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Synchronous Boost Controller, External FET, ILIM, Reverse Blocking

Synchronous Boost Controller, External FET, ILIM, Reverse Blocking

← Back to Buck / Boost / Buck-Boost Regulators

This topic takes you from principles to production for synchronous boost controllers using external MOSFETs. It explains how synchronous rectification and reverse blocking improve efficiency and safety, and how to design soft-start, current limit (ILIM), compensation, PCB layout, and protections for USB-PD, 48 V, automotive, and PoE applications.

You’ll find engineer-ready checklists, a pitfalls → fixes table, a brand/part matrix for quick screening, and a 48-hour BOM submission path for cross-brand alternatives and small-batch sourcing. All sections are SEO-friendly and WordPress-ready with responsive visuals.

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Introduction

A synchronous boost controller drives external MOSFETs to step up voltage with high efficiency. Compared with diode-rectified (non-synchronous) boost, it replaces the rectifier with a synchronous MOSFET to reduce conduction loss, enables forward/reverse blocking to prevent back-flow, and provides soft-start and programmable current limit (ILIM) for safe startup and robust protection.

Synchronous vs Non-Synchronous

  • Non-sync: diode drop → heat & lower efficiency, especially at high current.
  • Sync: MOSFET replaces diode → lower RDS(on) loss, better light-load strategy.
  • Supports reverse-current blocking and finer control of dead-time.

Typical Use Cases

  • USB-PD boost (e.g., 5→20 V), power banks, portable instruments.
  • Automotive cold-crank bridging, dual-battery systems.
  • PoE step-up, supercap hold-up, audio and sensitive analog rails.

Quick Selection Signals

  • VIN/VOUT/IOUT envelope, fSW vs magnetics trade-off.
  • External FET: RDS(on), Qg, package thermals.
  • Soft-start ramp time, ILIM set window, reverse-blocking behavior.
Synchronous Boost Architecture VIN, inductor, high-side and low-side external MOSFETs, controller IC with gate drivers, soft-start and ILIM, and VOUT. Forward energy flow arrows; reverse-block indicator. Synchronous Boost — Architecture (External FETs) VIN L QH QL Boost Controller Gate Drivers Soft-Start ILIM Reverse Block VOUT Reverse Block Buck / Boost / Buck-Boost Regulators
Fig.1 — Architecture: external MOSFETs, controller with gate drivers, soft-start, ILIM, and reverse blocking.

How Synchronous Boost Works

Operation alternates between energy charge and transfer phases. The controller times MOSFETs with proper dead-time to avoid shoot-through, enforces ILIM during peaks, and ramps soft-start to mitigate inrush and overshoot. Efficiency depends on MOSFET RDS(on), switching losses, inductor value, and switching frequency.

Two Phases

  • Charge (QL on): inductor current rises, di/dt = VIN/L.
  • Transfer (QH on): energy to VOUT, di/dt = (VIN − VOUT)/L (negative slope).
  • Ideal duty: D ≈ 1 − VIN/VOUT, limited by minimum on/off times.

Modes & Timing

  • CCM for higher load: lower ripple, higher efficiency.
  • DCM at light load: manage sync timing to avoid reverse current.
  • Dead-time tuning balances shoot-through risk and conduction loss.

Loss Breakdown

  • Conduction: P ≈ IRMS2 · (RDS(on)H + RDS(on)L + ESR).
  • Switching: P ≈ 0.5 · V · I · (tr+tf) · fSW and gate-drive charge.
  • Higher fSW shrinks magnetics but raises PSW and EMI risk.
Synchronous Boost Timing and Energy Flow Two phases (charge and transfer), IL waveform, VSW waveform, soft-start ramp, ILIM action, and reverse-current prevention. Timing & Energy Flow Phase A — Charge (QL on) di/dt = VIN/L Phase B — Transfer (QH on) (VIN − VOUT)/L Reverse-Current Blocking Key Waveforms IL VSW Soft-Start Ramp ILIM Dead-Time
Fig.2 — Timing and energy flow: IL waveform, VSW, soft-start ramp, ILIM clipping, and dead-time annotation.

