Center Idea: When to Choose High-Current, Low-VDO LDO
Use a 1–5 A low-VDO LDO only when the dropout headroom and thermal budget are provably safe, and the post-buck cleanup yields a measurable transient/ripple benefit.
Bottom line: Pass all three: VDO_margin safe, T_J within margin, and a clear, measured transient/ripple improvement. Otherwise, stick with buck optimization.
Principles: Where VDO Comes From & Why Transients Differ
Dropout is set by pass-device conduction and drive headroom; transient behavior follows the ESR zero, output pole, and internal compensation.
- Budget
V_dip_maxacross ESR spike (≈40%) and capacitive sag (≈60%). - Choose C/ESR within the device’s stability table; verify
f_cand phase margin. - Log step dip (%, μs) at hot/cold; compare against VDO(T) trend.
Design Rules: Copy-Ready Formulas & Checks
Run three checks in order: headroom, thermal, then SOA/transient. Keep variables and units consistent with your test logs.
1) Headroom
VDO_margin = VIN(min) − VOUT(min)
Rule: VDO_margin ≥ VDO@I_STEP + 0.10 V
Default: I_STEP ≈ 0.5·I_OUT(max)
2) Thermal
P_D,dc = (VIN − VOUT) · I_OUT(avg)
P_D = P_D,dc + (VIN − VOUT) · I_STEP · D_step
ΔT ≈ P_D · RθJA, T_J = T_A + ΔT ≤ T_J(max) − 10 °C
3) SOA / Transient
ΔV_step ≈ I_STEP·ESR + (I_STEP/C_out)·Δt_control
Split budget: ESR ≈ 40%, Cap ≈ 60%
t_rec ≈ 3 / (2π·f_c)
Example — FAIL
VIN(min)=1.90 V, VOUT=1.80 V, I_OUT(max)=3 A, I_STEP=1.5 A.
VDO@3 A=250 mV → VDO@1.5 A≈150 mV.
VDO_margin=100 mV < 150 mV+100 mV → Fail headroom.
Example — PASS
VIN(min)=2.05 V, VOUT=1.80 V, I_OUT(max)=3 A, I_STEP=1.5 A.
VDO_margin=250 mV ≥ 150 mV+100 mV → Headroom OK.
Next: compute P_D, ΔT, then ESR/C from dip target.
Measurement Checklist
Log: VIN/VOUT/I(t), dip (%), t_rec (µs), T_A/T_HOT, T_J estimate, D_step. Test 10/100/1000 µs steps at −40/25/85 °C and VIN(min/typ).
When LDO after Buck: Benefit vs Cost
Add a low-VDO LDO after a buck only if measurable ripple, spike, and step-response gains outweigh the headroom, thermal, and BOM costs.
Measurable Gains
Ripple attenuation: Att_ripple_dB = 20·log10(Vin_rip/Vout_rip)
Spike reduction: Spike_gain = Vspike_in/Vspike_out
Step improvement: Benefit_step = ΔV_buck/ΔV_buck+LDO
Costs
Efficiency: η_total ≈ η_buck · (VOUT/VINT)
Loss: P_D = (VINT − VOUT) · I_OUT(avg) + pulses
Thermal: ΔT ≈ P_D · RθJA (keep T_J ≤ T_J(max) − 10 °C)
Quick Decision
Adopt if ≥2 gains meet target and both headroom & thermal pass. Otherwise optimize the buck (loop, C, layout) or choose a better buck instead of adding an LDO.
Layout, Thermal & Validation
Keep the power loop short, sense at the load (Kelvin), spread heat with copper and vias, and validate with step/pulse testing across temperature.
Validation: Step / Pulse Method
Stimulus: 10/100/1000 µs steps, I_STEP ≈ 0.5·I_OUT(max). Log VIN/VOUT/I(t), dip (%), t_rec (µs), T_A/T_HOT, T_J estimate. Test at −40/25/85 °C and VIN(min/typ). Targets: dip ≤ 5%, recovery 50–100 µs (use device limits).
Typical Fix Order
Fix sense routing → move first Cout closer → add damping / adjust ESR → widen power loop → add copper/vias → revisit headroom/thermal if still failing.
