ULN2003A PCB Design Guide: Footprint, Pinout, and Alternatives
Seven Darlington sink drivers with built-in flyback diodes for relays, solenoids, stepper coils, and LED strings.
The ULN2003A is seven Darlington sink drivers in a 16-pin package: open-collector outputs that withstand 50 V in the OFF state and sink 500 mA peak each, with integral suppression diodes brought out to a common free-wheeling pin for inductive loads. Each input has a 2.7 kΩ series base resistor sized for 5 V TTL and CMOS, so a bare logic pin switches a relay coil, solenoid, stepper winding, or LED string with no external base resistor. Outputs may be paralleled for higher current, and the device spans five packages — from the breadboard-friendly PDIP-16 to the 4.2 × 2.0 mm SOT-23-THN-16 — at a 1ku price under $0.13 in SOIC.
It earns its place on cost and ruggedness, not performance. The Darlington output saturates around a volt, not millivolts: 0.9 V typ at 100 mA, up to 1.6 V max at 350 mA. The load loses that voltage, the package dissipates it, and thermal resistance varies wildly by package — from 66.7 °C/W in PDIP to 123.1 °C/W in the tiny SOT-23-THN. For relay coils running well above the dropout, none of that matters; for low-voltage loads, sustained high currents, or dense SMD boards, a DMOS array like the TPL7407L or discrete logic-level MOSFETs is the better answer.
The ULN2003A and ULN2803A are completely different lineages from different manufacturers. TI owns the ULN2003A (seven channels, 16-pin packages); ST owns the ULN2803A (eight channels, 18-pin DIP only). A ULN2003A in a schematic expecting a ULN2803A pinout will swap two channels and potentially misroute COM. Check pinout against the manufacturer you are actually buying from. Most field failures come from five classes of mistake, each covered below.
What breaks boards
The outputs are Darlingtons: budget up to 1.6 V of saturation drop
A Darlington pair cannot saturate like a single transistor or a MOSFET. VCE(sat) is 0.9 V typ / 1.1 V max at IC = 100 mA, II = 250 µA; 1.0 V typ / 1.3 V max at 200 mA; and 1.2 V typ / 1.6 V max at 350 mA. The load sees its supply minus that drop, and the package — not the load — dissipates drop times current, which is what makes the thermal ceiling bite. On 5 V loads the lost volt is real margin. If the drop disqualifies the part, a DMOS array such as the TPL7407L switches the same loads with far less on-voltage.
The per-channel ratings are not a package rating: thermal varies by package
Each Darlington is rated 500 mA peak, but total emitter-terminal current is capped at -2.5 A absolute maximum, and thermal resistance RθJA spans a huge range: 66.7 °C/W (PDIP-16), 88.6 °C/W (SOIC-16), 95.0 °C/W (SOP-16), 114.1 °C/W (TSSOP-16), 123.1 °C/W (SOT-23-THN-16). The SOT-23-THN package is especially unforgiving: sum VCE(sat) times IC over every simultaneously-on channel and the budget disappears fast. Derate for real ambient, stagger duty cycles, or parallel outputs to spread current — the heat still lands in the same package either way.
Floating the COM pin makes the built-in flyback diodes ornamental
The suppression diodes run from each output to a common free-wheeling pin (COM). They clamp inductive kickback only if COM is tied to the supply feeding the coils. The schematic symbol makes it easy to leave unconnected, and then every relay turn-off flies the output past the 50 V absolute maximum until channels start dying. Tie COM to the load supply, not the logic rail, when the two differ, and budget the clamp: the diode drops 1.7 V typ / 2.0 V max at IF = 350 mA while returning the kickback to the coil supply.
The 2.7 kΩ input is sized for 5 V logic; check VI(on) below that
Inputs are current-driven through a 2.7 kΩ series base resistor chosen for 5 V TTL and CMOS, drawing 0.93 mA typ / 1.35 mA max at VI = 3.85 V. The input voltage guaranteed to switch a channel rises with load current: VI(on) max is 2.4 V at IC = 200 mA, 2.7 V at 250 mA, 3.0 V at 300 mA (VCE = 2 V). A 3.3 V GPIO works at modest collector currents, but an input that only half-turns-on a Darlington parks the output in linear dissipation instead of saturation. The ULN2004A (10.5 kΩ input) is the higher-voltage-logic variant in the same datasheet.
Five packages, one MPN: check which package you're actually ordering
TI supplies the ULN2003A in five packages: PDIP-16 (N, RθJA 66.7 °C/W), SOIC-16 (D, 88.6 °C/W, $0.129/1ku), SOP-16 (NS, 95.0 °C/W), TSSOP-16 (PW, 114.1 °C/W), and the tiny SOT-23-THN-16 (DYY, 123.1 °C/W). The DYY package has the worst thermal and is best for low-duty-cycle, low-channel-count use. The PDIP has the best thermal and fits breadboards. Four orderables are OBSOLETE (DRG3, ANS tube, APW tube, PWRG4) — verify you're ordering an active reel or tube variant. Also note: TI offers the ULN2003B as a separate part (different datasheet SLVS222B) — it is not a drop-in without checking pinout and input specs.
