← Back to Low Dropout Regulators (LDOs)
1. Introduction: Why Thermal Path Matters for LDOs
A low dropout regulator turns the voltage difference (VIN − VOUT) into heat at the same current flowing to the load. Unlike a buck converter, this heat cannot be moved away by power-stage efficiency; it must be conducted through the package into the PCB and then into ambient. When the heat cannot leave fast enough, the LDO will reach its thermal shutdown (OTP) threshold and switch off.
In automotive, lamp housings, camera modules, or any enclosed space, the ambient temperature around the board is already high, so the board can provide much less thermal relief to the LDO. That is why the thermal path must be considered at device and package selection time, not only during layout review.
A practical LDO thermal review therefore links four elements together: expected power dissipation → chosen package → copper/thermal vias → allowed TJ margin. This page explains how to make that link.
2. Package Options for LDOs (D²PAK, TO-252, QFN/DFN with big pad)
Not every LDO package can dissipate the same power. Packages with large thermal tabs such as D²PAK / DPAK (TO-252) tolerate higher (VIN − VOUT) × IOUT heat without hitting thermal shutdown, even on average-quality PCBs. Smaller QFN/DFN packages can achieve similar performance only when they are mounted on a large copper pad with several thermal vias. Very small packages like SOT-223 / SOT-23 are power-limited and should be marked as such in the BOM so purchasing knows the thermal headroom is smaller.
When doing cross-brand replacement across TI, ST, NXP, Renesas, onsemi, Microchip, and Melexis, always compare the available package codes and the RθJA values measured on similar boards, not just the electrical pinout.
3. Power Dissipation First: P = (VIN − VOUT) × IOUT
For low dropout regulators, the very first step of thermal planning is to calculate how much power will be burned inside the device. Because the LDO is linear, it converts the voltage difference (VIN − VOUT) directly into heat at the same output current. That is why you must compute P = (VIN − VOUT) × IOUT before you talk about RθJA, copper area, or package size.
Always use the worst-case triple when doing this calculation: VINmax, IOUTmax, and TAmax. If the LDO sometimes runs close to dropout, the power may be lower at that moment, but you still need to check the case where the input is at its maximum and the load is at its maximum. Only then will you have a reliable junction temperature estimate.
Document this calculated dissipation in your BOM/RFQ so purchasing can compare TI, ST, NXP, Renesas, onsemi, Microchip, and Melexis parts against the same thermal requirement.
4. RθJA from Datasheet vs Real Board
RθJA values in LDO datasheets are commonly measured on JEDEC 2s2p reference boards with well-defined copper, controlled layer stack, and free-air convection. Real application boards rarely match such conditions, especially inside automotive housings or small enclosures. That is why the thermal resistance of your board should be assumed to be higher than the datasheet number.
A practical engineering method is to start from the datasheet RθJA, then multiply it by 1.5–2.0 to cover reduced copper, fewer thermal vias, nearby hot components, or poorer airflow. Packages with large exposed pads such as D²PAK / TO-252 or bigger QFNs will be closer to datasheet values; tiny SOT or DFN packages on a narrow board can be much worse.
After applying the factor, use TJ = TA + P × RθJA_est. If TJ approaches 150°C or the LDO’s OTP point, move to a larger package or increase the copper area.
5. TJ Margin Planning
Once you know the power dissipation P and you have estimated a realistic thermal resistance RθJA for your board, the junction temperature is obtained from the classic formula: TJ = TA + P × RθJA. This step closes the loop and tells you whether your LDO will run close to thermal shutdown (OTP) or stay in a safe region.
For automotive and hot, enclosed applications, always evaluate at least three ambient points: 85°C, 105°C, and 125°C. These reflect typical cabin, warm-compartment, and lamp/sensor enclosure temperatures. For each of these, calculate TJ and keep a 10–20°C safety margin to the device’s OTP/maximum rated junction temperature.
If your calculation gives a TJ that sits very close to 150°C (or to the LDO’s datasheet TJ(max)), you should either move to a larger package, add more copper/thermal vias, or reduce VIN / IOUT. Running permanently on the edge will make the device chatter in and out of OTP.
6. Board-Level Thermal Path: Big Pads + Thermal Vias
The LDO’s thermal performance is only as good as the copper and vias below it. For QFN/DFN LDOs with exposed pads, the EP must be soldered to a large copper area and stitched with at least 4–9 thermal vias into the inner or bottom planes. Otherwise, the real RθJA will be far above the datasheet value and your TJ planning will fail.