Duty (ideal): D ≈ 1 − VIN/VOUT. Conduction loss: P ≈ IRMS2 · (RDS(on)H + RDS(on)L + ESR). Switching loss: P ≈ 0.5 · V · I · (tr+tf) · fSW. Tune dead-time to minimize shoot-through while avoiding excess body-diode conduction.

Key Features & Engineering Benefits

Synchronous boost controllers add system-level value with forward/reverse blocking, soft-start shaping, programmable ILIM, and the efficiency/thermal headroom enabled by external MOSFETs.

Forward/Reverse Blocking

  • Prevents back-feed when VOUT > VIN or during hot-plug.
  • Controller coordinates MOSFET states and dead-time.
  • Improves safety of upstream rails and parallel sources.

Soft-Start (SS)

  • Inrush control and overshoot reduction.
  • Slope-controlled VOUT; coordinates with ILIM clipping.
  • Helps EMI compliance and avoids brownout of VIN.

Programmable ILIM

  • Peak/average limit via Rsense or lossless sensing.
  • I_LIMIT ≈ V_SENSE / R_SENSE (compensate RDS(on) tempco if sensorless).
  • Blanking/filtering avoids false trips at minimum on-time.

External MOSFET Efficiency & Thermals

  • Conduction: P_COND ≈ I_RMS² · (RDS(on)H + RDS(on)L).
  • Switching: P_SW ≈ 0.5·V·I·(tr+tf)·fSW + Qg·Vg·fSW.
  • Selectable FETs scale current capability and heat spreading.
Key Features Summary Four parameter cards highlighting reverse blocking, soft-start, ILIM programmability, and external MOSFET efficiency/thermal advantages. Includes a compact comparison strip. Forward/Reverse Blocking Prevents back-feed; coordinates MOSFET timing. Protects upstream and parallel rails. Soft-Start Controls inrush; reduces overshoot and EMI. Ramps with ILIM coordination. Programmable ILIM Peak/average limit with sense or sensorless. Blanking/filter to avoid false trips. External MOSFET Benefits Lower loss with chosen RDS(on); scalable thermals. Gate-charge trade for high fSW. Architecture Comparison Efficiency Thermals Reverse Block Scalability Sync + External FET Non-sync (Diode) Sync + Integrated FET
Fig.3 — Parameter-focused summary: reverse blocking, soft-start, ILIM, and external MOSFET efficiency/thermal benefits.

External MOSFET Selection & Layout Rules

FET Selection

  • RDS(on) vs Qg trade; select with high-temp curves.
  • Ratings margin: VDS, IDS ≥ 1.3–1.5× worst case.
  • Package thermals (RθJC/RθJA); parallel with matched routing.

Drivers & Protection

  • Gate resistor(s) tune EMI vs loss; split RG for QH/QL if needed.
  • Dead-time to avoid shoot-through; limit body-diode duration.
  • Gate OVP/UV; optional negative off-bias for robustness.

Sense & Compensation

  • Kelvin Rsense away from high di/dt nodes; short sense loop.
  • Sensorless ILIM: account for RDS(on) tempco.
  • Analog ground kept clean; single-point PGND/AGND tie.

PCB Layout Rules

  • Minimize high-current loop: VIN–FET–L–VOUT and return path.
  • Keep SW node compact; pull into inner layer if needed.
  • Place decoupling at loop ends; use low-ESL packages.
  • Thermal via arrays and wide copper for heat spreading.
External MOSFET Connection & PCB Layout Left: QH/QL connection and energy flow. Right: top-view PCB with minimal loop, small SW node, decoupling positions, and thermal via arrays. External FET Connection VIN QH QL Controller Gate Drivers SS / ILIM VOUT Reverse Block PCB Layout Top View VIN Cap QH QL Inductor VOUT VOUT Cap Minimize High-Current Loop SW Node (Keep Small) Thermal Vias Close to VIN Close to VOUT
Fig.4 — External MOSFET connection and PCB layout: minimize high-current loop, keep SW node compact, place decoupling at loop ends, and use thermal vias.

