LM3914 - Dot/Bar Display Driver

Overview

The LM3914 is a monolithic integrated circuit designed to sense analog voltage levels and drive 10 LEDs in either a linear bar graph or moving dot display format. This versatile IC provides a visual representation of analog signals, making it ideal for various measurement and display applications. The device features programmable LED current drive, eliminating the need for current-limiting resistors, and offers flexible display modes selectable via a single pin.

With its wide operating voltage range and robust input protection, the LM3914 serves as a reliable solution for analog-to-visual signal conversion in both consumer and industrial applications. The internal voltage reference and expandable architecture allow for customization to meet specific design requirements.

Key Features

• Drives LEDs, LCDs, or vacuum fluorescent displays
• User-selectable bar or dot display mode via external pin
• Expandable architecture for displays up to 100 steps
• Internal adjustable voltage reference (1.2V to 12V)
• Operates with single supply voltage below 3V
• Inputs capable of operating down to ground potential
• Programmable output current (2mA to 30mA)
• No multiplex switching or interaction between outputs
• Robust input protection (±35V withstand capability)
• Current-regulated, open-collector LED driver outputs
• Outputs compatible with TTL or CMOS logic interfaces
• Floating internal 10-step divider with wide voltage reference range

Applications

The LM3914 finds application in numerous fields requiring visual representation of analog signals:

• Audio level meters and VU meters
• Battery voltage monitoring systems
• Power supply voltage indicators
• Temperature measurement displays
• Pressure and strain gauge indicators
• Analog signal strength displays
• Process control instrumentation
• Automotive dashboard displays
• Test and measurement equipment
• Expanded-scale voltmeters

Electrical Characteristics

Absolute Maximum Ratings

• Supply Voltage (V+): 25V
• Input Voltage: ±35V
• Output Voltage: 25V
• Output Current: 30mA (per output)
• Reference Output Current: 3mA
• Power Dissipation: 1365mW
• Operating Temperature Range: 0°C to +70°C
• Storage Temperature Range: -65°C to +150°C

Typical Operating Characteristics (V+ = 5V, TA = 25°C)

• Reference Voltage: 1.25V (adjustable)
• Reference Voltage Temperature Coefficient: ±100ppm/°C
• Input Bias Current: 25nA
• LED Current Range: 2mA to 30mA
• LED Current Matching: ±0.5%
• Voltage Between Steps: 125mV (typical)
• Input Voltage Range: 0V to (V+ - 1.5V)
• Supply Current (no load): 2.4mA
• Output Saturation Voltage: 0.5V (at 10mA)

Functional Description

The LM3914 consists of a high-impedance buffer amplifier, a precision 10-step voltage divider, and ten independent comparators with open-collector outputs. The input signal is compared against the voltage divider taps, with each comparator driving its respective LED when the input voltage exceeds its reference level.

The display mode (bar or dot) is selected by the MODE pin (pin 9). Connecting this pin to V+ selects bar mode where all LEDs up to the input level illuminate. Leaving it open or connecting to ground selects dot mode where only the LED corresponding to the input level lights.

The internal voltage reference provides a stable 1.25V between the REF OUT (pin 7) and REF ADJ (pin 8) terminals. This reference can be adjusted using external resistors to set the full-scale display range. The LED current is set by a single resistor connected between pin 7 and ground.

Package Information

Available Packages

• 18-pin PDIP (Plastic Dual In-line Package)
• 18-pin SOIC (Small Outline Integrated Circuit)

PDIP Package Dimensions

• Package Type: DIP-18
• Body Length: 22.86mm (max)
• Body Width: 6.35mm (max)
• Body Height: 4.57mm (max)
• Lead Pitch: 2.54mm
• Lead Length: 3.3mm (min)

SOIC Package Dimensions

• Package Type: SOIC-18
• Body Length: 11.55mm (max)
• Body Width: 7.5mm (max)
• Body Height: 2.65mm (max)
• Lead Pitch: 1.27mm
• Lead Width: 0.4mm (nominal)

Design Considerations

Current Setting

The LED current is set by the resistor connected between pin 7 (REF OUT) and ground. The formula for calculating this resistor is:

R₁ = 12.5V / I_LED

where I_LED is the desired current per LED (2mA to 30mA).

Voltage Reference Adjustment

The full-scale voltage can be adjusted using the internal reference and external resistors. The reference voltage can be increased by connecting a resistor between REF OUT and REF ADJ pins:

V_ref = 1.25V × (1 + R₂/R₁)

where R₁ is the current-setting resistor and R₂ is the resistor between REF OUT and REF ADJ.

Cascading Multiple Devices

For displays requiring more than 10 steps, multiple LM3914 devices can be cascaded. The REF OUT of the first device connects to the REF ADJ of the second device, and so on. The signal input should be buffered and distributed to all devices.

Typical Application Circuit

A basic 0V to 5V bar graph meter can be implemented with the following connections:

• Pin 4 (GND) to circuit ground
• Pin 3 (V+) to positive supply (5V or higher)
• Pin 5 (SIGNAL IN) to analog input signal
• Pin 9 (MODE) to V+ for bar mode or open for dot mode
• Pin 7 (REF OUT) through current-setting resistor to ground
• LED anodes to V+ through appropriate current-limiting resistors (if needed)
• LED cathodes to output pins (pins 1, 18-10)