For D²PAK / TO-252 packages, connect the thermal tab directly to a wide GND or power plane, not through a thin trace. The tab is designed to dump heat into the board; if it is pinched by a narrow connection, the heat will stay around the device and trigger thermal shutdown earlier.
Keep sensitive analog, sense, or feedback traces away from the hottest copper zones. Route them around the pad or drop them quickly to a cooler layer. This prevents drift and measurement errors caused by local heating.
7. Package Choice vs Assembly/Space
Package selection for LDOs is part of the thermal design, not only a mechanical or BOM decision. Use D²PAK / DPAK (TO-252) when the voltage drop is large, the output current is high, the application is automotive or hot, or when the customer’s board conditions are unknown. Use QFN / DFN with exposed pad when you have multilayer boards with large copper planes and you can place multiple thermal vias. Always tell purchasing when the QFN version requires a big pad to meet datasheet RθJA.
A short BOM note like “QFN EP must be soldered to large copper with 4–9 thermal vias, otherwise derate current” will make TI ↔ ST ↔ NXP ↔ Renesas ↔ onsemi ↔ Microchip ↔ Melexis comparisons more realistic.
8. Validation & Thermal Test Playbook
Thermal validation should be done in two passes: first in air/room conditions to make sure the LDO is stable and no unexpected hotspots appear, and then in a chamber or hot enclosure (85°C / 105°C / 125°C) to confirm that the junction temperature calculated by TJ = TA + P × RθJA matches reality. Each run must record VIN, VOUT, IOUT, TA, TJ(est), and hotspot location.
Keep this dataset in your BOM or RFQ so purchasing can ask TI, ST, NXP, Renesas, onsemi, Microchip, and Melexis for parts that survive the same thermal profile.
9. Seven-Brand Notes (Thermal-Aware LDO Lines)
Below is a thermal-focused view of the seven brands you work with — TI, ST, NXP, Renesas, onsemi, Microchip, Melexis. This is not a full LDO listing and it intentionally does not include part numbers, so you can later plug in programmatic PN tables or ACF fields.
The goal is to tell purchasing who usually gives bigger packages, who documents QFN with exposed pad well, and who publishes multi-board RθJA tables. After reading, purchasing should be able to say: “OK, for this hot board, I will switch to the brand that shows more thermal data,” or “I will ask the same brand for the D²PAK/TO-252 version.”
TI
Often has automotive / big-package variants (D²PAK / TO-252) and may show different thermal conditions. Good when board copper is unknown.
ST
Many LDOs in QFN/DFN with large EP and notes about minimum copper. Good for small boards — but tell purchasing to reserve the pad.
NXP
Some power/automotive lines show thermal data across different boards. Useful when you want to apply the 1.5–2× factor clearly.
Renesas
Automotive LDOs often come in larger, thermally safer packages with clear 85/105/125°C operating ranges. Good for high-TA pages.
onsemi
Strong on lighting / camera / sensor feeds and often documents board-level thermal behavior. Good match for “the box is already hot.”
Microchip
Documentation often separates typ board vs customer board assumptions → purchasing can directly ask for customer-board guidance.
Melexis
Focused on automotive/sensor modules; packages are often mechanical + thermal aware, good for lamp housings and sealed units.
What the buyer can do after reading
- Swap to the bigger package of the same brand if TJ is too close to 150°C.
- Pick the brand that publishes multi-board / multi-copper RθJA if current datasheet only shows JEDEC.
- If the project cannot make a big copper pad, prefer vendors with D²PAK / TO-252 automotive variants.
- Ask vendors for board-specific thermal guidance when sending layout snippets.
Part numbers are intentionally omitted here to allow later, programmatic, per-brand PN expansion.
10. Submit your BOM (48h)
Send your BOM + VIN / VOUT / IOUT + target ambient (85 / 105 / 125°C), and we will check whether your current LDO package has enough RθJA margin. If you can also provide a layout snippet (EP pad area, number of thermal vias, copper pour) or stack-up, we can tell you if a larger package (D²PAK / TO-252) or a brand with richer thermal data is safer.
We will map it across TI, ST, NXP, Renesas, onsemi, Microchip, and Melexis and suggest: keep current package, switch to the big package, or switch to a vendor that publishes multi-board thermal tables.
Submit your BOM (48h) Optional: attach layout/stack-up for RθJA check.
Frequently Asked Questions
How do I estimate LDO RθJA on my own board?
