Introduction

Incandescent light bulbs, described as "heaters that give off a little light" ^[1], are the target of governments worldwide due to their inherent inefficiency. Higher wattage lamps may be doomed by evolving efficiency standards, but their miniature cousins might still have a bright future in industrial settings such as 24V programmable logic controller (PLC) systems.

Turning on a 24VDC incandescent lamp isn't a trivial job for an IC driver. Common tungsten filaments, when cold, are about 15x more conductive than when glowing hot. Consequently, while lighting the bulb the driver must cope with a near short circuit condition without overheating.

The LTC2874 can safely operate up to eight lamps. Here's how.

 

 

Not Just for IO-Link

The LTC2874 is a Quad IO-Link Master power and signaling interface to devices connected by cables up to 20m long. Operating from 8V to 30V, each CQ driver output sources or sinks 100mA. That's enough to light the miniature 1W bulbs (40mA typ.) or 2W bulbs (80 or 85mA typ.) sometimes used in 24VDC systems.

Each of the four CQ outputs can drive a lamp, while additionally supporting IO-Link when connected to a smart device. The four L+ Hot Swap power supply outputs can be put to work too, each sourcing current up to a limit set by a sense resistor. That means that one LTC2874 can operate eight incandescent lamps!

Lighting a 28V 1W Bulb using a CQ Driver

The LTC2874 CQ driver outputs (like their L+ power supply output counterparts) are protected by overcurrent circuit breakers and low-duty cycle auto-retry circuits that guard against overheating in the event of fault conditions or heavy loads. As shown below, each CQ pin easily lights a 1W bulb using these built-in functions. While the filament draws high inrush current as it heats up, the driver pulses on and off. Even at <1% duty cycle, the lamp turns on after only a few pulses.

 

 

Relevant SPI register bit settings:

 

 

Lighting a 24V 2W Bulb using a CQ Driver

Larger lamps, having even smaller cold filament resistance, don't heat up when pulsed at the low duty cycle defined by LTC2874 auto-retry function. However, the LTC2874 can light them if a microcontroller (via the SPI interface) defines a faster pulse-off interval during turn on.

 

 

Here's how this works. The CQ driver turns on and current-limits to 160mA TYP. After 0.48ms in the overcurrent condition, the /IRQ pin signals and the driver disables. Responding to the interrupt request, the microcontroller waits for a cooling interval TOFF and then clears register 0 × 4, which re-enables the driver to begin a new pulse. This repeating cycle pulses the output at a duty cycle of 0.48ms/(0.48ms + TOFF).

To avoid excessive heating of the IC, two precautions are prudent:

1. Avoid using a duty cycle higher than 5 - 10%.
2. Limit the total duration of the pulsing, perhaps to 1 second. If the /IRQ pin has stopped signaling by then, the bulb was successfully lit. If the /IRQ pin still signals, we must guard against the possibility of a hard short. It's time to keep the driver off and declare a fault condition.

Relevant SPI register bit settings:

 

 

Combining CQ Pins

To increase the guaranteed current capability, dot the CQ pins in any combination. Just be sure to also combine their respective driver inputs (TXD1-4 pins) and controls (TXEN1-4 pins or DVREN1-4 register bits).

 

 

The following example curve for a 2W bulb illustrates how a faster pulsing interval and dotted outputs both speed up the turn-on time.

 

 

Using this approach, three or four dotted CQ pins can handle even larger lamps.

Using L+ Hot Swap Outputs

The L+ power supply outputs can operate lamps in a similar manner, either using auto-retry mode or microcontroller-timed pulsing. In this application, these outputs add flexibility (current limits are set by resistors, and pulse ON- and OFF-times are both programmable) and a constraint (operation is via the SPI interface only).

For these outputs, it's the external Hot Swap MOSFET that must not be overheated. When defining the duty cycle, keep in mind the safe operating area (SOA) of this device.

Relevant SPI register bit settings:

 

 

Another Device for Lamp Applications

The LT3669/LT3669-2 can turn on incandescent lamps too. These dual-driver parts have a built-in pulsing scheme that automatically manages the duty cycle when the output current limit is exceeded.

Conclusion

The LTC2874 can safely operate miniature incandescent lamps. For 24VDC applications, the CQ driver pins can handle 1W bulbs using the built-in auto-retry pulsing function, 2W bulbs using a microcontroller to manage duty cycle, and somewhat larger sizes when outputs are combined. The L+ Hot Swap outputs can turn on incandescent bulbs too, making possible a microcontroller-operated octal lamp driver.