Answer: The LD1117S33TR is ST’s fixed 3.3 V, 800 mA LDO in SOT-223 on tape and reel. It is accurate to 3.267–3.333 V at 25 °C, drops out at 1.2 V max at 800 mA and needs a 10 µF output capacitor. Two limits catch people out: the standard grade is rated only for TJ 0 to 125 °C (the S33CTR covers −40 °C), and ST gives the SOT-223 a 110 °C/W junction-to-ambient figure. (ST DocID2572 Rev 38, Tables 1, 2, 6, 26.)
Key specifications
| Parameter | Value | Condition | Source |
|---|---|---|---|
| Output voltage | 3.267 / 3.3 / 3.333 V | Vin = 5.3 V, IO = 10 mA, TJ = 25 °C | Table 6, p.12 |
| Output voltage, load & line | 3.235 – 3.365 V | IO = 0–800 mA, Vin = 4.75–10 V, TJ 0–125 °C | Table 6, p.12 |
| Dropout voltage | 1 / 1.05 / 1.10 V typ; 1.1 / 1.15 / 1.2 V max | IO = 100 / 500 / 800 mA | Table 6, p.12 |
| Output current | 800 min / 950 typ / 1300 max mA | Vin = 8.3 V, TJ = 25 °C | Table 6, p.12 |
| Line regulation | 1 typ / 6 max mV | Vin = 4.75–15 V, IO = 0 mA | Table 6, p.12 |
| Load regulation | 1 typ / 10 max mV | Vin = 4.75 V, IO = 0–800 mA | Table 6, p.12 |
| Quiescent current | 5 typ / 10 max mA | Vin ≤ 15 V | Table 6, p.12 |
| Supply voltage rejection | 60 min / 75 typ dB | IO = 40 mA, 120 Hz, Vin = 6.3 V, 1 Vpp, TJ = 25 °C | Table 6, p.12 |
| Output noise | 100 µV typ | 10 Hz – 10 kHz, TJ = 25 °C | Table 6, p.12 |
| DC input voltage (abs. max) | 15 V (18 V if IOUT < 20 mA) | — | Table 1 and note 1, p.7 |
| Operating junction temperature | 0 to +125 °C (standard); −40 to +125 °C (C version) | — | Table 1, p.7 |
| Thermal resistance, SOT-223 | RthJA 110 °C/W, RthJC 15 °C/W | — | Table 2, p.7 |
| Minimum output capacitor | 10 µF | Test circuits use CO = 10 µF | p.1 Description; Table 6 header |
Pinout (SOT-223)
ST’s SOT-223 drawing (Figure 2, p.6) shows pin 1 = GND (ADJ on the adjustable part), pin 2 = VOUT, pin 3 = VIN, with the tab connected to VOUT. This is the same order as the AMS1117 and LM1117.
Thermal: the 110 °C/W figure changes the maths
ST quotes RthJA = 110 °C/W for SOT-223 (Table 2), noticeably higher than the 90 °C/W in the AMS datasheet or the 61.6 °C/W JEDEC figure in TI’s. Different test boards make these numbers hard to compare, but if you design with ST’s own figure:
- From 5 V at 0.5 A: PD = (5 − 3.3) × 0.5 = 0.85 W → 0.85 × 110 = 93.5 °C rise. At 25 °C ambient, TJ ≈ 118.5 °C, only 6.5 °C below the 125 °C limit.
- At 40 °C ambient the allowance is (125 − 40)/110 = 0.77 W, which is about 455 mA from 5 V.
- From 12 V the same budget gives about 89 mA. That is a job for a buck converter.
If you need more, ST lists the same 3.3 V die in DPAK (LD1117DT33TR, 100 °C/W) and TO-220 (LD1117V33, 50 °C/W) (Tables 2 and 26).
Design notes
- Headroom from USB. With 1.2 V maximum dropout at 800 mA, 4.5 V at the input is exactly enough at full load (4.5 − 3.3 = 1.2 V). At 500 mA you need 4.45 V; at 100 mA, 4.4 V. ST’s load-and-line output band (3.235–3.365 V) is specified from 4.75 V.
- Capacitor. ST asks for a “very common 10 µF minimum capacitor” for stability (p.1) and characterises with CO = 10 µF. It gives no ESR window in the electrical tables we checked, so validate a ceramic-only output on your board.
- Quiescent current goes to the load. ST points out that with the NPN pass transistor, most of the quiescent current flows into the load rather than to ground (p.1). The 5–10 mA figure is still drawn from the input when the load is idle.
- Input above 15 V. The 18 V absolute maximum applies only while the output current is below 20 mA (Table 1, note 1). Do not count on it for start-up transients with a real load.
Pitfalls specific to LD1117S33TR
- Outdoor or automotive cold starts. The standard part is rated 0 to +125 °C junction (Table 1). For −40 °C, order LD1117S33CTR, and note its looser limits: 3.24–3.36 V at 25 °C, 3.16–3.44 V over temperature, and up to 1.3 V dropout at 800 mA (Table 12, p.18).
- Copying the AMS1117 thermal numbers. ST’s SOT-223 figure is 110 °C/W, not 90 °C/W.
- Dropping the “TR”. In ST’s order table, the SOT-223 parts all end in TR (tape and reel). There is no separate tube orderable listed for SOT-223 (Table 26).
- Assuming the 1.3 A maximum output current is usable. That is the top of the current-limit spread at 8.3 V input and 25 °C, not a rating.
Part numbers (information only)
| ST order code | Package | Grade |
|---|---|---|
| LD1117S33TR | SOT-223 (tape and reel) | 0 to 125 °C |
| LD1117S33CTR | SOT-223 (tape and reel) | −40 to 125 °C |
| LD1117D33TR | SO-8 | 0 to 125 °C |
| LD1117DT33TR | DPAK (tape and reel) | 0 to 125 °C |
| LD1117V33 | TO-220 | 0 to 125 °C |
Source: Table 26, p.44 (ST also lists C-grade SO-8, DPAK and TO-220 codes).
FAQ
- What is the difference between LD1117S33TR and LD1117S33CTR? The C version is rated for −40 to +125 °C junction temperature instead of 0 to +125 °C, with slightly wider output and dropout limits (Tables 1 and 12).
- Can the LD1117S33TR run from 5 V USB? Yes. Its 1.2 V maximum dropout at 800 mA needs 4.5 V at the input; keep cable drop in mind.
- How much current can it deliver from 5 V without a heatsink? Using ST’s 110 °C/W figure at 40 °C ambient, about 455 mA. A larger copper area does better, but ST gives no copper curve in the tables we checked.
- What output capacitor does it need? At least 10 µF (p.1); ST’s test circuits use CO = 10 µF.
- Is the LD1117S33TR pin-compatible with the AMS1117-3.3? Yes: GND, VOUT, VIN on pins 1–3 with the tab on VOUT, in SOT-223.
Related (internal links)
- AMS1117 equivalents comparison
- AMS1117-3.3
- LM1117MP-ADJ/NOPB
- Power calculator
- Linear voltage regulators
- SMD package sizes
Sources (retrieved 2026-10-26)
- STMicroelectronics, LD1117 Adjustable and fixed low drop positive voltage regulator, DocID2572 Rev 38 (January 2025): https://www.st.com/resource/en/datasheet/ld1117.pdf
Disclaimer
Values are copied from ST DocID2572 Rev 38, read on 26 Oct 2026; worked numbers are our arithmetic. Verify against the current datasheet and ST product page before production.
Reviewed by: Jasper · Updated: 2026-10-26