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Transient & Line/Load Regulation

← Back to Low Dropout Regulators (LDOs)

This page explains how to hold VOUT inside a tight window when load current steps (25→75%), when VIN sags/jumps, or when the loop slows down at −40…125 °C. Output: a spec template, a tuning path (CFF/comp/Bode), and a validation checklist. It is different from start-up/PG pages — here we assume the LDO is already on.

Transient & Line/Load LDO outputs · keep VOUT in window during load/VIN steps Load-step response VOUT Target: ±2% VOUT undershoot overshoot ΔV target ±1–3% of VOUT use 2% for automotive Load step 25–50% IOUT_MAX 100% = lab only Recovery time ≤ 100–200 µs ≤ 500 µs (industrial) Figure – LDO transient test: keep VOUT in ±2% window for 25–50% load step, record tREC, and write all three items into BOM.

How to Set the Transient Target

First define what you want the scope picture to prove. Use the 3-item structure below — it is short enough for purchasing and precise enough for cross-brand replacement.

1) Voltage deviation

Target ΔV = ±(1–3)% VOUT.
Consumer nodes can relax to ±5%. Automotive camera/lighting stay at ±2%.

2) Load step condition

ISTEP = 25–50% IOUT,max. 100% step is for lab margin only, do not put it into BOM unless it is really needed.

3) Recovery time

tREC ≤ 100–200 µs back into the window. Industrial can go to 500 µs.

BOM Remark (copy-paste)

Transient is the first parameter that gets lost when somebody swaps the LDO for a cheaper brand. Put the requirement in the BOM so purchasing, QA, and EMS all see the same line.

BOM remark: “Transient spec: ±2% Vout for 50% Iload step, t_rec ≤ 200 µs, validated at VIN_MIN/VIN_MAX and –40…125°C.”

Short form (optional): Transient: ±2% / 50% step / 200 µs / VIN min–max / –40…125°C.

How Different Applications Phrase the Same Transient

Same test, different focus. Use the wording that matches your load domain — this prevents you from mixing this page with start-up/PG pages.

MCU / Digital rail

Focus on undershoot so the rail does not trigger brownout/reset.

Phrase: “Keep droop < 2% at 25→75% load step, VIN = min.”

Analog / RF / ADC / PLL

Watch overshoot and ringing first — they break linearity.

Phrase: “Overshoot ≤ 1–2%, no ringing, check at VIN ripple.”

Automotive / wide temp

Loop is slowest at low temp. Always test at VIN_MAX and –40 °C.

Phrase: “Validate transient at VIN_MAX, –40…125°C incl. harness impedance.”

Line Regulation vs Load Regulation

This section explains why line/load usually look “perfect” in datasheets but drift on your board, and how to write BOM remarks so replacements must re-test at your VIN range and temperature.

Line vs Load Regulation Test at VINmin / VINnom / VINmax, not only single-point datasheet value Line regulation VINmin → VINnom → VINmax ΔVout / ΔVin VIN increase → LDO loop slower Load regulation Iload: 0 → 50% → 100% ΔVout / ΔIload High load → COUT / ESR / layout rules Figure: separate line and load tests because each one breaks first under different conditions (VIN high vs. load high).

How to document it

BOM remark – line: “Line regulation to be re-verified at VIN_MIN, VIN_NOM, VIN_MAX with final output network and across –40…125°C.”

BOM remark – load: “Load regulation to be re-verified for 0→50% and 50→100% IOUT step, X7R derated COUT, and LDO discharge feature enabled.”

Output Network & CFF Impact

This part is just: when do we add a feed-forward capacitor, what changes after we add it, and which sentence should live in the handoff/BOM so the next person knows to re-measure Bode.

Output Network & CFF “CFF only helps if tuned to your divider & Cout ESR zero.” When do we add CFF? • FB divider is large → want faster small-signal • Big ceramic COUT → need HF help • Pin-to-pin change → loop no longer “just right” Don’t add CFF to fix bad layout ringing. What changes after adding CFF? 1) Mid-band response improves → Transient ΔV gets smaller (looks nicer in report) 2) HF phase may drop faster → Must re-measure Bode with that CFF value 3) Zero location moves with temperature → Do worst-case validation at MLCC min-cap and max-cap Figure: CFF helps transient only when tuned to FB upper resistor and Cout ESR zero; otherwise it may hurt phase margin.

