AMS1117-ADJ Calculator – Resistors, Dropout & Heat Check

Answer

VOUT = 1.25 V × (1 + R2/R1) + IADJ × R2, with IADJ = 55 µA typical. Keep R1 at or below 125 Ω so the divider alone draws the 10 mA minimum load. For 3.3 V, R1 = 120 Ω and R2 = 196 Ω give 3.302 V. Then check that VIN − VOUT is at least 1.3 V and that a SOT-223 stays below 125 °C (θJA 90 °C/W). Source: Advanced Monolithic Systems AMS1117 datasheet, pp.2–5.

Calculator

VOUT (V) R1 (Ω) Series VIN min (V) VIN max (V) Load (A) Package Ta (°C)

R2 exact (with IADJ): 195.77 Ω
R2 ignoring IADJ: 196.8 Ω
R2 nearest: 196 Ω
VOUT at nearest R2: 3.302 V
VOUT band (VREF 1.232–1.268 V): 3.255 V – 3.35 V
Divider current: 10.42 mA
Headroom: 1.7 V
Dissipation: 850 mW
Tj: 101.5 °C
I max at 125 °C: 653.59 mA
θJA: 90 °C/W
Output capacitor: 22 µF tantalum covers all conditions in the AMS datasheet.

pass Divider current (minimum load): Idiv = 1.25/R1 = 10.42 mA. Fail below 10 mA (maximum of the minimum-load spec).
pass Headroom vs dropout: VIN,min − VOUT = 1.700 V. Fail below 1.3 V at ≤0.8 A.
pass Input voltage limit: VIN,max = 5 V. Fail above 15 V.
pass Junction temperature: Tj ≈ 101.5 °C (θJA 90 °C/W). Fail above 125 °C.

How it works

The AMS1117 holds 1.25 V typical between OUT and ADJ. That reference drives current through R1. The same current flows through R2, and the ADJ pin adds IADJ through R2. Because the regulator needs at least 5 mA typical / 10 mA maximum of load, the divider itself should draw it. That is why R1 tops out at 1.25 V / 10 mA = 125 Ω. The datasheet also suggests R1 = 100–200 Ω, which is why this page fails a divider current below 10 mA rather than treating 5 mA as enough.

If the thermal check fails, a pin-compatible alternate will not fix the heat. The dissipation is still (VIN − VOUT) × I. Use the LM2596-ADJ calculator when a buck is the right next step.

Part numbers

Part	Package	Notes
AMS1117 (ADJ)	SOT-223	Tab = VOUT
AMS1117CD (ADJ)	TO-252	—
AMS1117CS (ADJ)	SO-8	θJA 160 °C/W

Fixed-output versions are listed in the same ordering table. Information only. Pin-compatible options are compared on the AMS1117 equivalents page.

Disclaimer

Values come from the Advanced Monolithic Systems datasheet above, as read on 26 Oct 2026. Results are calculations from datasheet values, not measurements. θJA depends on the PCB. Check the latest datasheet for the exact part, and test before production. Parts sold as “AMS1117” by other makers may differ. This page covers the Advanced Monolithic Systems datasheet only.

Updated: 2026-10-26 · Jasper

Parameters used on this page

Parameter	Value	Source
VREF	1.232 / 1.250 / 1.268 V at 10 mA; 1.2125 / 1.250 / 1.2875 V over 1.5 V ≤ VIN − VOUT ≤ 12 V	AMS DS1117 p.2
IADJ	55 µA typ / 120 µA max	p.3
Minimum load current	5 mA typ / 10 mA max	p.3
Dropout	1.1 V typ / 1.3 V max at 0.8 A	p.3
Current limit	900 / 1100 / 1500 mA	p.3
Maximum input voltage	15 V	p.2
Junction temperature	−40 to 125 °C	p.2
θJA	SOT-223 90, TO-252 80, SO-8 160 °C/W	p.2
θJC	15 °C/W	p.3
Thermal shutdown	165 °C	p.4
Output capacitor	22 µF tantalum	p.4
SOT-223 pinout	1 GND/ADJ, 2 VOUT, 3 VIN, tab = VOUT	p.1

Worked examples

For 3.3 V, R1 = 120 Ω draws 10.42 mA. Exact R2 is 196.8 Ω ignoring IADJ, or 195.8 Ω including it. 196 Ω (E96) gives 3.302 V; 200 Ω (E24) gives 3.344 V. For 5.0 V, R1 = 120 Ω and 357 Ω gives 4.988 V. At 5 V to 3.3 V in SOT-223 (θJA 90 °C/W, Ta 25 °C), 0.8 A is 1.36 W and about 147 °C (fail); 0.5 A is 0.85 W and about 101.5 °C. The current that keeps Tj at or below 125 °C is about 0.65 A at 25 °C.

Pitfalls

Copying the LM317 R1 of 240 Ω. 1.25 V / 240 Ω is 5.2 mA, below the 10 mA maximum minimum-load spec, so with no load VOUT can drift high.
Running 5 V to 3.3 V at "1 A". The headroom is only 1.7 V and the chip dissipates 1.7 W per amp. A SOT-223 at θJA 90 °C/W runs out of thermal margin around 0.65 A at 25 °C.
Swapping the 22 µF tantalum for a small ceramic without checking stability. The datasheet guidance assumes tantalum.
Too little copper under the tab. The datasheet shows θJA moving roughly 55–80 °C/W with copper area; the 90 °C/W figure is not a floor.
Mixing up ADJ and fixed versions. They share a pinout, but on fixed versions pin 1 is GND, so a divider there does nothing useful.

FAQ

What resistors give 3.3 V on an AMS1117-ADJ?

R1 = 120 Ω and R2 = 196 Ω give 3.302 V typical. With 200 Ω you get 3.344 V.

Why does my AMS1117 get so hot?

It burns (VIN − VOUT) × I. At 5 V to 3.3 V and 0.8 A that is 1.36 W, about 147 °C in SOT-223 at 25 °C ambient.

What is the AMS1117 dropout voltage?

1.1 V typical, 1.3 V maximum at 0.8 A.

Does the AMS1117 need a minimum load?

Yes: 5 mA typical, 10 mA maximum. A 120 Ω R1 already draws 10.4 mA.

What is the maximum input voltage of the AMS1117?

15 V.

Sources

Advanced Monolithic Systems, AMS1117 1A Low Dropout Voltage Regulator (DS1117; no revision code printed): http://www.advanced-monolithic.com/pdf/ds1117.pdf (retrieved 2026-10-26; page numbers are PDF page order).
Updated: 2026-10-26. Jasper.