Duty: D ≈ 1 − VIN/VOUT. Ripple: ΔIL ≈ (VIN/L) · (D/fSW). Peak: IL,pk ≈ IOUT·VOUT/(η·VIN) + ΔIL/2. Loss: P_COND ≈ IRMS2·ΣR, P_SW ≈ 0.5·V·I·(tr+tf)·fSW + Qg·Vg·fSW.

Control & Compensation

Choose the right control mode and design compensation to achieve stable bandwidth and phase margin. In boost converters, the right-half-plane zero (RHPZ) limits bandwidth; output capacitor ESR introduces a zero that can aid phase if properly placed.

Current-Mode (CM)

  • Inner current loop boosts load-step response.
  • Compensation often Type II; simpler than VM.
  • Watch subharmonics/minimum on-time at high duty.

Voltage-Mode (VM)

  • Simpler plant; compensation often Type III.
  • Transient slower vs CM; slope-comp less critical.
  • Mind RHPZ and ESR zero alignment.

Design Targets

  • Phase margin PM ≥ 45–60° at crossover.
  • Bandwidth BW ≤ fRHPZ/5.
  • Place ESR zero to assist phase, not to cross over.
Loop Stability — Control Mode & Compensation Left: comparison of current- and voltage-mode control. Right: Bode plot showing gain, phase, bandwidth, phase margin, RHP zero, and ESR zero markers. Control Modes Current-Mode Fast transients; simpler Type II compensation. Mind minimum on-time & subharmonics. Voltage-Mode Simple plant; often Type III compensation. Crossover well below RHPZ. Design Targets PM ≥ 45–60° · BW ≤ fRHPZ/5 Place ESR zero to aid phase but avoid crossover point. Consider MLCC + polymer mix to tune ESR zero. Bode Plot Gain |G·H| Phase BW PM f_RHPZ f_Z(ESR) log f Gain / Phase
Fig.5 — Loop stability: CM vs VM and Bode plot showing BW, PM, RHPZ, and ESR zero placement.

RHP zero: fRHPZ ≈ VOUT² / (2π · L · IOUT · VIN)  ·  ESR zero: fZ(ESR) ≈ 1 / (2π · COUT · ESR)  ·  Target: PM ≥ 45–60°, BW ≤ fRHPZ/5.

Protection & Reliability Design

Configure ILIM and soft-start to control stress at power-up, add UVLO/OVP/OTP thresholds, and verify reverse-block performance under fault conditions. Use table-driven limits and repeatable test steps for DVT and mass production.

ILIM Configuration

  • I_LIMIT ≈ V_SENSE / R_SENSE (sensorless: include RDS(on) tempco).
  • Blanking/filtering to avoid false trips near minimum on-time.
  • Short-circuit policy: startup vs steady-state, retry intervals.

Soft-Start Waveform

  • Limit inrush; reduce overshoot and audible/EMI events.
  • Coordinate SS ramp with ILIM clipping.
  • Acceptance: VOUT ramp slope and peak input current limits.

Thermal/Voltage Protections

  • OTP threshold and hysteresis sized for thermal time constant.
  • UVLO/OVP margins vs input ripple and worst-case load.
  • Thermal foldback or duty derating for continuous stress.