Metrics To Record
ΔV_dip(%), t_rec(µs), P_D(W), T_A/T_J(°C), RθJA(°C/W), Att_ripple(dB).
IC Selection Guide (1–5 A · Low VDO · Optional AEC-Q)
Shortlist across the seven brands. Values vary by option/temp; verify in the datasheet for your exact operating point and stability table.
| Brand | Part Number | IOUT (A) | VDO @ I (typ/max) | VIN / VOUT Range | Key Features | AEC-Q | Best-Fit Use |
|---|---|---|---|---|---|---|---|
| Texas Instruments | TPS7A85 | 4 | Low @ high I (see DS) | VIN wide / VOUT adj | Fast transient, PG, low noise options | — | CPU/FPGA assist, SerDes |
| Texas Instruments | TPS7A84A | 3 | Low (see DS) | VIN wide / VOUT adj | PSRR, soft-start, PG | — | SoC I/O/PLL |
| Texas Instruments | TPS7A53-Q1 / TPS7A54-Q1 | 3 / 4 | Low (see DS) | VIN/VOUT per variant | PG, protections, fast transient | AEC-Q100 | Automotive assist rails |
| STMicroelectronics | LD39200 | 2 | ≈130 mV@2 A (typ, ref) | VIN/VOUT per option | High PSRR, PG, reverse-current prot. | Some variants | PLL/SerDes clean-up |
| STMicroelectronics | LD59150 / LD59150-Q* | 1.5 | Very low (typ @ 1.5 A) | VIN low / VOUT adj | NMOS + BIAS, fast transient | AEC-Q options | Low headroom rails |
| Renesas (ex-Intersil) | ISL80103 / ISL80102 | 3 / 2 | Low @ rated I (see DS) | VIN/VOUT per option | PG, protections, transient focus | — / variants | General high-I assist |
| onsemi | NCV59745 | 3 | ≈115 mV typ (ref) | VIN/VOUT per option | Low noise, high PSRR, PG | AEC-Q100 | Automotive PLL/I/O |
| Microchip (Micrel) | MIC69502 | 5 | Low-to-mid (see DS) | Low VIN / low VOUT | High current linear; thermal focus | — | Post-buck clean-up @ 3–5 A |
| Microchip (Micrel) | MIC69302 | 3 | Low-to-mid (see DS) | Low VIN / low VOUT | Single-supply options, PG | — | CPU/FPGA assist, DDR aux |
| NXP | — (no standalone 1–5 A LDO) | — | — | — | Use PMIC-integrated rails or cross-brand | — | Cross to TI/REN/MCHP/ON/ST |
| Melexis | — (no standalone 1–5 A LDO) | — | — | — | Focus on sensors/actuators; cross-brand | — | Cross to TI/REN/MCHP/ON/ST |
≥3 A · Very Low VDO · Fast Transient
TPS7A85 / TPS7A54-Q1 · ISL80103 · MIC69302 · NCV59745
1–2 A · Low Headroom
LD39200 · LD59150(-Q*) · ISL80102
Automotive (AEC-Q)
TPS7A53-Q1 / TPS7A54-Q1 · NCV59745 · LD59150-Q*
Use-Case Cards
Match the rail’s noise/transient targets and thermal budget. Verify stability (C/ESR window) and VDO at your load/temperature.
CPU/GPU/FPGA Assist Rails
Target < 10–20 mVpp and fast recovery (<100 µs). Ensure VDO margin and thermal headroom at 3–5 A peaks.
- Primary picks: TPS7A85, MIC69302, MIC69502
- Automotive: TPS7A54-Q1, NCV59745
- Notes: log step dip (%), trec; Kelvin-sense at load.
SoC I/O / PLL / SerDes
Prioritize high PSRR and low ripple; headroom can be small; typical 1–3 A continuous.
- Primary picks: LD39200, TPS7A84A, ISL80102/03
- Automotive: LD59150-Q*, NCV59745
- Notes: confirm C/ESR stability table and PG behavior.
DDR Auxiliary Rails (non-VTT)
Moderate noise and step targets; ensure PG/sequence compatibility; typical 1–3 A envelope.
- Primary picks: TPS7A84A, ISL80102/03, MIC69302
- Automotive: TPS7A53-Q1, NCV59745
- Notes: verify start-up with downstream timing; check reverse-current behavior.
