Upgraded regulators offer adjustable current limiting
Article By : Bob Dobkin
The LT3080 architecture has the abilities to parallel the regulators for more output current and the regulator to operate down to zero.
The architecture of linear regulators has remained virtually unchanged since the introduction of the three terminal floating voltage regulators in 1976. Regulators were either a floating architecture (LT317) or else an amplifier loop with feedback from the output to the amplifier. Both of these architectures suffer from limitations on versatility, regulation and accuracy.
The feedback resistors set the output voltage and attenuate the feedback signal into the amplifier. So, the regulation at the output is a percentage of the output voltage, so higher output voltages have worse regulation in “Volts” while the percentage may be the same. Also, the bandwidth of the regulator changes with voltage. Since the loop gain is decreased, the bandwidth is decreased as well at higher output voltages. This makes transient response slower and ripple worse as output voltage goes up.
The regulator fixes current limiting and it has no adjustment. It is built into the IC and different devices must be used for different output currents. So, if the current limit needs to be matched to the application or accurate current limit is needed, an external circuit must be used.
An architecture was introduced by Linear Technology in 2007-the LT3080. It used a current source for the reference and a voltage follower for the output amplifier. Two advantages of this architecture are the ability to parallel the regulators for more output current and the ability for the regulator to operate down to zero. Since the output amplifier always operates at unity gain, bandwidth is constant as well as regulation. Transient response is independent of output voltage and regulation can be specified in millivolts rather than a percent of output.
Along with different output current variations, these regulators were specifically designed to add functional features not previously available in existing regulators. There are monitor outputs for temperature, current and external control of current limit. One device (LT3086) also has external control of thermal shutdown. A negative regulator provides monitoring and can operate as a floating regulator or an LDO. All of these new regulators can be paralleled for higher current, current sharing and heat spreading.
Therefore, Linear Technology’s latest regulators provide an order of magnitude better regulation against load and line changes compared to prior devices. Regulation specs as well as transient response do not change with output. New functionality in these devices provides temperature and current monitoring, as well as adjustable current limiting. Paralleling no longer requires external current balance circuitry to prevent current hogging.
Read more: Download the full application note
