LM3915N - Dot/Bar Display Driver

Overview

The LM3915N is a monolithic integrated circuit designed to drive ten LEDs, LCDs, or vacuum fluorescent displays in response to analog voltage levels. This versatile IC provides a logarithmic 3 dB/step display, making it particularly suitable for audio level meters and other applications requiring logarithmic scaling. The device features a single-pin control to switch between bar graph and moving dot display modes, offering flexibility in display configuration.

With built-in regulated and programmable LED current drive, the LM3915N eliminates the need for external current-limiting resistors, simplifying circuit design. The entire display system can operate from a single power supply, making it an efficient solution for various display applications.

Key Features

• Logarithmic 3 dB/step display with 30 dB range
• Drives 10 LEDs, LCDs, or vacuum fluorescent displays
• Selectable bar graph or moving dot display mode
• Programmable LED current (no external resistors required)
• Single supply operation
• Internal voltage reference: 1.25V to 12V
• Input operates down to ground
• Outputs are current regulated, open collectors
• Can drive LEDs of different colors
• Expandable to displays of 100 steps

Applications

The LM3915N is ideal for a wide range of applications requiring visual voltage level indication:

• Audio level meters (VU meters)
• Power level indicators
• Signal strength meters
• Expanded scale voltmeters
• Battery voltage monitors
• Analog sensor displays
• Industrial process control indicators
• Test equipment displays

Electrical Characteristics

Absolute Maximum Ratings

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

Typical Operating Characteristics (V+ = 12V, TA = 25°C unless otherwise noted)

• Supply Current (no load): 2.5mA to 6mA
• Reference Voltage: 1.28V (typical)
• Reference Voltage Adjust Range: 1.25V to 12V
• LED Current Range: 1mA to 30mA
• LED Current Matching: ±0.5% (typical)
• Voltage Between Steps: 3dB (0.3V typical at 1V reference)
• Input Bias Current: 25nA (typical)
• Input Voltage Range: 0V to V+ -1.5V

Functional Description

The LM3915N contains ten comparators with precision resistor dividers that establish the logarithmic voltage thresholds for each display element. The input signal is compared against these thresholds, illuminating the appropriate number of LEDs to represent the input level.

The device features a mode pin (pin 9) that selects between bar graph mode (all LEDs up to the signal level are illuminated) or dot mode (only the LED corresponding to the signal level is illuminated). The logarithmic scaling makes each step represent a 3dB change in signal level, which corresponds to a doubling/halving of power.

The LED brightness can be controlled through a single resistor that sets the current for all LEDs. The internal current regulation ensures uniform brightness across all LEDs regardless of supply voltage variations.

Package Information

18-Pin PDIP (N Package)

• Package Type: Plastic Dual In-Line Package (PDIP)
• Package Code: N
• Number of Pins: 18
• Body Width: 7.62mm (300 mil)
• Body Length: 22.86mm (900 mil)
• Lead Pitch: 2.54mm (100 mil)
• Mounting: Through-hole
• Operating Temperature: 0°C to +70°C

Typical Application Circuit

A basic application circuit for the LM3915N includes:

• Power supply connection (V+ to pin 3, GND to pin 2)
• Input signal connection to pin 5
• LED array connected to pins 1 and 10-18
• Current setting resistor between pin 7 and V+
• Reference voltage divider (optional) between pin 8, pin 4, and ground
• Mode selection (pin 9 connected to V+ for dot mode or left open for bar mode)

Design Considerations

When designing with the LM3915N, consider the following:

• The input signal should be within the specified voltage range (0V to V+ -1.5V)
• For audio applications, proper signal conditioning may be required before the LM3915N
• The reference voltage can be adjusted to scale the display range
• Multiple LM3915N devices can be cascaded for displays with more than 10 steps
• Ensure proper heat dissipation when driving LEDs at maximum current
• Use decoupling capacitors near the power supply pins for stable operation

Conclusion

The LM3915N provides a complete, efficient solution for driving logarithmic displays with minimal external components. Its flexible design allows for both bar graph and dot display modes, while the programmable current drive ensures consistent LED brightness. With its wide operating voltage range and logarithmic scaling, the LM3915N is particularly well-suited for audio level meters and other applications requiring logarithmic response displays.