LM2901DR

The LM2901DR from Texas Instruments is the automotive/industrial-qualified version of the industry-standard LM339 quad differential comparator family. It contains four independent voltage comparators in a single SOIC-14 package, designed to operate from a single power supply over a wide voltage range of 2 V to 30 V without requiring a dual supply.

The LM2901 is one of the most widely used comparators in the electronics industry, alongside its commercial-grade sibling the LM339. The primary difference is the extended operating temperature range: the LM2901 is specified for -40 to +125 degrees C (automotive grade), while the LM339 is specified for 0 to +70 degrees C (commercial grade). The LM2901 also has slightly relaxed offset voltage and bias current specifications to account for the wider temperature range.

The open-collector output architecture is a defining feature. Each comparator output is an NPN transistor with an open collector, meaning the output can only pull low (sink current to ground) but cannot pull high. An external pull-up resistor is required to establish the high logic level. This has several advantages: (1) the output can be pulled up to a voltage different from the comparator supply (wired-OR logic); (2) multiple open-collector outputs can be connected together to implement AND or NOR logic without additional gates; (3) the output can directly drive loads such as LEDs, relays, or optocouplers.

The common-mode input voltage range that includes ground (0 V) is critical for single-supply applications. Many comparators require the inputs to be at least 1-2 V above the negative rail, but the LM2901 allows both inputs to go to ground (or even slightly below) without phase inversion or output glitches. This makes it ideal for low-side current sensing, battery monitoring, and zero-crossing detection applications where the input signal may be near ground.

The differential input voltage range of plus or minus 38 V (for the B-version) exceeds the supply voltage range, meaning the two inputs can differ by up to 38 V without damaging the device. This is useful for comparing signals from different voltage domains, such as monitoring a 24 V industrial signal with a 5 V comparator supply.

TI offers the LM2901B as an upgraded drop-in replacement with significantly improved specifications: offset voltage reduced from 7 mV to 0.37 mV (19x improvement), input bias current from 250 nA to 3.5 nA (71x improvement), propagation delay from 1.3 us to 1 us, and ESD rating from 500 V to 2 kV HBM. For new designs, TI recommends the B-version. However, the original LM2901 remains in high-volume production due to its extremely low cost and established design heritage.

The LM2901DR is available in SOIC-14 with tape and reel packaging (R suffix), suitable for automated SMT assembly. Other package options include PDIP-14 (N suffix), TSSOP-14 (PW suffix), and SOP-14 (NS suffix).

The LM2901DR operates as four independent differential voltage comparators sharing a common power supply and ground.

Comparator Operation: Each comparator has two inputs (IN+ and IN-) and one open-collector output. The comparator continuously compares the voltage at the non-inverting input (IN+) with the voltage at the inverting input (IN-). When V(IN+) is greater than V(IN-) by at least the offset voltage plus the overdrive voltage, the output transistor is off (open collector), and the output is pulled high by an external pull-up resistor. When V(IN+) is less than V(IN-), the output transistor turns on, pulling the output low (saturating to near ground). The output state changes when the differential input voltage crosses the switching threshold, which is ideally zero but is affected by the input offset voltage (up to 7 mV).

Open-Collector Output: The output stage consists of an NPN transistor with its collector connected to the output pin and its emitter connected to ground. When the comparator detects that IN+ is less than IN-, the output transistor saturates, pulling the output pin toward ground (typically 0.25 V at 4 mA sink current). When IN+ is greater than IN-, the output transistor is off, and the output pin floats (high impedance). An external pull-up resistor (typically 1 kOhm to 100 kOhm) connected from the output pin to the logic supply voltage establishes the high logic level. The pull-up voltage can be different from the comparator supply voltage, allowing level shifting.

Input Stage: The input stage uses a PNP differential pair that allows the common-mode input voltage range to extend down to ground (0 V) and up to VCC – 1.5 V. The PNP input transistors source a small bias current (typically 25 nA, max 250 nA) that flows out of the input pins. This bias current must be considered when designing high-impedance input networks, as it can create voltage offsets across source resistances. The differential input voltage is clamped by back-to-back diodes in parallel with a 1 kOhm resistor, limiting the differential current to safe levels even when the input voltage difference exceeds the supply voltage.

Propagation Delay: The typical propagation delay is 1.3 us for a 100 mV input step with 5 mV overdrive. This is relatively slow compared to dedicated high-speed comparators (which can be 10-100 ns), but is adequate for most control and monitoring applications. The propagation delay increases with smaller overdrive voltages and larger load capacitances.

Wired-OR Configuration: Multiple open-collector outputs can be connected together to a single pull-up resistor to implement a wired-OR (actually wired-AND) function. If any comparator output is low, the combined output is low. This is useful for creating multi-channel window comparators or multi-input alarm circuits without additional logic gates.

Power Supply: The LM2901 operates from a single supply (VCC to GND) without requiring a negative supply. The quiescent current is approximately 0.8 mA total (0.2 mA per comparator) and is relatively independent of supply voltage. The comparators remain functional down to 2 V supply, making them suitable for battery-powered applications.