Key specifications
| Parameter | Value | Source |
|---|---|---|
| Output voltage (absolute max) | 50 V (VCE, collector-emitter); clamp diode reverse voltage also 50 V | SLRS027 Rev. T, Section 5.1 Absolute Maximum Ratings |
| Peak collector current (per channel) | 500 mA peak per Darlington pair; outputs may be paralleled for higher current | SLRS027 Rev. T, Section 5.1 Absolute Maximum Ratings + Description (Section 3) |
| Total emitter-terminal current (absolute max) | -2.5 A (sum of all channel currents through common emitter pin) | SLRS027 Rev. T, Section 5.1 Absolute Maximum Ratings |
| Saturation voltage VCE(sat) | 0.9 V typ / 1.1 V max at IC = 100 mA, II = 250 µA; 1.0 V typ / 1.3 V max at IC = 200 mA, II = 350 µA; 1.2 V typ / 1.6 V max at IC = 350 mA, II = 500 µA | SLRS027 Rev. T, Section 5.6 Electrical Characteristics: ULN2003A (TA = 25 °C) |
| Input drive (ON-state input voltage) | VI(on) max: 2.4 V at IC = 200 mA; 2.7 V at IC = 250 mA; 3.0 V at IC = 300 mA (VCE = 2 V). 2.7 kΩ series base resistor sized for 5 V TTL/CMOS. | SLRS027 Rev. T, Section 5.6 Electrical Characteristics: ULN2003A |
| Input current II | 0.93 mA typ / 1.35 mA max at VI = 3.85 V; 0.35 mA typ / 0.5 mA max at VI = 5 V | SLRS027 Rev. T, Section 5.6 Electrical Characteristics: ULN2003A |
| Clamp diode forward voltage VF | 1.7 V typ / 2.0 V max at IF = 350 mA | SLRS027 Rev. T, Section 5.6 Electrical Characteristics: ULN2003A |
| Operating free-air temperature range | -40 to 70 °C (ULN2003A); ULN2003AI extends to 105 °C; ULQ2003A extends to 85 °C | SLRS027 Rev. T, Section 5.1 Absolute Maximum Ratings |
| Thermal resistance (junction-to-ambient) | PDIP-16: 66.7 °C/W; SOIC-16: 88.6 °C/W; SOP-16: 95.0 °C/W; TSSOP-16: 114.1 °C/W; SOT-23-THN-16: 123.1 °C/W | SLRS027 Rev. T, Section 5.4 Thermal Information |
| Collector cutoff current ICEX | 50 µA max at VCE = 50 V, TA = 25 °C, II = 0; 100 µA max at TA = 70 °C, II = 0 | SLRS027 Rev. T, Section 5.6 Electrical Characteristics: ULN2003A |
| Input capacitance Ci | 15 pF typ / 25 pF max at VI = 0 V, f = 1 MHz | SLRS027 Rev. T, Section 5.6 Electrical Characteristics: ULN2003A |
| ESD ratings | HBM ±2000 V, CDM ±500 V | SLRS027 Rev. T, Section 5.2 ESD Ratings |
Verified against the manufacturer datasheet on 2026-07-10. Confirm the current revision before production use.
Alternatives
- ULN2803A: STMicroelectronics' 8-channel version in DIP-18; same Darlington architecture with one extra channel. ST-only, DIP-only. Pick when you need an extra driver and are already using through-hole.
- ULN2003B: TI's newer 7-channel Darlington array in SOIC-16, PDIP-16, TSSOP-16 with improved input thresholds; datasheet SLVS222B. Separate part from ULN2003A lineage, not a drop-in without checking pinout and input specs.
- TPL7407L: TI's 7-channel low-side NMOS driver marketed as a ULN2003A upgrade; MOSFET outputs give far lower on-voltage than the Darlington's ~1 V VCE(sat). 16-pin SOIC/TSSOP package.
- TBD62083A: Toshiba's 8-channel sink-type DMOS transistor array; same function with MOSFET-style low on-voltage. 18-pin packages only.
Common questions
- Can the ULN2003A source current or drive a load high-side?
- No. All seven channels are open-collector Darlington sink drivers: the load connects between its supply and the output pin, and the ULN2003A pulls the low side toward ground. Each channel sinks 500 mA peak, and outputs may be paralleled for more. For sourcing or high-side switching you need a different part entirely.
- Do I need external flyback diodes when driving relays with the ULN2003A?
- No; each output has an integral suppression diode to the common free-wheeling pin (COM). But they only work if COM is connected to the relay coil supply — left floating there is no clamp at all. The diode drops 1.7 V typ / 2.0 V max at 350 mA.
- What's the difference between ULN2003A, ULN2003AI, and ULQ2003A?
- Temperature range. The ULN2003A operates from -40 to 70 °C; the ULN2003AI extends to 105 °C for industrial use; the ULQ2003A is the automotive-qualified variant rated -40 to 85 °C. All three share the same datasheet (SLRS027) and the same Darlington output architecture. The ULN2003B is a separate part with a different datasheet (SLVS222B) — not interchangeable without checking pinout.
- How much total current can a ULN2003A switch at once?
- Each channel is rated 500 mA peak, but the limits stack: total emitter-terminal current is -2.5 A absolute maximum, and thermal resistance RθJA varies from 66.7 °C/W (PDIP) to 123.1 °C/W (SOT-23-THN). Sum VCE(sat) times current over every on channel (up to 1.6 V max at 350 mA each) and stay under the thermal cap after derating for your ambient and package. The SOT-23-THN package is especially restrictive — save it for low-duty-cycle, low-channel-count designs.