Negative rails don't disappear

background

Negative power rails are used with major IC building blocks such as digital-to-analog converters (DACs), analog-to-digital converters (ADCs), operational amplifiers, and GaAs FET bias circuits. Symmetrical power supplies (rails) process AC signals and do not produce DC offsets. Obviously, if there is only one positive power rail, the output signal can usually only take positive values ​​(depending on the circuit topology); however, if there are positive and negative power rails, the output signal can swing from negative voltage to positive voltage. Furthermore, negative power supplies are important in applications such as process control where the actual signal is bipolar. Taking motor control as an example, a positive voltage will cause "clockwise" rotation, a negative voltage will cause "counterclockwise" rotation, and the voltage amplitude controls the speed of the motor.

Negative  LDOs  are an elegant way to provide clean power to the negative rail. There are many negative LDO applications, including:

l Military - radar, sonar, thermal imagers, surveillance systems

l Industrial - CCD and IR cameras, laboratory/test equipment, data acquisition systems, blood/glucose equipment, motors, vibration control, power analyzers

l Medical - MRI equipment, dental X-ray systems, ultrasound equipment, blood analyzers, endoscopes

l Communications - optical modules, fiber optic transceivers

l Car- Antenna

l RF applications- radar, optical transceivers, power amplifiers, high-precision instruments

Traditionally, high-performance negative output switching regulators or negative output charge pumps have been used in these applications, which can provide lower output voltages and minimal heat buildup. The downside is usually cost, complexity (in some cases), and output noise. However, with improved LDO design methods (including lower reference voltages and lower dropout voltages for higher operating efficiency), the use of low-dropout regulators in such applications is now increasing. In addition, some negative power rails also need to be accurately regulated, and low noise, fast transient response, and good power supply rejection are also desirable characteristics in many applications.

An ideal solution to the above problems would be a low noise negative LDO with medium to high current capability and wide input/output voltage range. Linear Technology has introduced a new product designed to solve such problems.

1.5A Low Noise Negative LDO

The LT3015 is a 1.5A negative low dropout linear regulator with fast transient response, low noise and precise current limit. With its wide input voltage range of -1.8V to -30V and adjustable output voltage of -1.220V to -29.5V, the device's common emitter NPN power transistor design requires only a single power supply and achieves a low dropout voltage of 310mV (typical at full load). The output voltage noise is only 60μVRMS in the 10Hz to 100kHz bandwidth. The LT3015 has an operating current of 1.1mA and drops to <1μA in shutdown. In addition, the quiescent current is well controlled when the voltage drops. The LT3015 can be used as a negative complement to one of Linear Technology's popular positive LDOs (1.5A LT1963A), and is very suitable for negative logic supplies, low noise instruments, industrial power supplies, and post-regulation switching power supplies.

The LT3015's reference amplifier topology provides precise DC characteristics and good loop stability with a very wide range of output capacitors, including small and low-cost ceramic output capacitors. The device is stable with an output capacitor of only 10μF. These tiny external capacitors can be used without adding series resistance (ESR), which is common when using many other regulators. The LT3015 has a bidirectional shutdown function, allowing the device to operate with positive or negative logic levels. In addition, due to the high shutdown threshold accuracy of the LT3015, the /SHDN pin can also be used to set a programmable undervoltage lockout (UVLO) threshold for the regulator input supply. The IC's internal protection circuitry includes reverse output protection, precision current limiting with foldback (used to keep the power transistors in their safe operating area), and thermal limiting with hysteresis.

Figure 1: Typical Application Circuit of LT3015 The LT3015 is available in a low-profile (0.75mm high) 3mm x 3mm 8-pin DFN package with backside thermal pad, a 12-pin MSOP package with backside thermal pad, a through-hole TO-220 power package, and a surface-mount DD-Pak power package. The operating junction temperature is as wide as -55°C to +125°C (depending on the device grade).

Cooling characteristics

The LT3015's innovative design offers several features that enable simplicity and high performance in any design. A high gain-bandwidth error amplifier provides fast transient response and high power supply rejection ratio (PSRR). In addition, the device's PSRR performance is improved by 4 times over the leading positive PNP LDO regulator and high rejection levels (>25dB) are maintained even at 10MHz, see Figures 2 and 3.

Figure 2: Transient response of the LT3015

Figure 3: LT3015 input ripple rejection

The LT3015’s ±15% accurate current limit is particularly helpful in applications where the LDO is powered by a front-end switching supply. This accuracy enables tight regulation and limits the size of the inductor, making it easier to reduce solution size. The LT3015 achieves low output voltage noise (60μVRMS) but does not require an external bypass capacitor as is common with most low noise regulators, simplifying the design.

Parallel connection for higher output current

The LT3015 lends itself to a variety of innovative circuit implementations. As shown in Figure 4, two LT3015s can be paralleled in a master-slave configuration to provide higher output currents. Any differences in the regulator input currents are amplified by the LT1366 op amp, which in turn drives current into the slave regulator’s resistor divider to balance the current sharing between the regulators. Note that if one were to simply parallel the two regulators directly without using an op amp, small differences between the two reference voltages would result in the regulator with the highest reference providing most of the load current.

Resistor R5 should be chosen so that it does not produce excessive current swings at the output of the op amp, allowing the ADJ pin of the slave device to reach a balanced state. R5 also helps loop stability by decoupling the compensation capacitor C3 from the ADJ pin of the slave regulator. It is also quite easy to connect multiple regulators in parallel. All that is needed is to add an op amp and a sense resistor for each additional regulator, which is basically a "copy" of the slave regulator in Figure 4.

Figure 4: LT3015 configured in parallel

Linear Technology offers a broad line of negative LDOs and combined dual negative/positive LDOs with output currents ranging from 200mA to 3A, see Table 1 below.

Table 1: Linear Technology's negative LDO products

in conclusion

A well-designed negative LDO requires a little more thought than its positive LDO counterpart. Negative LDOs are used to power key building block ICs such as op amps, ADCs and DACs in a wide range of applications across a variety of market segments. Linear Technology has a growing portfolio of negative LDOs, and our new LT3015 is an innovative 1.5A device that offers low dropout, high accuracy output voltage, low output voltage noise, fast transient response, high PSRR, and the ability to be paralleled for higher output currents, all in a variety of thermally enhanced packages. This provides designers with a simple, easy-to-use choice for their negative rails.