Device guidance

LM61440-Q1 is the selected primary buck regulator for Hengshun's current 12 V nominal automotive reference design. In that design it operates from a protected node—not directly from an unbounded vehicle battery interface—and converts a calculated 6–28 V continuous input envelope to a 5 V / 3 A / 15 W target. The exact LM61440AASQRJRRQ1 variant, 400 kHz setting, Auto Mode and enabled spread spectrum are locked choices for that reference design, not universal recommendations for every LM61440-Q1 application.

Thermal considerations

  • Calculate IC switching and conduction loss together with inductor and capacitor loss across line, load, mode and tolerance; no board-level 3 A thermal evidence exists yet.
  • Use the package thermal land, vias and board copper as an intentional heat path and confirm temperatures in the actual enclosure and operating profile.

Protection considerations

  • LM61440-Q1 is downstream of the selected LM74930-Q1 protection controller, pass MOSFETs and TVS; it is not the raw battery-side surge or reverse-polarity barrier.
  • The current calculation has a positive but tight 0.358259 A screen between the 3.641741 A worst calculated inductor peak and a conservative 4.0 A published current-limit floor. Duty dependence, temperature and transient behavior require hardware characterization.
  • Dynamic load dump, protected-node overshoot, reverse polarity, TVS energy and pass-MOSFET SOA have no completed system evidence in the current record.

Application contexts

  • Downstream of a separately engineered automotive surge, reverse-polarity and disconnect front end
  • Non-isolated 5 V rails in ECUs or related automotive electronics when the protected source and load envelope fit the reviewed design

Typical architectures

  • Protected battery interface feeding local input capacitance, an integrated synchronous buck stage and a supervised 5 V rail
  • Distributed ECU power architecture in which the protection boundary and point-of-load conversion are reviewed as distinct functions

Electrical considerations

  • Start from the protected-node minimum, maximum and event residual rather than using the nominal battery voltage as the complete input requirement.
  • Check conversion ratio, minimum on/off timing, inductor ripple and current-limit behavior across the full approved input and load envelope.
  • Treat the current reference design's 400 kHz, Auto Mode and spread-spectrum choices as one calculated operating point that still needs hardware verification.

Input and output design

  • The current Hengshun calculation targets a protected 6–28 V continuous input and 5 V / 3 A / 15 W output; the 36 V protected-node event target is not a continuous regulation point.
  • Its 100 kΩ / 24.9 kΩ feedback pair calculates 5.016064 V from the nominal 1 V reference before reference and resistor tolerances are applied.

Inductor selection

  • The current design selects a 10 µH Coilcraft XAL1080-103MED after ripple, saturation, RMS-current and tolerance-floor screening.
  • The worst calculated peak at the −20% inductance floor is 3.641741 A; magnetic loss, temperature rise and the final board environment remain measurement tasks.

Capacitor selection

  • Keep the two mandatory high-frequency VIN bypass capacitors at their corresponding VIN/PGND pin pairs and minimize their switching loops.
  • The four-part 4.7 µF / 100 V TDK CGA8N3X7S2A475K230KB input bank remains provisional: exact effective capacitance at 28 V is unresolved and the part carries Production (NRND) lifecycle caution.
  • The calculated output starting point is four 22 µF / 16 V automotive X7R capacitors with a 33 pF feed-forward capacitor and 1 kΩ series resistor; transient response still requires verification.

Layout considerations

  • Place local input ceramics, the IC power pins and ground returns to minimize the hot-loop area, and keep switch-node copper only as large as electrical and thermal needs require.
  • Route feedback, RT and other quiet signals away from the switch node; spread spectrum supports the EMI plan but does not replace filtering, grounding, layout or measured emissions evidence.

Suitable applications

  • Calculated 12 V nominal automotive ECU 5 V rails behind an approved protection stage
  • Automotive auxiliary conversion where requirements, passives, layout and validation are reviewed for the actual module
Official manufacturer product documentation ↗Official datasheet ↗

01

Selected role in the current reference design

Hengshun selected LM61440-Q1, orderable variant LM61440AASQRJRRQ1, as the primary regulator in the calculated 12 V nominal automotive buck reference design. The device receives power from VPROTECTED after the separate input-protection stage and is responsible for the local 5 V conversion. Existing TPS54260-Q1, LM76002-Q1 and LM65625-Q1 guides remain useful comparison material, but those parts are not presented as the primary implementation in this design.

Input boundary
6–28 V continuous at the protected node
Output target
5 V / 3 A / 15 W, calculated
Switching choice
400 kHz; Auto Mode; spread spectrum enabled
Architecture boundary
Not connected directly to an unbounded battery input

02

Requirements before component selection

Define normal battery states, cold crank, jump start, reverse battery, load-dump residual, continuous protected voltage, output load and allowable dropout before finalizing a buck stage. For the current reference design, 9–18 V is the normal battery context, while the regulator calculation uses the separately bounded 6–28 V protected continuous range. A protected-node event target no higher than 36 V is an architecture goal; dynamic overshoot has not been established by simulation or capture.

03

Power-stage and feedback choices

The current calculation uses a 33.2 kΩ RT resistor for the 400 kHz setting and the AAS factory option for Auto Mode with spread spectrum. Four operating points at 6 V, 12 V, 18 V and 28 V were checked for ripple and timing. The 100 kΩ / 24.9 kΩ feedback pair calculates 5.016064 V nominally. These are traceable choices for the referenced design; a different input range, switching plan, rail tolerance or load requires a fresh calculation.

04

Magnetics, capacitors and current margin

The selected 10 µH XAL1080-103MED clears the calculation's inductance bounds and produces a worst tolerance-floor peak of 3.641741 A. Compared with the conservative 4.0 A published threshold floor used by the review, the resulting +0.358259 A margin is positive but tight. The value is a screening result, not a guarantee of dynamic current-limit behavior. Output and local bypass networks follow the calculated starting point, while the input MLCC bank remains a visible evidence gap.

Provisional CIN boundary

  • Four CGA8N3X7S2A475K230KB 4.7 µF / 100 V candidates are retained.
  • Exact effective capacitance per part and for the bank at 28 V remains unresolved.
  • The selected TDK part has Production (NRND) lifecycle caution.
  • HGS25 does not close the ≥10 µF effective-bank gate or reopen the selection.

05

Thermal and low-EMI implementation

Compact synchronous conversion concentrates switching current and heat in the IC, local capacitors and inductor. Preserve the intended pin-level loops, keep the switch node controlled, separate quiet signal returns and provide copper and vias for heat spreading. The selected mode and spread spectrum can support an emissions strategy, but neither establishes board temperature margin or electromagnetic performance. Those conclusions depend on the final stackup, enclosure, cable set and test conditions.

06

Protection boundary and evidence still required

Coordinate converter UVLO, current limit and restart with the LM74930-Q1-based upstream disconnect and clamp functions. Before a release decision, review the final schematic and layout and obtain line/load transient, startup, short-circuit, thermal and EMI evidence. The current system has no completed bench, thermal, EMI, dynamic load-dump, dynamic overshoot, reverse-polarity, MOSFET-SOA, TVS-energy or compliance evidence, so this page remains a calculated application guide.