← Back to Low Dropout Regulators (LDOs)
Introduction: Why LDO Needs a Protection Set
LDOs are linear devices: any difference between VIN and VOUT turns directly into heat. That makes them more sensitive to long current-limit events than switching regulators. At the same time, most LDO rails today sit behind a DC/DC stage, which often starts with a pre-biased output or powers down slowly. This upstream behavior can feed energy back into the LDO if the device has no reverse protection.
For that reason, engineers and purchasers should look at the whole LDO protection set instead of a single “OCP” line in the datasheet. A practical set for LDO rails consists of: ILIM/foldback, short-circuit behavior, OTP/thermal shutdown, active output discharge, and reverse-current / reverse-battery guards. Only when these five are checked can we safely swap parts across TI, ST, NXP, Renesas, onsemi, Microchip, and Melexis.
ILIM / Foldback in LDOs
Not all LDOs limit current in the same way. A fixed current limit holds the output current at a defined ceiling, while a foldback limit intentionally pulls the allowed current down as the output collapses. Foldback is particularly useful for LDOs because the device must dissipate all excess power as heat, and long shorts can quickly drive the junction into OTP.
Use foldback when you expect long-duration shorts, high ambient temperatures (lamp housing, engine bay), or rails feeding camera / sensor loads that may look like a short at start-up. Foldback gives the LDO time to survive until thermal shutdown or until the upstream DC/DC recovers.
To make protection comparable across TI, ST, NXP, Renesas, onsemi, Microchip, and Melexis, capture these fields in the BOM:
- I_OUT_NOM — rated output current in normal operation.
- I_LIM — current-limit threshold before full collapse.
- I_SC — short-circuit current after foldback (often 25–40% of nominal).
- TJ @ SC — temperature/condition for the short-circuit test.
Note: some automotive LDOs apply a temperature-dependent foldback; record this in validation so purchasing can evaluate cross-brand substitutes safely.
Short-Circuit Behavior
LDOs do not react to short circuits in the same way. Some hold a continuous current limit, some combine current limit with output fold-down, and some will auto-retry or require an EN/PG action to recover. Because LDOs dissipate power linearly, continuous current at a shorted output will heat the die rapidly, so a downstream thermal shutdown is often needed as a final safety net.
When documenting or purchasing LDOs, describe the short-circuit behavior explicitly, not just “SCP: yes”. This allows cross-vendor replacement between TI, ST, NXP, Renesas, onsemi, Microchip, and Melexis without changing the power-up or recovery sequence.
For enclosure or high-temperature automotive cases, the safest option is the second type (limit plus fold-down), because the device actively reduces dissipation under a hard fault. If your chosen LDO is of the first type (continuous current limit), you must confirm in the next section that the OTP is present and repeatable.
Write these fields into BOM / test notes:
- SC_MODE: continuous / limit-folddown / auto-retry / EN-linked.
- SC_I: actual current during short (mA or % of IOUT_nom).
- SC_T_HOLD: how long it can hold short at room temp.
- PG_BEHAVIOR_SC: PG stays high / drops / delayed drop.
- EN_RECOVER_REQ: yes/no (needed for some auto-retry parts).
OTP (Thermal Shutdown) for LDOs
Because LDOs are linear, any sustained short-circuit or heavy overload will turn into junction heating. Thermal shutdown (OTP) is the layer that prevents permanent damage once current limiting has done all it can. LDO suppliers often choose slightly different trip points and recovery temperatures, so for cross-vendor replacements you must log both values.
Typical LDO OTP trip points cluster around 125°C, 150°C, or 175°C. However, the recovery temperature can be 10–30°C below the trip point, and that difference affects how quickly a rail comes back in a hot lamp housing or engine compartment. Automotive LDOs normally offer wider operating temperature and more consistent OTP, but you should still record the actual measured values.
Record these OTP fields for cross-brand replacement:
- OTP_TRIP: e.g. 150°C (typ).
- OTP_RECOVERY: e.g. 130°C (typ) or “auto, 20°C hysteresis”.
- TEST_COND: VIN, Iload, ambient / chamber temperature.
- OTP_LINKED_TO_SC: yes/no (enter yes if short-circuit easily pushes into OTP).
- AUTOMOTIVE_NOTE: verified in enclosure / lamp / low-airflow if for AEC-Q100 use.
Active Discharge / Output Pulldown
Many LDOs stop regulating when EN = 0 but they do not automatically discharge the output capacitor. With a large COUT or when rails are paralleled
(MCU, ADC, sensor bias), this looks like “the rail is still alive” and can break your power-down sequence. You must therefore check what discharge option the LDO uses:
internal discharge, external/bleeder discharge, or conditional discharge (only when EN = 0).
