Steve Knoth, Senior Product Marketing Engineer, Power Products Group at Linear Technology Corporation explores the dynamics of a 1.5A negative low dropout linear regulator and its suitability for application demand
High performance inverting switching regulators or inverting charge pumps provide lower output voltages and minimal heat buildup. The downside has typically been cost, complexity and (in some cases), output noise. However, with improved LDO design techniques – including lower reference voltages and lower dropout voltages for higher efficiency operation – low dropout regulators now have a growing presence in many applications.
Additionally, some negative rails require accurate regulation and in many cases, low noise, fast transient response, and good supply rejection are also desirable.
A negative, low noise LDO with medium to high-level current capability and wide input/output voltage range was required by the industry and Linear Technology has a new product to address these needs. The LT3015 is a 1.5A negative low dropout linear regulator featuring fast transient response, low noise and precision current limit.
With a wide input voltage range of -1.8V to -30V and adjustable output voltage from -1.220V to -29.5V, the device’s common emitter NPN power transistor design requires only a single supply and enables a low dropout voltage of 310mV (typical at full load). Output voltage noise is 60µVrms over a 10Hz to 100kHz bandwidth range.
The device features operating current of 1.1mA and drops to <1µA in shutdown. Quiescent current is also well controlled in dropout. The regulator is the negative complement to one of the company’s positive LDOs, the 1.5A LT1963A, and is ideal for negative logic supplies, low noise instrumentation, industrial supplies, and for post-regulating switching supplies.
The reference-amplifier topology provides precision DC characteristics, as well as good loop stability with an extremely wide range of output capacitors, including small, low-cost ceramic output capacitors. It is stable with only a 10µF output capacitor.
The tiny external capacitors can be used without the necessary addition of series resistance (ESR) as is common with many other regulators. The device exhibits bidirectional shutdown capability that enables it to operate with either positive or negative logic.
As the device has a 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.
Cool feature specifications
The LT3015 is available in a low profile (0.75mm) 3mm by 3mm 8-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. Operating junction temperature is as wide as -55°C to 125°C depending on the grade.
The design offers several features that enable simplicity and high performance. The high gain-bandwidth error amplifier allows for fast transient response and high power supply rejection (PSRR).
Additionally the device’s PSRR has demonstrated a four-times improvement over other positive PNP LDO regulators, and maintains a high-level of rejection (>25dB) even out to 10MHz, (see Figures 1 and 2).
A precision current limit of ±15 percent 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 low output voltage noise (60uVrms) without requiring an external bypass capacitor as in common with most low-noise regulators, simplifying the design.
This regulator is well suited to a variety of innovative circuit implementations. Parallelling two of the devices in a master-slave configuration can 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 cause the regulator with the highest reference to deliver the lion’s share of the load current.
A resistor R5 is chosen so 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 de-coupling compensation capacitor C3 from the ADJ pin of the slave regulator. Parallelling multiple regulators is also fairly easy. All it takes is an additional op amp and a sense resistor for each additional regulator.
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 of applications across various market segments.
Linear Technology has a growing portfolio of negative LDOs and the company’s new LT3015 is an innovative 1.5A device that offers low dropout, high accuracy output voltage, low output voltage noise, fast transient response and high PSRR.
It also offers the ability to be paralleled for higher output current – and all this in a wide variety of thermally enhanced packages. This gives designers an easy and simple choice for their negative power rails.