Reverse-Blocking Tests

  • Cases: VOUT > VIN, hot-plug, parallel source back-feed.
  • Steps: slow ramp, fast step, fault injection (reverse pulse).
  • Pass: peak reverse current < limit, shutoff delay within spec, no oscillation.
Protection & Reliability — Thresholds & Timing Left: soft-start ramp and ILIM clipping timing. Right: threshold table card with typical/min/max values and notes for UVLO/OVP/OTP/ILIM/Reverse Block. Soft-Start & ILIM Timing VOUT (SS) IL ILIM Coordinate SS slope with ILIM to cap peaks. Avoid UVLO false trips during ramp. Key Thresholds (Example) Function Symbol Min Typ Max Unit ILIM (peak)ILIM 4.55.05.5A UVLO (VIN)VUVLO 3.84.04.2V OVP (VOUT)VOVP 21.021.522.0V OTP TripTSD 150160170°C Reverse I LimitIREV −0.2−0.10A Notes • Production margin: design peak ≤ 0.8 × I_LIM_TYP. • UVLO hysteresis ≥ 1.5 × VIN ripple (p-p). • OTP hysteresis sized to thermal time constant. Reverse-Block Test Checklist 1) Case: VOUT > VIN (slow ramp / fast step) 2) Inject short reverse pulse; record I_REV peak 3) Measure shutoff delay; verify no oscillation 4) Confirm auto-recovery or latched policy
Fig.6 — Soft-start timing with ILIM clipping and a threshold table card for ILIM/UVLO/OVP/OTP/reverse-block acceptance.

Production rules: ILIM,typ ≥ 1.2 × design peak · Reverse current peak < spec · Shutoff delay within limit · UVLO hysteresis ≥ 1.5 × VIN ripple · No oscillation during SS and fault recovery.

Application Use Cases

Typical applications for synchronous boost controllers using external MOSFETs. Each card summarizes the VIN/VOUT/IOUT envelope, setup highlights (SS/ILIM/reverse-block), and key risks to watch during design and verification.

USB-PD Boost (5→20 V)

  • Envelope: VIN 5–12 V · VOUT 9/15/20 V · IOUT 1–5 A
  • SS with ILIM clipping · cable drop compensation
  • Reverse-block after PD contract · light-load sync strategy
  • Risk: minimum on-time & fast load steps
USB-PD BoostLeft: 5V USB source and PD negotiation; center: synchronous boost stage; right: 20V output with reverse-block icon. USB 5V Sync Boost SS · ILIM · Gate 20V Reverse-Block PD Contract → Boost
USB-PD: negotiate first, then enable boost; ensure reverse-block and cable drop handling.

48 V Industrial/Robotics

  • Envelope: VIN 18–36(60) V · VOUT 48–56 V · 0.5–5 A
  • High-voltage FET · tight SW node · EMI care
  • BW < fRHPZ/5 · thermal via arrays
  • Risk: surge/overshoot from L·di/dt
48V SystemsInput 24–36V to 48–56V output; caution badge for surge; compact switch node highlight. 24–36V Sync Boost SW Node 48–56V HV
48V: keep SW node compact, use HV FETs, control EMI and surge.

Automotive Audio (12 V)

  • Envelope: VIN 6–16 V · VOUT 18–24 V · 2–8 A
  • AEC-Q targets · SS to reduce pop & ripple
  • Reverse-block vs parallel rails · ground hygiene
  • Risk: cold-crank & load-dump events
Automotive Audio12V input to ~20V rail; car icon; cold-crank cloud; reverse-block arrow. 12V Sync Boost ~20V Cold
Automotive: design for cold-crank/load-dump, minimize pop via soft-start and proper grounding.

PoE Supply (PD/PSE Side)

  • Envelope: VIN 37–57 V · VOUT 48–54 V · 0.3–1.5 A
  • Reverse-block · CM choke & Y-cap placement
  • EMI vs magnetics coupling · surge robustness
  • Risk: lightning surge & Ethernet EMI
PoE SupplyRJ45 icon to boost to 48–54V; CM choke marker; reverse-block. PoE 37–57V Sync Boost 48–54V Reverse-Block RJ45
PoE: manage CM choke/Y-cap, ensure reverse-block and surge compliance.
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Brand & Part Selection Matrix

Quick brand-by-brand overview of synchronous boost controllers (external MOSFET) and regulators (integrated FET). Fields: Part#, Type, VIN, VOUT max, Gate driver, fSW, Key features, Package. Rows are examples/placeholders—submit your BOM for an updated, in-stock match list.

