1.5A, Negative Regulator Expands Family of CurrentReference Linear Regulators Dawson Huang The LT3080, introduced in 2007, represented a new linear regulator architecture featuring a current source as reference and a voltage follower for the output amplifier. This new architecture has 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. Table 1 summarizes the family of devices that use this architecture. The LT3091, the latest addition to this family, is a 1.5A low dropout negative linear regulator featuring adjustable current limit and current monitor. The LT3091 is similar to the other negative linear regulator in Figure 1. 1.5A, negative linear regulator with current limitation and monitor 0.1µF LT3091 the family, the LT3090, but with more than double the LT3090’s 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 HOW IT WORKS TO ADC (IMON) RSET 49.9k RMON 6.65k SET GND 10µF VOUT –2.5V MAX IOUT 1.5A IMONP OUT + – 10µF VIN –3V TO –10V 50µA IN ILIM SHDN IMONN 3.3V 0.1µF RLIM 5k Table 1. Some of Linear’s regulators featuring the current reference architecture LT3091 LT3090 LT3081 LT3080 Output current 1.5A 600mA 1.5A 1.1A I SET −50µA −50µA 50µA 10µA Adjustable current limit/current monitor Yes/Yes Yes/Yes Yes/Yes No/No LDO (low dropout) Yes Yes No Yes Positive/Negative voltage Negative Negative Positive Positive 20 | May 2016 : LT Journal of Analog Innovation 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 −50µ A 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 the SET pin. With this architecture, all of the internal operating current flows in from the output pin. Only a 20µ A load is required to maintain regulation at all output voltages. Figure 1 shows the basic hookup for the 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, design features This regulator is easy to parallel to increase output current. It can be used for power supplies capable of sinking and sourcing current. as shown in Figure 1. This function can effectively protect the load and limit the temperature of the IC. 0.1µF LT3091 U1 With 3.3V feeding the IMONN pin, the IMONP pin sources current equal to ¼000 of the output current. This current source is measured by tying a 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 ½000 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 DEVICES FOR MORE CURRENT Paralleling LT3091s is easy with this new current source reference regulator. 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 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, now has twice the set current flowing through it, so the output is −100µ A times RSET. The 10m Ω output resistors, ROUT1,2 ensure ballasting at full current. There is no limit to the number of devices that can be paralleled for higher current. RSET 24.9k 1% 22µF SET GND IMONN OUT ROUT1 10mΩ + VOUT –2.5V MAX IOUT 3A – 22µF VIN –3.3V 50µA IN IMONP SHDN ILIM 5k GND LT3091 U2 SET IMONN OUT ROUT2 10mΩ + Figure 2. 3A negative linear regulator with paralleled LT3091 – 50µA IN SHDN IMONP ILIM 5k Figure 3. Thermal performance of two paralleled LT3091s U1 52°C U2 53°C May 2016 : LT Journal of Analog Innovation | 21 Figure 3 shows the thermal distribution of the design of Figure 2—U1 and U2 reach similar temperatures, indicating equally shared current. and quiet solution. Figure 5 shows the transient response of the two output voltages. Figure 6 shows the thermal performance of the entire system. LOW NOISE POSITIVE-TO-NEGATIVE CONVERTER LOW NOISE POSITIVE AND NEGATIVE POWER SUPPLY 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-andnegative 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 an available 7mm × 4mm DFN package. It can generate both a positive and a negative output from a single input. Figure 4 shows a low noise coupledinductor positive-to-negative converter. The inverting converter is based on LT3581, a PWM DC/DC converter with built-in power switch. Its 4mm × 3mm DFN package and tiny externals can be combined with the LT3091 in a compact Figure 7 shows a 1.5A 12V-to-±3.3V low noise power supply using the LT8582, LT3081 and LT3091. Figure 8 shows the transient response Figure 4. 1.5A low noise and fast transient positive-to-negative converter C2 1µF L1A 3.3µH VOUT1 –5V 1.5A L1B 3.3µH CONCLUSION The LT3091 is a 1.5A, low dropout, current reference negative linear regulator. This regulator 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. n 0.1µF 49.9k 1% LT3091 SET 10µF 1206 D1 SW1 SW2 VIN C1 22µF C1: 22µF, 25V, X7R, 1210 C2: 1µF, 50V, X7R, 1206 C3: 22µF, 16V, X7R, 1210 D1: CENTRAL CMSH3-40FL L1: COILCRAFT MSD7342-332MLB This setup can be used as an operational amplifier power supply—where a high speed operational amplifier requires a low noise, high speed ±3.3V power supply. • • VIN 12V of the negative rail. Figure 9 shows the temperature of the entire system. 100k 124k LT3581 FB SHDN GATE FAULT CLKOUT RT VC SYNC SS GND C3 22µF 0.1µF IMONN OUT + 60.4k 56pF GND 6.8k – 10µF 1206 50µA IN 3.3nF SHDN IMONP ILIM 5k Figure 5. Transient response for positive-to-negative converter VOUT1 100mV/DIV (AC COUPLED) Figure 6. Thermal image for positive-to-negative converter LT3581 54°C D1: 54°C VOUT2 100mV/DIV (AC COUPLED) L1: 64°C LT3091 83°C IOUT −1A/DIV 100µs/DIV 22 | May 2016 : LT Journal of Analog Innovation VOUT2 –2.5V MAX IOUT 1.5A design features 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, making it ideal as a post regulator. Low dropout helps keep it from overheating when supporting 1.5A loads. Figure 7. 12V to ±3.3V low noise power supply 10µF C1 2.2µF L1A 4.7µH VOUT1' 5V D1 • VIN 12V CIN1 22µF SWA1 SWB1 • VIN1 100k PG1 LT8582 100k CLKOUT1 RT1 13k IMON ILIM 1.5nF 0.1µF 66.5k 1% 0.1µF 66.5k 1% 22µF GND 115k RT2 SHDN2 SS2 VIN2 VC2 SWA2 SWB2 2.2nF 0.1µF 47pF 22µF 18.7k C2 2.2µF • SET GND IMONN OUT + VOUT2 –3.3V MAX IOUT 1.5A – L2B 4.7µH • CIN1, CIN2: 22µF, 25V, X7R, 1210 COUT1, COUT2: 22µF, 16V, X7R, 1210 C1, C2: 2.2µF, 50V, X7R, 1206 D1, D2: CENTRAL CMSH3-40FL L1, L2: WÜRTH WE TDC 74489440047 LT3091 COUT2 22µF ×2 60.4k FBX2 Figure 8. Transient response at VOUT2 load transient TEMP VOUT2' 3.3V MAX IOUT 1.5A 47pF 0.1µF GATE2 CIN2 22µF OUT SET 115k PG2 L2A 4.7µH – COUT1 22µF ×2 VC1 SS1 SYNC2 + 45.3k GATE1 SYNC1 CLKOUT2 ISET 50µA L1B 4.7µH FBX1 SHDN1 IN LT3081 D2 VOUT1 –5V 50µA 10µF IN SHDN IMONP ILIM 5k Figure 9. Thermal image for 12V to ±3.3V low noise power supply LT3081 75°C VOUT1 100mV/DIV (AC COUPLED) VOUT2 100mV/DIV (AC COUPLED) LT8582 81°C IOUT −1A/DIV LT3091 83°C 100µs/DIV May 2016 : LT Journal of Analog Innovation | 23