“CFF improves transient only if placed across the upper FB resistor and tuned to match Cout ESR zero.”

“Do not increase CFF to fix layout-caused ringing.”

Compensation & Bode Verification

This section is only to confirm that the loop still behaves after you change Cout / add CFF / swap LDO vendor. We are not doing a full “control theory” page — just the three checks you must re-run.

Three points to check

  • Crossover frequency (fc): keep it in 100 Hz – 20 kHz depending on how fast you want the transient. Too high → easier to couple to fsw.
  • Phase margin (PM): target ≥ 45°, automotive / camera / harsh temperature target ≥ 60°.
  • Gain @ 0.5 · fsw: should not be high; prefer it to be at or below 0 dB to avoid switching-frequency artefacts.
Bode verification for LDO transient tuning Check fc, phase margin, and gain near half of switching frequency. 1) Crossover (fc) 100 Hz – 20 kHz Higher = faster but riskier Keep away from fsw / 10 Faster transient needs higher fc 2) Phase margin PM ≥ 45° typical PM ≥ 60° for automotive OK zone Low PM → ringing / slow recovery 3) Gain @ 0.5·fsw Prefer ≤ 0 dB Do not let this be high Otherwise PWM noise shows up Figure: minimum Bode verification set after changing Cout / CFF / vendor part.

Injection points

Single buck / boost: inject at the EA OUT / COMP node. LDO or fully integrated buck: inject at the FB divider node. Always measure a baseline (no CFF) → add CFF → re-measure.

Add to report/BOM: “Bode taken at VIN_MIN / VIN_MAX and –40°C / 25°C / 125°C”

Typical mistakes to call out

  • Measuring while the LDO is still in cold-start / soft-start → the loop is not in final state.
  • Using long ground leads on the injection setup → looks like instability even when it is stable.
  • Forgetting that system has ESR / polymer capacitors in parallel → lab result ≠ SPICE result.

Measurement / Validation Playbook

Goal: give the test / validation engineer a repeatable set of waveforms and a copy-paste BOM remark so that future vendor changes can be checked without re-reading the whole datasheet.

Test environment

  • Use load cables of the same length to keep output impedance identical.
  • Oscilloscope: ≥ 100 MHz / 1 GSa/s.
  • Probes: short ground spring, no flying ground clip.
  • Any change in cable / fixture / probe → invalidate previous transient data.
Validation waveforms to capture keep them identical across vendor / Cout / CFF changes. 1) VIN step VINmin → VINmax, see line regulation record ΔV_peak + t_rec 2) I_load up 0.25 → 1.0 Iout_max (undershoot) note: worst at VIN_MIN / low temp 3) I_load down 1.0 → 0.25 Iout_max (overshoot) check: stays inside ±2% window 4) Cold / VIN_MIN repeat –40°C, VIN_MIN, same load steps for automotive report Figure: four waveform families to store with the LDO transient report — VIN step, load-up, load-down, low-temp repeat.

Fields to write into BOM / report

Copy–paste template (English, engineer tone):

Transient test @ LDO output
VIN_test: VIN_MIN = 9 V, VIN_MAX = 36 V
I_step: 50% Iout_max (0.25 → 1.0 A and back)
ΔV_peak: ≤ ±2% Vout (record mV)
t_rec: ≤ 200 µs to ±2% window
Cout: 22 µF X7R 25 V, Murata GRM series, derated @ 125°C
CFF: 100 pF across upper FB resistor
Note: data valid only for identical load fixture and probe setup

If the supplier LDO is replaced, run the 4 waveforms again and update only the ΔV_peak / t_rec rows — other fields stay the same.

Vendors with Transient-Friendly Parts (placeholders)

Below are real parts from the 7 brands you track. All of them appear in documents or app notes where load/line transient or compensation tuning is shown. Do not skip re-validation with your own Cout/CFF and temperature corners.

TI

Transient-capable / CFF documented

  • TPS62130A
  • TPS62840
  • LM73605-Q1

Note: CFF tuning required for ceramic-only output.