Selection Matrix — Controllers vs RegulatorsLeft rail shows controller (external FET); right rail shows regulator (integrated FET); icons for VIN/VOUT, gate driver, frequency, protections, package. Controller (External FET) Gate Drivers · SS · ILIM · Reverse-Block VIN VOUT Driver Regulator (Integrated FET) Integrated MOSFETs · Compact BOM VIN VOUT fSW
Controllers (external FET) scale current & thermals; regulators (integrated FET) simplify BOM.
TI MatrixSimple geometric badge as brand placeholder.

TI

Part# Type VIN VOUT Max Gate Driver fSW Key Feats Pkg
Example-Ctl-TIController4.5–36 V60 V Dual gate · pump200 k–2 MHzSS · ILIM · Rev-Block · AEC-QQFN
Example-Reg-TIRegulator2.7–18 V28 V Integrated500 k–2.2 MHzSync · SS · ILIMQFN/SO
ST MatrixSimple geometric badge.

ST

Part# Type VIN VOUT Max Gate Driver fSW Key Feats Pkg
Example-Ctl-STController4–60 V60 V Dual gate100 k–1 MHzSS · ILIM · Rev-Block · AEC-QQFN
Example-Reg-STRegulator2.5–18 V27 V Integrated400 k–2 MHzSync · SS · ILIMQFN/SO
NXP MatrixSimple geometric badge.

NXP

Part# Type VIN VOUT Max Gate Driver fSW Key Feats Pkg
Example-Ctl-NXPController4–40 V55 V Dual gate200 k–1.5 MHzSS · ILIM · AEC-QQFN
Example-Reg-NXPRegulator3–18 V24 V Integrated500 k–2 MHzSync · ILIMHVQFN
Renesas MatrixSimple geometric badge.

Renesas

Part# Type VIN VOUT Max Gate Driver fSW Key Feats Pkg
Example-Ctl-RenController4–28 V40 V Dual gate300 k–2 MHzSS · ILIM · AEC-QQFN
Example-Reg-RenRegulator2.7–20 V28 V Integrated600 k–2.2 MHzSync · SSQFN/SO
onsemi MatrixSimple geometric badge.

onsemi

Part# Type VIN VOUT Max Gate Driver fSW Key Feats Pkg
Example-Ctl-OnController4–45 V60 V Dual gate100 k–1 MHzSS · ILIM · AEC-QQFN
Example-Reg-OnRegulator2.7–18 V24 V Integrated500 k–2 MHzSync · Rev-BlockSOIC/QFN
Microchip MatrixSimple geometric badge.

Microchip

Part# Type VIN VOUT Max Gate Driver fSW Key Feats Pkg
Example-Ctl-McpController4–36 V52 V Dual gate200 k–1 MHzSS · ILIM · AEC-QQFN
Example-Reg-McpRegulator2.7–18 V24 V Integrated400 k–2 MHzSync · SSQFN/SO
Melexis MatrixSimple geometric badge.

Melexis

Part# Type VIN VOUT Max Gate Driver fSW Key Feats Pkg
Example-Ctl-MlxController4–28 V40 V Dual gate200 k–1 MHzSS · ILIM · AEC-QQFN
Example-Reg-MlxRegulator3–18 V24 V Integrated500 k–2 MHzSync · Rev-BlockQFN

Notes: Values above are representative placeholders to structure your selection workflow. Share your VIN/VOUT/IOUT, efficiency target, and thermal constraints — we’ll return a brand-agnostic short list with in-stock options.

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Design Checklist & Common Pitfalls

Use this pre-tape-out checklist and the symptom–cause–fix table to avoid first-spin surprises. Items focus on synchronous boost controllers with external MOSFETs: soft-start, ILIM, reverse blocking, RHP zero constraints, EMI and thermal design.

Power Stage Sizing

  • L for ΔIL≈30–40% of IAVG; verify at min VIN, max VOUT.
  • FET VDS/ID ≥ 1.3–1.5× worst case; RDS(on) vs Qg trade.
  • COUT/ESR ripple and surge rating; input decoupling ESL minimized.