Start from the datasheet RθJA (usually on a JEDEC 2s2p board), then multiply it by 1.5–2.0 to cover your smaller copper, fewer vias, and hotter enclosure. Use this scaled RθJA in TJ = TA + P × RθJA. If layout changes, re-estimate it.
What data do I need before running TJ = TA + P × RθJA?
You must know VIN_max, VOUT, IOUT_max, ambient temperature to check (85°C / 105°C / 125°C), and an estimated RθJA for your board. First compute P = (VIN − VOUT) × IOUT, then plug it into the TJ formula. Always use the worst combination.
How much TJ margin should I keep for automotive LDOs?
Keep 10–20°C below the device’s maximum junction temperature or OTP point. For 125°C ambient tests, try to land TJ around 130–140°C, not 149°C. If your result is “too close,” switch to a bigger package or reduce VIN/IOUT to regain margin.
Is D2PAK always better than QFN for LDO thermal?
D²PAK/DPAK is safer when ΔV and IOUT are high or the target ambient is 105–125°C, especially when the customer PCB is unknown. A QFN with a large exposed pad and multiple thermal vias can match it, but only if layout follows the thermal note.
Do I need thermal vias under an LDO exposed pad?
Yes. For QFN/DFN LDOs, the exposed pad is the main thermal path. Use at least 4–9 vias right under or next to the pad and tie them into inner or bottom planes. Without these vias, real RθJA rises and TJ can hit thermal shutdown quickly.
What if my board cannot fit a large copper pad?
Then you must derate: limit IOUT, reduce VIN, or move the LDO to a bigger package that can dump heat somewhere else. Also tell purchasing in the BOM note that “QFN used on small copper → current must be limited” to avoid later overloading.
Can I put feedback/sense traces across the hot pad?
It is better not to. The hot copper around the LDO can introduce drift or noise, and dense vias can disturb sensitive traces. Route FB/sense around the pad or drop to a cooler layer. Keep the thermal area primarily for heat spreading.
Why does my LDO hit thermal shutdown even though current is within spec?
Because the current limit in the datasheet was validated on a reference board with better cooling. On your board, with higher VIN, higher TA, and smaller copper, the same power dissipation pushes TJ to OTP. Re-run TJ = TA + P × RθJA with real values.
Do I have to test in a chamber, or is room test enough?
Do both. Room test (≈25°C) confirms stability and obvious hotspots. Chamber or lamp-housing test at 85/105/125°C confirms worst-case TJ and whether OTP is triggered. Relying on room-only data is usually too optimistic for automotive or sealed enclosures.
Which parameters should I log during thermal validation?
Log at least: VIN, VOUT, IOUT, ambient TA, estimated TJ, hotspot location, and package used. Put the same dataset into the BOM/RFQ so suppliers can reply with compatible LDOs or bigger packages. This makes cross-brand thermal comparison much easier.
Can I derate the LDO instead of changing package?
Yes, as long as you write the derated condition explicitly, for example “IOUT ≤ 350 mA @ 125°C, VIN = 12 V.” This keeps TJ below the limit on your board. Also ask purchasing to look for the same LDO family in a bigger package as a backup.
Is lowering VIN a valid way to improve LDO thermal?
Yes. Since LDO power is P = (VIN − VOUT) × IOUT, even a small reduction in VIN can noticeably lower dissipation. But this only works if the system allows lower input. If VIN is fixed by vehicle battery or pre-regulator, then use a bigger package.
When should I switch to a brand that publishes more thermal data?
Switch when your PCB is far from JEDEC conditions and you must prove TJ stays inside the limit. Brands that show multiple RθJA tables or layout examples make this easier. It also helps purchasing argue for bigger packages or alternative families.
How do I tell purchasing what copper/thermal-via requirement the LDO needs?
Add a BOM note like: “QFN LDO: exposed pad to large copper, 4–9 thermal vias to inner/bottom planes, assumed RθJA = 80°C/W (≈2× datasheet). If copper cannot be guaranteed, choose D²PAK/TO-252 automotive version.” This removes guesswork for buyers.
Can I compare TI, ST, NXP, Renesas, onsemi, Microchip, and Melexis on the same thermal test log?
Yes. Use one log format for all brands: VIN, VOUT, IOUT, TA, TJ(est), hotspot, package. Send this with your RFQ. Suppliers can then propose their bigger-package or automotive variants, and you can see which brand gives clearer thermal guidance.