Routing & Anti-Crossing Note
This page excludes DDR VTT/Vref specifics, wideband PSRR “ultra-low-noise” lists, and ultra-low-Iq topics. For those, route to their own sub-pages to avoid content overlap.
Troubleshooting
Diagnose by symptom → confirm with measurements → apply the shortest fix first. Keep records of VIN/VOUT/I(t), dip (%), t_rec, T_A/T_J, and layout notes.
VDO Insufficient
Confirm VDO_margin = VIN(min) − VOUT(min) ≥ VDO@I_STEP + 0.10 V. Measure at the LDO pins and at the load (two probes). Fix: raise upstream setpoint or shorten/ thicken the feed; consider a lower-VDO LDO or BIAS topology.
Thermal Shutdown
Estimate P_D = (VIN − VOUT)·I_avg + (VIN − VOUT)·I_STEP·D_step. Keep T_J ≤ T_J(max) − 10 °C. Fix: copper pours + via array, reduce Vdrop, choose better θJA package; if still hot, prefer buck.
Oscillation / Squeal
Use capacitor series from the stability table; adjust ESR/damping. Keep sense lines paired and away from SW areas; add minimum load if required. Measure with a short ground spring to avoid false ringing.
Cable Droop
V_drop ≈ I_load · R_line and L_line spikes degrade steps. Move the LDO close to the load, place the first Cout tight to OUT–GND, add local bulk + small ESR resistor for damping, or shorten the harness.
Parallel Imbalance
Avoid direct paralleling of generic LDOs. If redundancy is needed, use OR-ing/ideal diode; for true sharing, adopt a controller with current-balance. As a last resort, add small mΩ ballast resistors (with efficiency penalty).
Mini CTA
Submit your BOM (48h)
Attach rail specs (VOUT, VIN(min), Imax, ISTEP@µs, headroom, temps, package limits). We reply with two ready-to-buy options.
Submit BOM (48h)Cross-Brand “Two Alternatives”
You’ll get 1 same-brand and 1 cross-brand alternative matched by current class, VDO, AEC-Q, and availability. Automotive and small-batch friendly.
- Pin-to-pin where possible
- Stability table + PG behavior checked
- Thermal and headroom margins verified
PAA FAQs (Engineer Tone)
When do low-VDO benefits outweigh thermal loss?
Adopt a low-VDO LDO after the buck when measured gains meet thresholds: ripple attenuation ≥6 dB, spike ratio ≤0.5, and step-recovery improvement ≥2×. Confirm junction rise is acceptable: ΔT ≈ P_D·RθJA with P_D = (VIN−VOUT)·I_avg + pulses. Keep T_J ≤ T_J(max) − 10 °C across −40/25/85 °C and worst-case headroom.
Is paralleling 1–5 A LDOs reliable, and what’s required?
Generic LDOs should not be directly paralleled: current hogging and thermal run-away are likely. If redundancy is required, use OR-ing or ideal-diode controllers. For true current sharing, select devices with balance control or add mΩ ballast resistors (with efficiency penalty). Keep each path ≤70–80% rated I and ensure symmetric thermal layout.
Low-VDO vs high-PSRR LDO: how to trade noise and transients?
Low-VDO devices minimize headroom and heat at high current but may offer lower PSRR at 100 kHz–1 MHz and higher broadband noise than high-PSRR types. For PLL/SerDes, favor PSRR/noise. For CPU/FPGA assists, favor VDO and transient response. Verify PSRR at 1 kHz and 100 kHz, noise (µV_RMS), and step dip/recovery on your capacitor set.
How much ripple/spike reduction can LDO-after-buck deliver?
Test with identical stimulus before/after: calculate Att_ripple_dB = 20·log10(Vin_rip/Vout_rip), spike ratio = Vspike_in/Vspike_out, and step improvement versus recovery time. Targets: ≥6 dB ripple cut, spikes ≤0.5×, step dip or recovery ≥2× better. If metrics fall short, optimize the buck loop and layout before adding an LDO.
Which load and temperature points should VDO be checked at?
Use the step current the rail actually sees (I_STEP), not just I_OUT(max). Check VDO at −40 °C, nominal, and hot, including process tolerance. Enforce: VDO_margin = VIN(min) − VOUT(min) ≥ VDO@I_STEP + 0.10 V. If the worst-case point fails this guard band, select a lower-VDO device or increase upstream setpoint.
