Answer: The AMS1117-3.3 gives 3.300 V ±49 mV at 25 °C (3.201–3.399 V over temperature) and needs up to 1.3 V of headroom at 0.8 A, so its input must stay at or above about 4.6 V under full load. A healthy 5 V USB rail is enough; a sagging one (4.5 V at the board) is not. (AMS1117 datasheet pp.1–3.)
Key specifications
| Parameter | Value | Condition | Source |
|---|---|---|---|
| Output voltage | 3.251 / 3.300 / 3.349 V | VIN = 4.8 V, IOUT = 0 mA, TJ = 25 °C | p.2 Electrical Characteristics |
| Output voltage, full range | 3.201 – 3.399 V | VIN = 4.8 V, full operating temperature range (bold row) | p.2 Electrical Characteristics |
| Line regulation | 0.5 typ / 10 max mV (1.0 / 10 mV full range) | 1.5 V ≤ VIN − VOUT ≤ 12 V | p.2 Electrical Characteristics |
| Load regulation | 3 typ / 15 max mV (7 / 25 mV full range) | VIN = 4.75 V, 0 ≤ IOUT ≤ 0.8 A | p.3 Electrical Characteristics |
| Dropout voltage | 1.1 typ / 1.3 max V | IOUT = 0.8 A, ΔVOUT = 1 % | p.3 Electrical Characteristics, Note 4 |
| Current limit | 900 min / 1100 typ / 1500 max mA | VIN − VOUT = 1.5 V | p.3 Electrical Characteristics |
| Quiescent current | 5 typ / 11 max mA | VIN − VOUT = 1.5 V | p.3 Electrical Characteristics |
| Ripple rejection | 60 min / 72 typ dB | 120 Hz, COUT 22 µF tantalum, IOUT 1 A, VIN 4.75 V | p.3 Electrical Characteristics |
| Input voltage (abs. max) | 15 V | — | p.2 Absolute Maximum Ratings |
| θJA, SOT-223 | 90 °C/W | Soldered; 46 °C/W to > 90 °C/W depending on copper | p.2 Absolute Maximum Ratings (footnote) |
| Thermal shutdown | 165 °C | — | p.4 Application Hints |
| Junction temperature range | −40 to 125 °C | — | p.1 Ordering Information |
Pinout (SOT-223)
| Pin | Function |
|---|---|
| 1 | Ground |
| 2 | VOUT |
| 3 | VIN |
Source: p.1 Pin Connections. On the fixed version, pin 1 goes straight to ground; the divider is inside and Kelvin-connected (p.4).
Will it regulate from your input?
Headroom is VIN − 3.3 V and has to cover the dropout at your load current. The 1.3 V maximum is specified at 0.8 A (p.3) and is higher above that (Note 4).
| Input at the regulator pin | Headroom | Against 1.3 V max dropout |
|---|---|---|
| 5.0 V | 1.70 V | OK, with 0.40 V to spare |
| 4.75 V (low end of USB 2.0) | 1.45 V | OK, with 0.15 V to spare |
| 4.6 V | 1.30 V | On the limit |
| 4.5 V | 1.20 V | Not guaranteed: below the maximum dropout |
A cable and polyfuse dropping 0.4–0.5 V at the board is enough to land you in the last row. Measure at pin 3 under the heaviest load, not at the source.
Design notes
- Output tolerance in context. At 25 °C the output is ±1.5 % (3.251–3.349 V). Over the full temperature range it widens to 3.201–3.399 V (p.2), before adding line and load regulation. If an ADC reference or a 3.3 V-only sensor needs tighter limits, account for that band.
- Load step budget. Load regulation is 15 mV max at 25 °C and 25 mV over temperature for a 0–0.8 A step (p.3). That is small next to the tolerance band; ripple and thermal drift usually matter more.
- Ripple. 72 dB typical at 120 Hz (p.3) means 1 Vpp of input ripple becomes about 0.25 mVpp. The minimum, 60 dB, gives 1 mVpp.
- Quiescent current. 5 mA typical and 11 mA maximum (p.3). This matters for battery designs: an idle board draws 11 mA × 3.3 V…5 V of standby power before any load.
- Heat. From 5 V at 0.5 A the part dissipates (5 − 3.3) × 0.5 = 0.85 W. With 90 °C/W that is a 76.5 °C rise, so at 40 °C ambient the junction reaches about 116.5 °C, close to the 125 °C limit (p.5).
- Capacitors. 22 µF solid tantalum on the output covers all cases (p.4).
Pitfalls specific to AMS1117-3.3
- Powering it from a 3.7 V Li-ion cell. 3.7 V − 3.3 V = 0.4 V, far below the 1.1 V typical dropout. The output follows the battery down; it does not regulate.
- Treating it as 3.3 V from a weak USB port. See the table above: below about 4.6 V at the pin under 0.8 A, regulation is not guaranteed.
- Forgetting the quiescent current in sleep budgets. It can be up to 11 mA, which is much more than a CMOS LDO.
- Mixing up the ADJ and fixed footprints. Same pin order, but pin 1 is ground here. A board laid out for the adjustable part with R1/R2 can take the fixed part only if R1/R2 are removed and pin 1 is tied to ground.
Part numbers (information only)
AMS lists AMS1117-3.3 (SOT-223), AMS1117CD-3.3 (TO-252) and AMS1117CS-3.3 (SO-8), all for −40 to 125 °C (p.1). The other fixed voltages are −1.5, −1.8, −2.5, −2.85 and −5.0.
FAQ
- Can an AMS1117-3.3 run from USB 5 V? Yes, if the voltage at its input pin stays at or above about 4.6 V at 0.8 A. At 4.5 V the 1.3 V maximum dropout is no longer covered (p.3).
- Can it run from a single Li-ion cell? No. 0.4–0.9 V of headroom is below the 1.1 V typical dropout, so the output will sag with the battery.
- How accurate is the 3.3 V output? 3.251–3.349 V at 25 °C with no load; 3.201–3.399 V over the full temperature range (p.2); line and load regulation add up to 10 mV and 25 mV more.
- What is its idle current? 5 mA typical, 11 mA maximum (p.3).
- Is the AMS1117-3.3 pin-compatible with the adjustable AMS1117? The pin order is the same (1, 2, 3 = ground/adjust, VOUT, VIN), but the fixed part grounds pin 1 and needs no divider.
Related (internal links)
- AMS1117-ADJ (SOT-223)
- AMS1117-5.0
- LD1117S33TR (ST 3.3 V)
- AMS1117 equivalents
- Battery life calculator
- Linear voltage regulators
Sources (retrieved 2026-10-26)
- Advanced Monolithic Systems, AMS1117 1A Low Dropout Voltage Regulator (DS1117; no revision code printed): http://www.advanced-monolithic.com/pdf/ds1117.pdf (page numbers are PDF page order). Checked against a mirror copy of this PDF (sha256 189a2651…a3f0); AMS prints no revision code, so we cannot prove it is the newest issue.
Disclaimer
Values are copied from the AMS1117 datasheet listed above, read on 26 Oct 2026. The headroom table and power figures are our arithmetic on those values. Verify against the current datasheet and measure your design before production.
Reviewed by: Jasper · Updated: 2026-10-26