Negative power rails are not going away

Negative LDOs power key building block ICs differently than their positive counterparts

BY STEVE KNOTH
Senior Product Marketing Engineer
Linear Technology
www.linear.com

Negative power rails are used with key IC building blocks such as digital-to-analog converters (DACs), analog-to-digital converters (ADCs), operational amplifiers, and GaAs FET biasing. Symmetrical power supplies (rails) handle ac-signals without dc-offsets.

Obviously, if only a positive rail is present, the output signal can generally assume only positive values depending on the circuit topology; however, with positive and negative rails, the output signal swings from negative to positive voltages. Further, negative supplies are important in applications such as process control where real-world signals are bipolar. For example, in motor control, positive voltages cause clockwise rotation and negative voltages cause counter-clockwise rotation, and the voltage amplitude controls the velocity.

Negative LDOs elegantly supply clean power for negative power rails. A variety of negative LDO applications exist, including:

Military: radar, sonar, thermal imagers, surveillance systemsIndustrial: CCD and IR cameras, lab/test equipment, data acquisition systems, blood/glucose equipment, motors, vibration control, power analyzersMedical: MRI equipment, dental x-ray systems, ultrasound equipment, blood analyzers, endoscopesCommunications: optical modules, fiber-optic transceivers Automotive: antennasRF applications: radios, optical transceivers, power amplifiers, precision instrumentation

High-performance inverting switching regulators or inverting charge pumps have traditionally served these applications, providing lower output voltages and minimal heat buildup. The downside has typically been cost, complexity (in some cases), and output noise. However, with improved LDO design techniques including lower reference voltages and lower dropout voltages for higher-efficiency operation low-dropout regulators can now have a growing presence in these applications. Additionally, some negative rails require accurate regulation and in many cases, low noise, fast transient response, and good supply rejection are desired as well.

An ideal solution to the problems outlined above would be a negative, low-noise LDO with medium- to high-level current capability and a wide input/output voltage range. Linear Technology has a new product offering to address this problem.

Negative 1.5-A low-noise LDO

The LT3015 1.5-A negative low-dropout linear regulator features fast transient response, low noise, and precision current limit. With its wide input voltage range of 1.8 to 30 V and adjustable output voltage from 1.220 to 29.5 V, the device’s common emitter npn power transistor design requires only a single supply and enables a low-dropout voltage of 310 mV (typical at full load).

Output voltage noise is 60 µVrms over a 10-Hz to 100-kHz bandwidth. The LT3015 features operating current of 1.1 mA and drops to

The LT3015’s reference-amplifier topology provides precision dc characteristics, as well as loop stability with a wide range of output capacitors, including small, low-cost ceramic output capacitors. It is stable with only a 10-µF output capacitor. These external capacitors can be used without the necessary addition of series resistance (ESR) as is common with many other regulators.

The LT3015 exhibits bidirectional shutdown capability that allows the device to operate with either positive or negative logic levels. Moreover, due to the LT3015’s high shutdown threshold accuracy, the SHDN pin can be used to set a programmable undervoltage lockout (UVLO) threshold for the regulator input supply. Internal protection circuitry for the IC includes reverse-output protection, precision current limiting with foldback to keep the power transistor in its safe operating area, and thermal limiting with hysteresis.

Fig. 1: LT3015 typical application circuit.

The LT3015 is available in a low-profile (0.75-mm) 3 x 3-mm eight-lead DFN with backside thermal pad, a 12-lead MSOP package with backside thermal pad, a through-hole power TO-220 package, and a surface mount power DD-Pak package. The operating junction temperature extends from 55 to 125C, depending on the grade.

Cool features

The LT3015’s design offers several features that enable simplicity and high performance in any design. The high gain-bandwidth error amplifier allows for fast transient response and high power supply rejection (PSRR). Additionally the device’s PSRR shows over a 4X improvement over leading edge positive PNP LDO regulators, and maintains a high level of rejection (>25 dB) even as high as 10 MHz, (see Figs. 2 and 3 ).

Fig. 2: LT3015 transient response.

Fig. 3: LT3015 input ripple rejection.

The LT3015’s current limit of ±15% is especially helpful in applications where the LDO is powered by a front-end switcher. This precision enables tight regulation and limits sizing on the inductor, for a smaller solution size. The LT3015 achieves 60-µVrms output voltage noise without requiring an external bypass capacitor as is common with most low-noise regulators.

Paralleling for higher output current

The LT3015 lends itself well to a variety of innovative circuit implementations. Figure 4 illustrates paralleling two LT3015’s in a master-slave configuration to accommodate higher output current.

Any difference in the input currents of the regulators is amplified by the LT1366 op amp, which in turn drives current into the resistor divider of the slave regulator to balance current sharing between the regulators. Note that if one were to just parallel the two regulators directly without an op amp, tiny variations between the two reference voltages would cause the regulator with the highest reference to deliver the lion’s share of the load current.

Resistor R5 is chosen such that it does not take excessive current swing at the output of the op amp to bring the ADJ pin of the slave device into balance. Moreover, R5 also helps with loop stability by decoupling compensation capacitor C3 from the ADJ pin of the slave regulator. Paralleling multiple regulators is also fairly easy. All it takes is an additional op amp and a sense resistor for each additional regulator — essentially a replica of the slave regulator in Fig. 4.

Fig. 4: LT3015 configured in parallel.

Well-designed negative LDOs take a bit more thought than their positive counterparts. Negative LDOs are used to power key building block ICs such as op amps, ADCs, and DACs in a wide variety applications across various market segments. Negative LDOs are used to power key building block ICs. ■