Schematic Symbol for LM3671 3.3V Buck Converter Breakout: LM3671...

LM3671 Buck Converter Overview

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Gathering circuit details.

Here’s a clear explanation of your “LM3671 3.3V Buck Converter Breakout” project: what it does, what’s inside, how it works, and how to improve or extend it.

What it is and purpose

A compact, high-efficiency 3.3 V power module intended to replace linear regulators (like LM1117-3.3) with a switching buck converter for much higher efficiency from USB or a single Li‑ion source.
Regulates a higher input (typ. USB/Li‑ion) down to 3.3 V at up to 600 mA.

Core components and their roles

Controller IC: U1 (TI LM3671MF-3.3/NOPB)
- Features (from the TI datasheet): fixed 3.3 V output, 600 mA load capability, 2 MHz PWM switching (typ), automatic PFM/PWM mode, synchronous rectification, internal soft-start, very low quiescent and shutdown current. Designed for single Li‑ion/USB inputs.
Output capacitor: C1 22 µF MLCC (0805)
Input capacitor: C2 10 µF MLCC (0805)
Inductor: L1 2.2 µH (0805)
Enable pull-up: R1 100 kΩ to VIN
I/O header: J1 1x4 2.54 mm (VIN, VOUT, GND, spare)

How they interact (design and implementation)

Input path: J1 Pin_2 is VIN. It feeds U1:VIN and the input bypass C2 (C2:P1 to VIN, C2:P2 to GND). Short, low-impedance placement here reduces input ripple current path.
Switching node: U1:SW drives L1 (L1:P1 on SW). The inductor’s other end (L1:P2) is the 3.3 V output node.
Output rail: The output node ties to C1 (22 µF to GND) and J1 Pin_3 (3.3 V). This provides transient support and ripple filtering.
Feedback/sense: U1:FB is tied to the output node (same net as L1:P2 and C1:P1). For fixed-output variants, TI notes the output sense is internally configured for the target voltage; ensure your symbol/pin mapping matches the fixed 3.3 V guidance in the datasheet for this exact MPN.
Enable: U1:EN is pulled up to VIN by R1 (100 kΩ), so the regulator is on when VIN is present. J1 Pin_1 is currently a spare; you could optionally rewire it to EN for external on/off control.

Key technologies and dependencies

2 MHz switching: enables very small magnetics and capacitors while pushing switching energy above much of the audio band, simplifying ripple filtering.
Synchronous buck with PFM/PWM: synchronous rectification improves efficiency at medium/high load; automatic PFM at light load cuts quiescent loss. The datasheet lists 16 µA typ quiescent and 0.01 µA typ shutdown current.
Magnetics and MLCCs: success depends on correct inductor value/current rating and low-ESR ceramic capacitors placed very close to the IC pins.

Why these design choices (trade-offs and optimizations)

2.2 µH inductor: a common sweet spot for 2 MHz, balancing ripple current, transient response, and DCR losses. Knowledge-based guidance: choose inductor saturation current ≥ 1.2× peak load, keep DCR low, and target ripple 20–40% of full load.
22 µF output MLCC: more than the typical 10 µF recommendation for this family, reducing output ripple and improving transient response at the cost of a slightly larger footprint and potential DC-bias derating considerations.
10 µF input MLCC: provides input surge and ripple current buffering right where the current pulses occur.
EN default-high with 100 kΩ: zero-cost auto-enable without burning appreciable current; can be overridden if you expose EN.

Performance considerations and potential challenges

Layout criticality: keep the hot loop (U1 SW pin -> L1 -> C1 -> U1 GND) as tight as possible. Place C2 directly at VIN and C1 at VOUT, with short, wide traces to GND.
EMI and ringing: fast edges at the SW node can radiate. If ringing is observed, consider a snubber or layout iteration. Avoid adding LC or π filters at the output without stability analysis; low-cutoff filters (e.g., around 10 kHz) can interfere with the regulator feedback loop.
Component derating: for ceramics, ensure operating voltage is ≤ 60% of rating; for buck outputs, a simple rule is Vrating ≥ 1.5× VOUT. For 3.3 V, 10 V MLCCs are a safe default to counter DC-bias derating.
Inductor current: verify both Isat and Imax; use the lower limit. Evaluate DCR losses (I_DC × DCR) and thermal rise at your max load.
Thermal: at 600 mA, synchronous bucks run cool compared to LDOs, but still check copper area and ambient to keep junction temperatures in range.

Real-world applications

3.3 V rail for MCUs, BLE/Wi‑Fi modules, sensors, radios, and logic.
USB- or Li‑ion-powered gadgets where efficiency and battery life matter, or where linear regulators would overheat.

Areas for improvement and future enhancements

Control and monitoring
- Wire J1 Pin_1 to EN with a small series resistor for external on/off or a microcontroller GPIO.
- Add a “power good” indicator LED (with series resistor) on VOUT and/or a test pad set for VIN, SW, VOUT, GND.
Robustness
- Add input protection: reverse-polarity protection, optional TVS or resettable fuse for harsh plug-in environments.
- Consider a small 0.1 µF ceramic in parallel with C2 to reduce high-frequency input impedance.
EMI/layout refinements
- Keep the SW copper minimal and avoid routing under sensitive nets. If needed, add an RC snubber from SW to GND (per bench validation).
Scalability
- Offer an adjustable-output variant of LM3671 family on the same footprint (jumper/selectable BOM) to support 1.8/2.5/5.0 V rails.
- Provide alternate inductor footprints to accommodate higher Isat parts for sustained 600 mA loads with lower DCR.
- For higher current needs, design a sister module using a higher-current pin-compatible family and replicate the pin header footprint.

