SUPPLEMENT POWER ELECTRONICS


NEGATIVE REGULATOR FOR HIGH CURRENT, NEGATIVE VOLTAGE APPLICATIONS


Dawson Huang, applications engineer, power management products at Linear Technology, discusses the arrival of a 1.5A, negative regulator and its place alongside current-reference linear regulators


the SET pin. With this architecture, all of the internal operating current flows in from the output pin. Only a 20µA load is required to maintain regulation at all output voltages. Figure 1 shows the basic hookup for the


A


new linear regulator featuring a current source as reference and a


voltage follower for the output amplifier introduced in 2007 by Linear Technology Corporation represented a new linear regulator architecture. This new architecture features a number of advantages, including easy regulator paralleling for increased output current and operation down to zero output voltage. Since the output amplifier always operates at unity gain without a resistor-setting divider, bandwidth and absolute regulation are constant across the output voltage range. Transient response is independent of output voltage and regulation can be specified in millivolts rather than as a percent of output. The LT3091, the latest addition to this


family, is a 1.5A low dropout negative linear regulator featuring adjustable current limit and current monitor. This device is similar to the other negative linear regulator in the family, the LT3090, but with more than double the current rating. The LT3091 is useful in high current,


negative voltage applications requiring low noise or precision output. It features fast transient response, high PSRR and low output noise. Low dropout helps keep it from overheating when supporting loads up to 1.5A. Built-in protection includes reverse output protection, internal current limit with foldback and thermal shutdown with hysteresis. This versatile negative regulator architecture can operate down to zero volts out and as a negative floating regulator. The negative output voltage is set with


a −50µA precision current source driven through a single resistor RSET


from


ground to the SET pin. The internal follower amplifier forces the output voltage to match the negative voltage of


S10 NOVEMBER 2016 | ELECTRONICS Figure 3:


Thermal performance of two paralleled LT3091s


Figure 1:


1.5A, negative linear regulator with current limitation and monitor


LT3091. It provides 1.5A of output current, can be adjustable to zero output voltage, and features both positive and negative monitors for output current. It is also reverse protected, when output voltage is lower than input. The current limit can be reduced below


1.5A by connecting an external resistor RLIM


between ILIM and IN pins, as shown


in Figure 1. This function can effectively protect the load and limit the temperature of the IC. With 3.3V feeding the IMONN pin, the


IMONP pin sources current equal to 1/4000 of the output current. This current source is measured by tying a


Figure 2:


3A negative linear regulator with paralleled LT3091


are at the same voltage, only a few milliohms of ballast, ROUT1,2


, are required


to allow them to share current. Figure 2 shows a schematic of two


LT3091s paralleled to obtain 3A output. The set resistor, RSET


set current flowing through it, so the output is −100µA times RSET


output resistors, ROUT1,2 , now has twice the . The 10mΩ ensure ballasting


at full current. There is no limit to the number of devices that can be paralleled for higher current. Figure 3 shows the thermal distribution


of the design of Figure 2 - U1 and U2 reach similar temperatures, indicating equally shared current. Inverting converters generate a


negative voltage from a positive input, and feature low output ripple. If combined with a high bandwidth LDO such as the LT3091, the overall converter can have very high transient response with even lower noise. A high current positive-to-positive-


and-negative converter can be built with a positive 1.5A LT3081 linear regulator and its negative 1.5A linear counterpart, the LT3091. The LT8582 is a dual-channel PWM DC/DC converter with internal switches in a 7mm by 4mm DFN package. It can generate both a positive and a negative output from a single input. This device is a 1.5A, low dropout,


resistor, RMON, to ground in series with


the current source and reading the voltage across the resistor. With the IMONP pin tied to VIN


, the IMONN pin


sinks current equal to 1/2000 of the output current. In this way, positive or negative output current can be monitored with minimal components, no additional sense resistors or amplifiers required. Paralleling is useful for


increasing output current or spreading heat. Since the LT3091 is set up as a voltage follower, tying all the SET pins together makes the outputs the same voltage. If the outputs


/ ELECTRONICS


Linear Technology (UK) Ltd. www.linear.com T: 01628 477 066


current reference negative linear regulator. It is easy to parallel to increase output current. It also features fast transient response, high PSRR and low output noise, making it ideal as a post regulator. It can be used for power supplies capable of sinking and sourcing current.


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