LM3914N

The LM3914N from Texas Instruments is a classic monolithic integrated circuit that senses analog voltage levels and drives 10 LEDs to provide a linear analog display. It is one of the most iconic and widely-used display driver ICs in the electronics industry, having been in continuous production since its introduction by National Semiconductor (now part of TI) in the early 1980s.

The LM3914 is fundamentally a 10-step linear voltmeter on a chip. It contains an input buffer, 10 precision comparators, a 10-step resistor divider, an adjustable voltage reference, and 10 current-regulated LED driver outputs. The only external components needed for a basic application are the 10 LEDs, one programming resistor (which sets the LED brightness), and a power supply.

The bar or dot display mode is selected by a single pin (pin 9). In bar mode, all LEDs up to the level corresponding to the input voltage are lit, creating a bar graph. In dot mode, only the single LED corresponding to the input voltage level is lit, creating a moving dot. The dot mode has a small overlap (about 1 mV) between adjacent segments to ensure that at least one LED is always lit, preventing ambiguous display states.

The internal voltage reference generates a nominal 1.25 V between the REF OUT (pin 7) and REF ADJ (pin 8) pins. Using two external resistors (R1 and R2), the reference voltage can be adjusted from 1.2 V to 12 V. The same resistor that sets the reference voltage also programs the LED current: LED current is approximately 10 times the current flowing out of the REF OUT pin. This elegant design means that a single resistor sets both the full-scale voltage and the LED brightness.

The floating 10-step voltage divider is a key feature. Both ends of the divider (RHI at pin 6 and RLO at pin 4) are accessible, allowing the display to span any voltage range, not just 0 to VREF. For example, an expanded-scale meter can display 4.5 V to 5.0 V full-scale by connecting RHI to 5.0 V and RLO to 4.5 V. The 10 LEDs then represent 50 mV steps, providing much finer resolution than a 0-5 V display.

The LM3914 can be cascaded (chained) to create displays with 20, 30, or more segments. In dot mode, pin 9 of the lower-voltage LM3914 is connected to pin 1 of the next higher LM3914, and special internal circuitry ensures that only one LED is lit across all cascaded devices. In bar mode, the outputs of multiple LM3914s operate independently.

The LM3914N is the PDIP-18 package version, popular for prototyping and educational applications. The through-hole package allows easy breadboard use and hand soldering. The LM3914 is also available in PLCC-20 (LM3914FN) for surface-mount applications.

IMPORTANT: TI has designated the LM3914 as not recommended for new designs (NRND), and the product is no longer in production. However, it remains widely available from distributors and second-source manufacturers. For new designs, alternatives include the LM3914 from other manufacturers, discrete LED driver solutions with MCU control, or integrated LED driver ICs with I2C/SPI interfaces.

The LM3914N operates as a linear 10-step analog-to-LED display converter with an internal voltage reference and precision resistor divider.

Input Buffer: The signal input at pin 5 (SIG) enters a high-impedance buffer amplifier that presents very low input bias current (25 nA typical). The buffer accepts input voltages from ground (0 V) up to VCC – 1.5 V, and is protected against overvoltage up to plus or minus 35 V. The buffer output drives the non-inverting inputs of all 10 comparators simultaneously.

Precision Resistor Divider: The internal resistor string consists of 10 equal-value resistors connected in series between the RHI (pin 6) and RLO (pin 4) pins. The total resistance is approximately 10 kOhm (1 kOhm per step). The voltage at each tap of the divider provides the reference voltage for one comparator. Because the resistors are monolithically integrated and laser-trimmed, their matching is very precise (typically 0.5 percent non-linearity), ensuring even spacing between LED thresholds.

Comparator Array: Each of the 10 comparators compares the buffered input voltage (at the non-inverting input) with the voltage at its respective divider tap (at the inverting input). When the input voltage exceeds the tap voltage, the comparator output activates its corresponding LED driver. In bar mode, all comparators with tap voltages below the input are activated, lighting all LEDs up to the current level. In dot mode, only the comparator corresponding to the highest activated tap is allowed to drive its LED, with adjacent comparators inhibited by the mode control logic.

Mode Pin Logic: Pin 9 (MODE) controls the display mode and the dot-mode carry function. When pin 9 is connected to VCC (pin 3), the chip is in bar mode: all comparators below the input level drive their LEDs. When pin 9 is left open, the chip is in dot mode: only one LED is lit. When pin 9 is pulled low by an external signal (such as the output of a higher LM3914 in a cascaded chain), the current LED is forced off to implement the dot-mode carry.

Voltage Reference: The internal bandgap reference generates a nominal 1.25 V between REF OUT (pin 7) and REF ADJ (pin 8). The reference voltage is set by external resistors R1 and R2: VREF = 1.25 x (1 + R2/R1) + IADJ x R2, where IADJ is typically 75 uA. The current flowing through R1 and R2 is also the programming current for the LED drivers: each lit LED draws approximately 10 times the reference current. For example, if the reference load is 1 mA, each LED draws approximately 10 mA.

LED Current Regulation: Each LED driver is a current-regulated open-collector NPN transistor. The current through each lit LED is held constant regardless of the LED supply voltage (VLED) from about 2 V to 17 V. This eliminates the need for individual LED series resistors. The LED current matching between outputs is typically within 0.12 mA at 2 mA and 1.2 mA at 20 mA.

Cascading: Multiple LM3914s can be chained to create displays with more than 10 segments. In dot mode, pin 9 of the lower-voltage device connects to pin 1 of the higher-voltage device. When an LED in the higher device lights, the current through pin 1 pulls pin 9 of the lower device below VLED, which activates comparator C2 internally, turning off LED 10 of the lower device. This ensures only one LED is lit across all cascaded devices.