In MCU/ADC/bias multi-rail systems, inconsistent fall time causes “ghost power”: one rail has stopped regulating but still sits at a few volts, so the controller thinks the device is alive. To avoid that, record discharge type and target fall time in your validation.
Fields to capture for purchasing / cross-brand use:
- DISCHARGE_TYPE: internal / external / EN-tied.
- R_DISCHARGE_EQ: internal ≈ 150–300 Ω, or external
100 kΩto GND. - T_FALL@COUT: e.g. “<50 ms @ 10 µF, no load”.
- EN_DEPENDENT: yes/no (discharges only when EN = 0).
- MULTI_RAIL_NOTE: tested with MCU 3V3 + ADC 2V5 → OK.
Reverse-Current / Reverse-Battery Guards
LDOs placed behind a DC/DC converter or in multi-rail boards often see the situation “upstream off, downstream still powered”. If the LDO does not have reverse-current protection, the downstream rail can backfeed the LDO and even the DC/DC, causing unwanted current paths, slow power-down, or – in worst cases – damage. Automotive rails add another stress case: 12 V reverse battery / negative input, which must be handled explicitly.
If your design has more than one 3.3 V source, or the board sometimes boots with a pre-biased rail, always select an LDO that explicitly supports reverse-current tolerance. For automotive 12 V subsystems, use a part that states reverse-battery capability in the datasheet — do not assume every AEC-Q100 LDO includes it.
Fields to capture:
- RC_PROTECTION: yes/no (output above input safe?).
- RC_MAX_DELTAV: e.g. OUT up to VIN+0.3 V OK.
- RB_PROTECTION: yes/no (reverse-battery capable).
- RB_SPEC: e.g. −14 V, 60 s, no damage.
- PREF_BIAS_OK: yes/no (can start with pre-biased output).
- LINKED_TO_DISCHARGE: yes/no (LDO pulls down faster to avoid backfeed).
Validation & Test Playbook (for LDO Protection Set)
This playbook turns datasheet-level protection claims into measured values that purchasing can copy directly into the BOM notes and compare across TI, ST, NXP, Renesas, onsemi, Microchip, and Melexis-related sensor rails. The sequence is fixed: measure ILIM → force short → move to hot box → verify reverse/backfeed.
For every step, record trigger point, recovery point, ambient/chamber temperature, and board/tester ID. This makes it possible to later replace a TI device with an ST or onsemi part without retesting the whole ECU.
Log these into BOM / test notes:
- VIN / VOUT / COUT (as tested)
- I_LIM_25C and I_LIM_HOT (85–125°C)
- I_SC_25C, SC_MODE (continuous / fold-down / auto-retry), T_HOLD_SC
- OTP_TRIP (e.g. 150°C) and OTP_REC (e.g. 130°C), plus chamber temperature
- RC_PROT (pass/fail) and RC_DELTAV (OUT − VIN during test)
- RB_SPEC_TESTED (e.g. −14 V, 60 s, no damage)
- BOARD_REV / TESTER_ID for traceability
Seven-Brand Selection Pointers
Field-level guidance only — short and comparable. Each card gives: what protection combo this brand commonly offers, 2–3 real part numbers you can map in your BOM, and which rails to target. You can expand this list later into your master IC dictionary.
TI
Automotive LDO, reverse-current aware, OTP well documented.
- TPS709-Q1 (150 mA, 30 V, reverse current, OTP)
- TPS737-Q1 (1 A class, thermal, reverse-aware)
- LM2940 (5 V auto LDO, classic OVP/OCP/OTP)
Fields: I_LIM, OTP_TRIP, REV_CURRENT=YES, EN=YES.
ST
Many parts with EN + active discharge + SCP.
- LDQ40 (250 mA, EN low → discharge)
- LDL40 / LDH40 (200 mA, discharge on disable)
- LDF families for low-noise rails
Fields: EN+DISCH=YES, R_DISCH_EQ, SC=YES, OTP=YES.
NXP / Renesas (body & camera)
Bias rails for camera/ADAS/body ECUs, AEC-Q100 oriented.
- NXP PF3001 LDO rails (with UV/thermal)
- Renesas RAA214220 (20 V, 150 mA, EN)
- Renesas ISL80510 / ISL80505 (high-performance LDO)
Fields: LDOx_ILIM, OTP, CAM/BODY=YES, PRE-BIAS=OK?.
onsemi
Wide VIN + automotive, often with reverse/OV.
- NCV47711 (350 mA, adj. current, thermal)
- NCV4276C (400 mA, 45 V, reverse input)
- NCV4266 / NCV8718 (auto LDO basics)
Fields: VIN_PEAK, REV_IN=YES, ILIM_ADJ?, OTP=YES.