Component and net map (at-a-glance)

Input: J1 Pin_2 -> VIN net -> C2:P1, U1:VIN; C2:P2 -> GND.
Switch path: U1:SW -> L1:P1; L1:P2 -> VOUT net.
Output: VOUT net -> C1:P1, U1:FB, J1 Pin_3; C1:P2 -> GND.
Enable: U1:EN -> R1:P2; R1:P1 -> VIN.

Notes tied to the TI LM3671 datasheet

Fixed 3.3 V variant, 600 mA max load, 2 MHz PWM typ, automatic PFM/PWM, synchronous rectification, internal soft-start, 16 µA typ quiescent, 0.01 µA typ shutdown. Output MLCCs are expected (low-ESR ceramic). If you plan to change values, follow TI’s inductor and output capacitor guidelines for stability and ripple.

If you want, I can review the PCB layout loops for EMI/thermal risk, or propose a small ECO set (test pads, EN header option, input protection, snubber footprint) and update the design.

53 days

Ready.

VIN

Manufacturer Part Number

LM3671MF-3.3/NOPB

VIN

Resistance

100k Ω

VIN

Inductance

2.2u H

GND

3.3V

VIN

Capacitance

10u F

FID1

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Electrical Rule Checks

Missing Footprints

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Design Rule Checks

Overlapping Copper • 33

Des	Net	Des	Net
Trace 3b83	GND	U1:GND	None
Trace d8db	GND	C2:P2	None
Trace 1a42	Net - (9a33 - undefined)	3.3V:3.3V	None
Trace 1a42	Net - (9a33 - undefined)	J1:Pin_3	None
Trace 6598	Net - (9a33 - undefined)	L1:P2	None
Trace 132e	Net - (88ec - 404d)	U1:SW	None
Trace 709c	Net - (88ec - 404d)	L1:P1	None
Trace 01fe	Net - (244a - ab35)	C2:P1	None
Trace 9dc5	Net - (244a - ab35)	C2:P1	None
Trace aae9	Net - (244a - ab35)	C2:P1	None
Trace 3f7f	Net - (244a - b714)	EN:EN	None
Trace 55af	Net - (244a - b714)	EN:EN	None
Trace 1fa0	Net - (546e - f9d5)	R1:P2	None
Trace 2eeb	Net - (546e - f9d5)	U1:EN	None
C1:P1	None	Trace 269c	Net - (9a33 - undefined)
C1:P1	None	Trace bbbc	Net - (9a33 - undefined)
C1:P2	None	Trace 6b99	GND
GND:GND	None	Trace f8e4	GND
J1:Pin_1	None	Trace 3f7f	Net - (244a - b714)
J1:Pin_1	None	Trace 55af	Net - (244a - b714)
J1:Pin_1	None	Trace 5db0	Net - (546e - f9d5)
J1:Pin_2	None	Trace 54f8	Net - (244a - ab35)
J1:Pin_2	None	Trace 60cc	Net - (244a - ab35)
J1:Pin_2	None	Trace 67d3	Net - (244a - ab35)
J1:Pin_3	None	Trace ffa3	Net - (9a33 - undefined)
J1:Pin_4	None	Trace 357b	GND
J1:Pin_4	None	Trace f8e4	GND
R1:P1	None	Trace fc50	Net - (244a - ab35)
R1:P2	None	Trace d352	Net - (546e - f9d5)
U1:FB	None	Trace cb41	Net - (9a33 - undefined)
U1:GND	None	Trace a3af	GND
U1:VIN	None	Trace dfee	Net - (244a - ab35)
VIN:VIN	None	Trace 60cc	Net - (244a - ab35)

Invalid Layer

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Component Overrides • 12

	Designator	Ruleset
	＊｀REF｀＊	Override rule for ＊｀REF｀＊
	＊｀REF｀＊	Override rule for ＊｀REF｀＊
	＊｀REF｀＊	Override rule for ＊｀REF｀＊
	＊｀REF｀＊	Override rule for ＊｀REF｀＊
	＊｀REF｀＊	Override rule for ＊｀REF｀＊
	＊｀REF｀＊	Override rule for ＊｀REF｀＊
	＊｀REF｀＊	Override rule for ＊｀REF｀＊
	＊｀REF｀＊	Override rule for ＊｀REF｀＊
	＊｀VALUE｀＊	Override rule for ＊｀VALUE｀＊
	＊｀VALUE｀＊	Override rule for ＊｀VALUE｀＊
	＊｀VALUE｀＊	Override rule for ＊｀VALUE｀＊
	＊｀VALUE｀＊	Override rule for ＊｀VALUE｀＊

Airwires

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Deprecated Rules

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Layers with Multiple Fills

Reports layers that have multiple copper fills of different nets. Make sure the Connected Layers rule value of nets with fills is valid.

Floating Copper • 6

	Name
	Via
	Via
	Via
	Via
	Trace 7eb0
	Trace 5a8e, Trace 5ce3

Protected Intrusions

Reports intrusions from objects of other nets into polygons or fills that have the Protected layout rule applied.

Manufacturing

Manufacturer Part Number • 5

	Designator	MPN
	C1	None
	C2	None
	J1	None
	L1	None
	R1	None

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