When should you switch to a small-duty low-drop buck instead of an LDO?
Prefer a buck when P_D makes ΔT uncontrollable or efficiency falls below your minimum target. Typical cutovers: ΔT margin <10 °C under representative duty, or η_total significantly lower than budget. Validate that buck ripple/EMI meets the rail’s tolerance; if not, add a clean-up filter or post-LDO with sufficient headroom.
Which AEC-Q100 items matter most for high-current LDOs?
Focus on temperature grade (e.g., Grade 1), HTOL, temperature cycling, ESD, latch-up, and PG/RESET repeatability. Confirm PG thresholds and delays across corners, reverse-current behavior, and short-to-battery/ground protections. Sequence compatibility with downstream ECUs must be verified during crank/brown-out, at VIN(min) and thermal hotspots.
How do Kelvin-sense and power loops avoid “inner-loop zeros” side effects?
Sense at the load with a paired, low-noise route separated from the power loop. Place the first Cout tight to OUT–GND. Avoid mixing sense returns with high di/dt grounds. Keep wiring inductance low; if a sense filter is needed, select RC values that do not introduce a pole/zero inside the control bandwidth.
How to prevent reset chatter in power-up/down sequencing?
Align PG/RESET thresholds and delays with downstream supervisors. Ensure monotonic rise and adequate hold-time on power-down. Verify PG tolerance over temp and load transients. If chatter appears, increase delay or hysteresis, and reduce upstream noise. Test with worst-case ramp rates and brown-out events to confirm stability.
Root-causing VOUT jitter: source impedance, ESR, ground bounce, harness?
Probe both the LDO pins and the load node under 10/100/1000 µs steps. High source impedance, off-spec ESR/ESL, ground bounce, and harness L can all add jitter. Use a ground spring or coax to avoid false ringing. Add damping, shorten loops, and move the regulator closer to the load.
Thermal protection trips frequently—what’s the tuning order?
First, lower VOUT if permitted, then reduce I_LIMIT to curtail peaks. Next, add copper area and via arrays; improve enclosure thermal paths. If ΔT remains high, reduce (VIN−VOUT) or switch to a buck. Always log T_A and estimated T_J under realistic pulse duty to validate improvements.
DDR auxiliary rails (non-VTT): what should I watch?
Noise and transient needs are moderate, but PG timing must align with the main memory and controller rails. Confirm stability table on your capacitors, and check reverse-current behavior. Keep this topic separate from VTT/Vref specifics to avoid overlap and ensure the dedicated VTT page holds termination details.
Minimal delta and sharing strategies for LDO paralleling?
If parallel operation is unavoidable, use small mΩ ballast resistors to force sharing and maintain symmetrical copper/thermal paths. Monitor imbalance factor and hotspot delta. Prefer redundancy via OR-ing rather than current pooling. For true sharing, adopt parts with active balance control rather than generic LDOs.
Test script: step box, sampling rate, window, and logging fields?
Apply 10/100/1000 µs steps at I_STEP ≈ 0.5·I_OUT(max). Sample ≥10× the loop bandwidth. Log VIN, VOUT, I(t), dip (%), t_rec (µs), T_A/T_J (°C), Att_ripple_dB, and Spike_ratio. Test across −40/25/85 °C and VIN(min/typ). Use two-point probing (pins vs load) for harness effects.
Stocking plan: same-package dual options and one cross-brand?
Return two picks by default: one same-brand and one cross-brand, prioritizing package pinout, stability-table fit, PG behavior, and availability (AEC-Q optional). Document replacement cautions (sense routing, C/ESR windows). This keeps samples and small-batch builds flowing despite supply fluctuations.
CTA & Routing
Submit your BOM (48h)
Attach rail specs (VOUT, VIN(min), Imax, ISTEP@µs, headroom, temps, package limits). We’ll return one same-brand and one cross-brand alternative tailored to availability and AEC-Q needs.
Submit BOM (48h)Back to LDO Hub
Explore non-overlapping sub-pages (ultra-low-Iq, high-PSRR, VTT/Vref) and keep this page focused on 1–5 A low-VDO use.
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