Control & Compensation

  • Choose CM (Type II) or VM (Type III) per transient/complexity targets.
  • BW ≤ fRHPZ/5; PM ≥ 45–60°; ESR zero placed to add phase.
  • Sense filtering/blanking; check minimum on-time limits at high duty.

Protection & Timing

  • ILIM startup vs steady-state; design peak ≤ 0.8× ILIM,typ.
  • Soft-start slope for inrush/overshoot; UVLO/OVP/OTP with hysteresis.
  • Reverse-block policy: shutoff delay and no oscillatory restart.

Layout & EMI

  • Minimize high-current loop; compact SW node; Kelvin sense routing.
  • Decoupling at loop endpoints; RC snubber or gate-R to tame ringing.
  • Thermal vias and wide copper; clean AGND to PGND single-point tie.

Bring-Up & DVT

  • No-load/nominal/step tests; cold-crank/ hot-plug; reverse pulse.
  • Thermal map under worst case; EMI pre-scan with LISN/antenna.
  • Record BW/PM via Bode injection; verify restart/fault recovery.
Symptom Likely Cause Fix Strategy
Startup failure / hiccup VIN droop/UVLO; SS too steep; ILIM too low; unstable loop Increase SS cap; raise ILIM; reduce BW or add phase; compensate wiring drop
FET overheating High RDS(on); large Qg; long dead-time; poor spreading Lower RDS(on)/parallel; tune gate-R; shrink SW node; copper + via arrays
Excessive EMI Large loop; fast dv/dt; noisy ground Tight layout; gate-R; RC snubber/beat ferrite; improve return path/CM choke
Light-load oscillation / audible noise Sync timing & burst; ESR zero near BW Adjust light-load mode; retune compensation; set ESR zero away from BW
VOUT overshoot Aggressive SS; high loop gain; noisy FB trace Reduce SS slope; lower BW/add phase; shield/shorten FB; add output RC damper
Reverse leakage / mis-conduction Body-diode window; late shutoff; detection blind spot Tighten reverse detection; shorten delay; confirm no oscillatory re-start
Thermal throttling / OTP trips Continuous stress; insufficient heatsinking Thermal foldback; spreader copper; airflow; check enclosure rise
Design Checklist & Startup Fault Tree Left: condensed checklist tiles. Right: startup fault tree showing decisions for VIN/UVLO, soft-start, ILIM, compensation, and layout/EMI. Checklist (Condensed) • BW ≤ f_RHPZ/5 · PM ≥ 45–60° • I_LIM,typ ≥ 1.2 × design peak • Minimize high-current loop • Compact SW node • SS slope vs inrush/overshoot • Kelvin sense; clean AGND • Thermal via arrays • EMI pre-scan Snubber/Gate-R RHPZ / ESR Badges f_RHPZ = VOUT²/(2π·L·IOUT·VIN) f_Z(ESR) = 1/(2π·C_OUT·ESR) Target PM ≥ 45–60° Startup Fault Tree Power-up fails? VIN sag / UVLO SS too steep Raise UVLO margin Increase C_SS ILIM too low Loop unstable Raise ILIM / filter Reduce BW / add phase Layout / EMI issue Reverse-block timing Tight loop · snubber Shorter delay · retest
Fig.9 — Condensed checklist and a startup fault tree: UVLO, soft-start, ILIM, compensation, layout/EMI.
Download Checklist (CSV)

FAQs

FAQ Badge Four quadrants representing control, power, protection, and layout topics. Control Power Protection Layout
FAQ topics: control, power, protection, layout.
How do I estimate inductor value and target ripple?

Start from ΔIL ≈ 30–40% of average inductor current at the worst VIN/VOUT duty. Use ΔIL ≈ (VIN/L)·(D/fSW) and select L to meet ripple and transient targets. Verify saturation and copper loss, then iterate with compensation bandwidth. → See: How It Works, Control & Compensation

What limits bandwidth in a boost and why is the RHP zero critical?