Microchip
Industrial/automotive mix, good reverse-battery notes.
- MIC29712 (reverse battery + current limit)
- MIC2940A family (auto, 1 A)
- MCP1755S (lower current, protected)
Fields: VIN_MAX>45 V, REV_BATT=YES, SC=YES, OTP=YES.
Melexis (sensor bias)
Target load = automotive sensors, so LDO must be safe for backfeed and high temp.
- MLX90377 (magnetic position)
- MLX92235 (low-power hall)
- MLX90514 (temp/pressure style sensor)
Use LDOs from above brands but record: REV_CURRENT=YES, EN-tied discharge, OTP @ 150°C, tested in lamp/engine enclosure.
Submit your BOM (48h)
Tell us which LDO protections you need: ILIM/foldback, short-circuit, thermal shutdown (OTP), active discharge, and reverse-current / reverse-battery. We will map against TI, ST, NXP, Renesas, onsemi, Microchip, and Melexis sensor-bias use.
Basic rail info
VIN / VOUT / IOUT / COUT (as tested)
Protection set
ILIM, foldback, SCP, OTP, active discharge, reverse-current, reverse-battery
Brand preference
TI / ST / NXP / Renesas / onsemi / Microchip / Melexis
Environment
Room / 85°C / 105°C / lamp housing / engine bay
Frequently Asked Questions — LDO Protection Set
What is the difference between LDO current limit and foldback?
A fixed current limit holds the output at roughly one current value even if VOUT collapses. Foldback reduces that current once the output is shorted, so the die runs cooler and can survive a long short or high ambient tests.
When do I have to select an LDO with foldback current limit?
Use foldback when the rail can stay shorted for seconds, sits in a hot enclosure, or powers camera/sensor bias in automotive. Foldback reduces dissipation and lets the OTP work less aggressively.
Why does my LDO output stay high after disable?
Because disable often only turns off the pass element. If the device has no active discharge and your Cout is large or paralleled with other rails, the voltage decays very slowly. Choose parts with built-in discharge or add an external pulldown.
Can I add my own discharge resistor instead of the internal one?
Yes. Adding a resistor to GND guarantees a fall time on any brand, but it creates a permanent load. Document the value in the BOM so purchasing can check that replacements support the same shutdown behavior.
How do I know if the LDO has reverse-current protection?
Check the datasheet for phrases like “output can be forced above input” or “reverse-current protected.” If it is not explicit, bench test it by forcing VOUT while VIN is low. Record the result in the protection set notes.
What is the purpose of reverse-battery protection on automotive LDOs?
It allows the LDO to survive accidental −12 V/−14 V battery events or wrong polarity connection without permanent damage. Not every AEC-Q100 LDO has this, so always check the exact reverse-battery rating and duration.
Can I use an LDO without active discharge in a multi-rail system?
Only if all rails power down together or you add an external pulldown to guarantee fall time. Otherwise one rail may stay high and confuse MCU/ADC reset logic.
What happens when an LDO hits thermal shutdown?
The device turns its pass element off to protect itself. After the junction cools down to the recovery temperature, it restarts automatically or follows the internal auto-retry sequence. Different vendors recover at different temperatures.
Why is the OTP recovery temperature important?
If one brand recovers at 130°C and another at 110°C, the power-up/power-down sequence in a hot box may change. Logging the recovery point in BOM makes cross-brand replacement safer.
Are automotive LDOs always reverse-battery protected?
No. AEC-Q100 mainly covers qualification and temperature range. Reverse-battery is an extra feature and must be stated. If it is missing, add an external protection stage or pick another part.
How should I test reverse/backfeed cases on the bench?
Disable or remove VIN, then force VOUT to the normal operating voltage with a lab supply. Monitor current into the IN pin and check that the LDO does not latch or overheat. Log delta V and duration.
Can I power an LDO from a pre-biased DC/DC output?
Yes, but only if the LDO is pre-biased/startup-safe or has reverse-current blocking. Otherwise the pre-biased rail will feed back into the LDO input path.
Which parameters should I put into BOM remarks?
At minimum: I_LIM_25C, I_LIM_HOT, SC_MODE, T_HOLD_SC, OTP_TRIP/REC, DISCHARGE_TYPE, RC_PROT, RB_SPEC, and test environment. This lets purchasing match seven brands without re-testing everything.
What if the LDO needs EN toggle to recover?
Note it explicitly as EN_RECOVER_REQ = yes. Firmware or test rigs can then pulse EN after a fault. Without this note a replacement part may appear “dead” in production.
Can I mix parts from different brands and keep protection behavior?
Yes, if you match the logged fields: current limit type, OTP trip/recovery, discharge type, and reverse-current/battery capability. That is why this page standardizes the protection set.