Validate @ VIN_MIN/VIN_MAX, 25–50% Iout step, log ΔV_peak & t_rec.

STMicroelectronics

Has load-transient app note

  • A6986F
  • L7987L

Note: Use ST’s transient test setup and match Cout type.

Re-verify at system output network, not just datasheet conditions.

NXP

Multi-rail → test per rail

  • PF5024
  • MC34713

Note: Multi-output transients are not symmetric.

Log which rail was stepped and which rail was monitored.

Renesas

Wide-VIN transient

  • ISL85410
  • RAA211250

Note: “Check line reg at 42 V” still needs to be written in BOM.

Do VIN_min → VIN_nom → VIN_max while stepping 50% Iout.

onsemi

Automotive front / pre-reg

  • NCV8876
  • NCP3066

Note: Line transients are critical in crank/brown-out.

Test with cable/loom inductance to see real overshoot.

Microchip

Compensation examples available

  • MCP16331
  • MIC28512

Note: Use their COMP/CFF values as “no-CFF baseline”.

Then re-measure Bode after your output network swap.

Melexis

Automotive sensor supply rails

  • MLX81330 (sensor/actuator supply)
  • MLX81206
  • MLX81116

Note: Fast recovery, low overshoot for sensor chains.

Validate with real harness + temp chamber (–40…125°C).

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Transient / Line / Load Regulation – FAQs

What ΔV is acceptable for a 3.3 V rail powering MCUs and sensors?

Use ±2% as the main target; ±3% is still workable for non-critical sensors. Always bind it to a load step: “±2% for 50% IOUT,max, t_rec ≤ 200 µs, VINmin/VINmax.”

Why does my transient look worse at VIN = 36 V than at 12 V?

Loop bandwidth effectively drops at high VIN, so the same I-step causes deeper undershoot and longer recovery. Always re-verify at 9→14→36 V.

Does adding a feed-forward capacitor (CFF) always improve transient response?

No. It helps only when placed across the upper FB resistor and tuned near the output pole/ESR zero. Oversized CFF can eat phase margin, so measure Bode again.

How do I map my transient spec (±2%, 200 µs) to a Bode target?

200 µs recovery usually means loop crossover around 3–10 kHz with ≥50° phase margin for ceramic Cout. If undershoot stays large, raise fc slightly or increase Cout.

Why is the overshoot large only when the load releases (I → 0)?

Stored energy in Cout plus a controller that cannot sink fast. Use parts with active discharge/sink, or add a small preload to absorb the energy.

How to test line regulation when the front DC-DC has ripple?

First step VIN with a clean supply to get the DC number, then repeat behind the real pre-reg and note “validated with upstream ripple”. Don’t mix ripple and DC drift.

Can I validate load regulation without a Bode analyzer?

Yes. Sweep 0 → IOUT,max with an electronic load and log Vout. If ΔVout/ΔIload fits the target at VINmin and VINmax, it passes. Bode is needed only after network changes.

Why do cold-temperature tests show bigger undershoot?

Ceramic caps drop value in cold, wiring losses change, some ICs derate current. The same 50% step will dip more. Log “–40…125°C, actual board Cout”.

How to write a BOM remark so purchasing won’t pick a slow part?

Write it hard: “Transient: ±2% Vout for 50% Istep, t_rec ≤ 200 µs, VINmin/VINmax, –40…125°C, Cout = 22 µF X7R, CFF = 100 pF across Rupper.”

Which TI / ST / NXP parts are OK with 22 µF ceramic?

TI TPS62840 / TPS62130A, ST A6986F / L7987L, NXP PF5024 per rail. Use their Cout/CFF table and re-verify with your layout.

How to tell layout-induced ringing from real loop instability?

If it changes with probe ground length or cable length, it’s layout/measurement. If it’s repeatable across VIN/temp/load, it’s loop. A quick Bode with/without CFF will tell.

When should I use “pre-reg + LDO” instead of one regulator?

When VIN swing is wide (9–36 V) or load steps are big but the rail still needs ±2%. Let the fast DC-DC take the hit, then let the LDO clean ripple and noise.

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