LM3915 - Dot/Bar Display Driver

Overview

The LM3915 is a monolithic integrated circuit designed to sense analog voltage levels and drive ten LEDs, LCDs, or vacuum fluorescent displays with a logarithmic 3 dB/step analog display. This versatile IC offers a flexible solution for creating visual voltage level indicators, with the ability to switch between bar graph and moving dot display modes via a single control pin. The LM3915 eliminates the need for external current-limiting resistors by providing regulated and programmable LED current drive. It operates from a single supply voltage, making it suitable for a wide range of applications.

Key Features

• Logarithmic 3 dB/step display with 30 dB range
• Drives LEDs, LCDs, or vacuum fluorescent displays
• User-selectable bar or dot display mode
• Expandable to displays of 90 dB
• Internal voltage reference adjustable from 1.2V to 12V
• Wide operating voltage range: 3V to 25V single supply
• Inputs can operate down to ground potential
• Programmable output current from 1 mA to 30 mA
• Robust input withstands ±35V without damage or false outputs
• Current-regulated, open collector outputs
• Direct interface capability with TTL or CMOS logic
• Floating internal 10-step divider for wide voltage reference flexibility

Applications

The LM3915 is ideal for various applications requiring visual voltage level indication:

• Audio level meters and VU meters
• Power level indicators
• Battery voltage monitors
• Signal strength indicators
• Industrial process control displays
• Test and measurement equipment
• Automotive dashboard displays
• Medical instrumentation
• Expanded-scale voltmeters

Electrical Characteristics

(Typical values at 25°C, V+ = 5V unless otherwise specified)

Reference Section

• Reference voltage: 1.28V (typical)
• Reference voltage adjust range: 1.2V to 12V
• Reference output current: 3 mA (max)
• Reference temperature coefficient: ±100 ppm/°C

Display Section

• Logarithmic step accuracy: ±0.5 dB (typical)
• Input voltage range: 0V to V+ - 1.5V
• Input bias current: 25 nA (max)
• Output saturation voltage: 0.5V (max at 8 mA)
• Output leakage current: 50 µA (max)

Power Requirements

• Supply voltage range: 3V to 25V
• Supply current (no load): 2.5 mA (typical)
• Supply current (all LEDs on): Depends on programmed LED current

Package Information

The LM3915 is available in an 18-lead molded Dual In-line Package (DIP):

LM3915N Package Details

• Package type: 18-pin PDIP (Plastic Dual In-line Package)
• Package dimensions: 22.86mm x 6.35mm (0.9" x 0.25")
• Lead pitch: 2.54mm (0.1")
• Operating temperature range: 0°C to +70°C
• Storage temperature range: -65°C to +150°C
• Moisture sensitivity level (MSL): MSL 1 (Unlimited floor life at <30°C/85% RH)

Functional Description

The LM3915 contains a precision voltage reference, a high-input-impedance buffer amplifier, ten comparators, and a resistor divider network with logarithmic steps. The input signal is compared against the internal reference voltages established by the resistor divider. Each comparator turns on its corresponding output when the input signal exceeds its reference level.

Display Modes

The IC offers two display modes selectable via the MODE pin (Pin 9):

• Dot Mode: Only the LED corresponding to the highest active level is illuminated
• Bar Mode: All LEDs up to and including the highest active level are illuminated

Current Programming

The LED current is set by a single resistor connected between the RLO pin (Pin 7) and ground. The current is approximately 10 times the current through this resistor. This allows precise control over LED brightness while eliminating the need for individual current-limiting resistors for each LED.

Design Considerations

Power Supply Bypassing

A 0.1 µF ceramic capacitor should be placed close to the V+ pin to ensure stable operation and prevent oscillation. For applications with long power supply leads or noisy environments, additional bulk capacitance (1-10 µF) may be required.

Input Signal Conditioning

For AC signal applications, a rectifier and smoothing circuit should be used before the LM3915 input. The input impedance is high (typically >1MΩ), but for high-frequency signals or in noisy environments, a small capacitor (10-100nF) across the input may be beneficial.

Cascading Multiple Devices

For displays requiring more than 10 steps, multiple LM3915s can be cascaded. The internal resistor divider is floating and can be referenced to different voltages in each device, allowing creation of displays with 20, 30, or more steps.

Absolute Maximum Ratings

Stresses beyond those listed may cause permanent damage to the device:

• Supply voltage (V+ to GND): 25V
• Input voltage: ±35V
• Output voltage: 25V
• Output current: 30 mA (per output)
• Total power dissipation: 1.25W (at 25°C)
• Operating temperature range: 0°C to +70°C
• Storage temperature range: -65°C to +150°C
• Lead temperature (soldering, 10 sec): 260°C