ADI LTC3115-1: 40-V Wide-Input-Range Buck-Boost Converter
Overview
The LTC3115-1 from Analog Devices (formerly Linear Technology) is a monolithic synchronous buck-boost DC-DC converter that works over a very wide 2.7 V to 40 V input and output range. A single inductor and four integrated switches let it regulate an output that is above, below, or equal to the input, while a low 50 uA no-load quiescent current keeps it suitable for always-on and battery-backed systems. Its combination of high input voltage, wide output range and low transition noise makes it a natural choice for automotive and industrial rails that must survive cold-crank and load-dump events. This article covers its verified specifications and design notes.
Part Overview
The LTC3115-1 integrates the four power switches and the control circuitry in a thermally enhanced 20-pin TSSOP package. A proprietary switching algorithm optimizes efficiency whether the input is above, below, or equal to the output, and moves the converter between buck, buck-boost and boost regions with low audible and RF noise. It provides true output disconnect in shutdown, so the load is isolated from the input source when disabled. The LTC3115-2 is a related fixed-frequency variant; the family targets the gap between battery-powered low-voltage parts and high-voltage controller designs.Key Features
Wide 2.7 V to 40 V input and output ranges.
Monolithic synchronous four-switch buck-boost with integrated switches.
Low 50 uA quiescent current at no load for always-on systems.
True output disconnect in shutdown.
Low transition noise for audio and RF sensitive loads.
Programmable switching frequency set by a resistor.
Thermally enhanced 20-pin TSSOP power package.
Key Specifications
Input voltage range: 2.7 V to 40 V (per datasheet)
Output voltage range: 2.7 V to 40 V (per datasheet)
Output current: 2 A in step-down operation for VIN at least about 6 V; about 0.8 A when regulating 5 V from VIN at least 3.6 V (per datasheet)
Quiescent current: about 50 uA at no load (per datasheet)
Topology: monolithic synchronous four-switch buck-boost
Switching frequency: programmable by resistor, roughly 100 kHz to 2.2 MHz (typical - refer to official datasheet)
Shutdown: true output disconnect between input and load
Package: 20-pin thermally enhanced TSSOP
(Values per the official LTC3115-1 datasheet; confirm current capability for your Vin/Vout.)Applications and Design Notes
The wide input and output ranges make the LTC3115-1 suited to automotive and industrial supplies where the battery or bus can dip well below the target rail (cold-crank) or spike well above it (load-dump). It also fits multi-chemistry battery systems, distributed 24 V industrial rails, and always-on instrumentation that must keep running with little quiescent drain. Because the output can be programmed from a few volts up to 40 V, the same part can generate a 5 V, 12 V, or 24 V rail.
Design guidance:Choose the inductor for the buck-boost transition region, where switch currents are highest; its saturation rating must exceed the peak current across the operating window.
Place a low-ESR input capacitor close to the VIN pins to handle the pulsed current of the boost stage, especially during cold-crank.
Set the output voltage with the feedback divider to the internal reference, and use the frequency-setting resistor to place the switching frequency away from sensitive bands.
Use the enable pin for sequencing and shutdown; remember the output disconnects the load in shutdown, which protects the battery.
Provide adequate copper area under the thermal-enhanced TSSOP exposed pad, since the integrated switches dissipate appreciable power at 2 A.
Design Considerations and Common Mistakes
The 2 A rating applies mainly in step-down at higher input voltage. At low input and a 5 V output the available current is closer to 0.8 A; check the datasheet curves for your Vin/Vout pair rather than designing to 2 A everywhere.
A wide input range invites transients. Even though the part is rated to 40 V, add input protection such as TVS or a pre-filter for automotive load-dump, and verify the abs-max margin.
The buck-boost region is where efficiency dips and stress peaks. Design the worst-case input condition (Vin near Vout) rather than the nominal.
Because the part stays on at no load with 50 uA IQ, it is ideal for always-on rails, but the same light-load behavior must be checked for noise at the final load.
Feedback traces must be short and away from the switch node, or the wide input transients couple into the loop and degrade regulation.
Conclusion
The LTC3115-1 packs a 2.7 V to 40 V monolithic buck-boost into a TSSOP, giving designers a single part that holds a regulated rail through the deepest cold-crank and the highest load-dump. Its 50 uA no-load current and true output disconnect fit always-on and battery-backed systems. The design effort centers on inductor sizing for the buck-boost corner, input transient protection, and a solid thermal layout, all of which are standard for a wide-input converter of this class.
FAQ
Q: What is the input voltage range of the LTC3115-1?
A: 2.7 V to 40 V, which covers 12 V and 24 V automotive and industrial rails including cold-crank and load-dump transients (per datasheet).
Q: How much output current can it deliver?
A: Up to 2 A in step-down when the input is at least about 6 V; for a 5 V output from a 3.6 V or higher input, expect about 0.8 A. Check the curves for your exact condition.
Q: Why is the quiescent current important here?
A: The 50 uA no-load current keeps the converter efficient in always-on and standby systems that spend most of their life lightly loaded.
Q: What happens when the part is disabled?
A: The output is disconnected from the input, which prevents the battery or bus from feeding the disabled load and minimizes drain.
Q: What is the difference between the LTC3115-1 and LTC3115-2?
A: They are related wide-input buck-boost family members with different switching-frequency options; consult the datasheets for the exact frequency and mode differences.
For exact specifications and design guidance, refer to the official Analog Devices datasheet for LTC3115-1.
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