A complete microstepping driver for bipolar stepper motors in QFN-28, the chip on the ubiquitous red 'A4988' 3D-printer and CNC driver modules. Its built-in translator means you drive a motor by pulsing STEP and setting DIR, with no phase tables: three mode pins (MS1-MS3) select full, half, quarter, eighth, or sixteenth microstepping. Two DMOS full-bridges deliver up to 2 A per coil (with a heatsink) from a 8-35 V motor supply, with an adjustable current limit (VREF + sense resistors), automatic decay-mode selection, and built-in over-current and thermal shutdown. ENABLE, RESET, and SLEEP round out the control. Pinout is DMOS bridges plus the STEP/DIR/MS logic.

Pinout

Pin	Name	Type	Function
1	OUT2B	output	DMOS Full Bridge 2 Output B. Motor phase 2 output; connects to one terminal of stepper motor coil B. Up to ±2 A at up to 35 V (VBB2).
2	ENABLE	input	Active-low output enable logic input. Logic low enables the DMOS FET outputs; logic high disables all outputs. Translator inputs (STEP, DIR, MSx) remain active regardless of ENABLE.
3	GND	power_in	Device ground. Must be tied together externally with pin 18 by connecting to the exposed thermal PAD ground plane.
4	CP1	passive	Charge pump capacitor terminal. A 0.1 µF ceramic capacitor is connected between CP1 and CP2 for the high-side gate-drive charge pump.
5	CP2	passive	Charge pump capacitor terminal. Companion of CP1; 0.1 µF ceramic capacitor tied to CP1.
6	VCP	passive	Charge-pump reservoir capacitor terminal. A 0.1 µF ceramic capacitor is required between VCP and VBB to act as the reservoir for the high-side FET gate drive supply.
7	NC	unconnected	No internal connection. Leave floating or tie to ground for layout convenience.
8	VREG	power_out	Internal regulator decoupling terminal. Nominal 7 V internally generated supply used to drive the sink-side FETs; decouple with a 0.22 µF ceramic capacitor to ground. Monitored for fault protection.
9	MS1	input	Microstep resolution select input 1. Combined with MS2/MS3 selects full, half, quarter, eighth, or sixteenth step. Internal 100 kΩ pull-down.
10	MS2	input	Microstep resolution select input 2. Combined with MS1/MS3 to select step size. Internal 50 kΩ pull-down.
11	MS3	input	Microstep resolution select input 3. Combined with MS1/MS2 to select step size. Internal 100 kΩ pull-down.
12	RESET	input	Active-low reset input. Drives the translator to the predefined Home state and turns off all FET outputs. STEP inputs are ignored until RESET goes high.
13	ROSC	passive	Fixed off-time / decay-mode timing terminal. Resistor to GND sets t_OFF ≈ R_OSC/825 µs (with automatic mixed decay). Tied to VDD sets 30 µs off-time and mixed decay; tied to GND sets 30 µs off-time and 100% mixed decay.
14	SLEEP	input	Active-low sleep input. Logic low disables output FETs, regulator, and charge pump for low-power standby; logic high enables normal operation (allow 1 ms for charge pump stabilization before first STEP).
15	VDD	power_in	Logic supply input. Operating range 3.0 V to 5.5 V (compatible with 3.3 V and 5 V logic). Decouple with a 0.22 µF ceramic capacitor to GND.
16	STEP	input	Step-pulse logic input. Each low-to-high transition advances the translator/motor by one microstep as selected by MSx. Minimum HIGH/LOW pulse width 1 µs; setup/hold to input changes 200 ns.
17	REF	input	Gm current-set reference voltage input (V_REF). Sets peak coil current I_TripMAX = V_REF / (8 × R_S). Range 0 V to 4 V (abs max 5.5 V).
18	GND	power_in	Device ground. Must be tied together externally with pin 3 by connecting to the exposed thermal PAD ground plane.
19	DIR	input	Direction logic input. Selects rotation direction of the motor; changes take effect on the next STEP rising edge.
20	NC	unconnected	No internal connection. Leave floating or tie to ground for layout convenience.
21	OUT1B	output	DMOS Full Bridge 1 Output B. Motor phase 1 output; connects to one terminal of stepper motor coil A. Up to ±2 A at up to 35 V (VBB1).
22	VBB1	power_in	Load (motor) supply for Bridge 1. Operating range 8 V to 35 V; requires local bulk decoupling (typically 100 µF near VBB1/VBB2).
23	SENSE1	passive	Bridge 1 current sense resistor terminal. External R_S1 to GND sets phase-1 peak current together with V_REF. Voltage across R_S1 must not exceed 0.5 V (absolute max ±0.5 V).
24	OUT1A	output	DMOS Full Bridge 1 Output A. Motor phase 1 output; connects to the other terminal of stepper motor coil A. Up to ±2 A at up to 35 V (VBB1).
25	NC	unconnected	No internal connection. Leave floating or tie to ground for layout convenience.
26	OUT2A	output	DMOS Full Bridge 2 Output A. Motor phase 2 output; connects to the other terminal of stepper motor coil B. Up to ±2 A at up to 35 V (VBB2).
27	SENSE2	passive	Bridge 2 current sense resistor terminal. External R_S2 to GND sets phase-2 peak current together with V_REF. Voltage across R_S2 must not exceed 0.5 V (absolute max ±0.5 V).
28	VBB2	power_in	Load (motor) supply for Bridge 2. Operating range 8 V to 35 V; must be tied to VBB1 at the same rail, sharing the 100 µF bulk decoupling.
PAD	PAD	power_in	Exposed thermal pad for enhanced thermal dissipation. Solder to a ground plane; the GND pins (3 and 18) must be tied to this pad.

Key specifications

Parameter	Value
Type	Bipolar stepper motor driver
Output	2 DMOS full-bridges
Current	up to 2 A/coil (with heatsink)
Microstepping	full, 1/2, 1/4, 1/8, 1/16 (MS1-3)
Control	STEP + DIR translator
Motor supply	8-35 V
Protection	current limit + thermal + crossover
Package	QFN-28 (5x5 mm, exposed pad)

Ordering variants

A4988SETTR-T (QFN-28)
A4988 module (Pololu-style breakout)

CAD (KiCad official)

Symbol: A4988 (generated from datasheet; KiCad has no symbol)
Footprint: Package_DFN_QFN:QFN-28-1EP_5x5mm_P0.5mm_EP3.35x3.35mm
3D model: QFN-28 (KiCad packages3D)

Provenance

Data extracted from the manufacturer datasheet, not a distributor listing:

Datasheet: Allegro MicroSystems datasheet (20 pp, sha256 d1dfba3bb7be2cbf, retrieved 2026-08-17, tier: manufacturer)
Extractor: ds2sf (Claude). Every pin and dimension is cited to a datasheet page; it refuses to fabricate.
CAD: KiCad official libraries (CC-BY-SA 4.0).

Datasheet PDF