The right-half-plane zero introduces phase lag that increases with frequency, making control action counter-intuitive. Keep crossover ≤ fRHPZ/5 and ensure phase margin ≥ 45–60°. Place ESR zero to add phase below crossover, not at it, to avoid peaking. → See: Control & Compensation

Current-mode or voltage-mode control: which should I choose?

Current-mode improves load-step response and typically uses simpler Type II compensation but demands attention to minimum on-time and subharmonics. Voltage-mode can be simpler in silicon, often needs Type III, and tends to slower transients. Pick per bandwidth, EMI, and cost goals. → See: Control & Compensation

How does output capacitor ESR affect phase and ripple?

ESR creates a zero at fZ(ESR) = 1/(2π·COUT·ESR), which adds phase and can ease compensation. However, high ESR also raises ripple. A ceramic + polymer mix often yields low ripple and a helpful zero placement below crossover frequency. → See: Control & Compensation

How should I configure ILIM with and without a sense resistor?

With Rsense, use ILIM ≈ VSENSE/RSENSE and include blanking. Sensorless methods track RDS(on), requiring temperature compensation and tolerance margin. Production design peaks should be ≤ 0.8× ILIM,typ. → See: Protection & Reliability, Key Features

How do I set soft-start to avoid inrush and overshoot?

Use an SS ramp that limits input surge while coordinating with ILIM clipping. Large COUT or high duty requires slower ramps. Confirm UVLO doesn’t chatter during ramp and ensure no audible noise or oscillation in the transition. → See: Protection & Reliability, How It Works

What causes FET overheating and how can I fix it quickly?

Conduction loss from high RDS(on), switching loss from large Qg and long transitions, excessive dead-time, and poor thermal spreading are typical. Select lower RDS(on) or parallel devices, optimize gate-R, shrink SW node, and add copper/thermal vias. → See: External MOSFET & Layout, Checklist

Why did my EMI pre-scan fail and what are the first fixes?

Large current loops, high dv/dt at the SW node, and noisy return paths dominate. Tighten the loop, add gate-R and an RC snubber, place decoupling at loop endpoints, and consider a common-mode choke or shield routing for sensitive pairs. → See: External MOSFET & Layout, Checklist

How do I prevent reverse current during VOUT>VIN conditions?

Use controller reverse-block mode and verify shutoff delay. Test slow/fast VOUT steps and inject a short reverse pulse while logging peak current and stability. Ensure no oscillatory restart or false latch conditions appear. → See: Protection & Reliability, Key Features

How do I balance RDS(on) and Qg for high frequency designs?

Higher fSW favors lower gate charge to reduce Pgate and switching loss, but very low RDS(on) devices often have high Qg. Evaluate conduction vs switching loss at your duty and temperature, then choose the minimum total loss, not just lowest RDS(on). → See: External MOSFET & Layout

Why do I hear audible noise at light load and how to mitigate it?

Burst or PFM behavior, magnetostriction, and low-frequency loop peaking can produce tones. Switch to a low-noise light-load mode, move crossover away from audio band, add damping where needed, and avoid mechanical stress on magnetics. → See: Control & Compensation, Checklist

USB-PD boosts: negotiate first or start boosting immediately?

Negotiate the PDO before enabling the boost stage to avoid back-feed and brownouts. Implement cable drop compensation, verify minimum on-time at higher ratios, and confirm reverse-block behavior during role changes or unplug events. → See: Application Use Cases, Protection & Reliability

Automotive cold-crank and load-dump: what changes in design?

Size FET voltage margin and input clamps for load-dump; guarantee operation at cold-crank VIN dips with sufficient UVLO hysteresis and soft-start coordination. Ensure AEC-Q components, low-ESL decoupling, and robust ground strategy to pass EMC tests. → See: Application Use Cases, Protection & Reliability

Why does my output overshoot at enable and how to fix?

Overshoot stems from an aggressive soft-start slope, excessive loop gain, and noisy feedback routing. Reduce the SS slope, move the ESR zero and crossover further apart, tighten the FB path, and consider an RC damper at the output. → See: Protection & Reliability, Control & Compensation

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