in an energy saving PFM mode without sacrificing the ripple performance. Since the current mode LDO limits the load current, the system is inherently short circuit protected