LED-driver level-shifter includes fault detection

The relatively low operating voltage of many LED-driver ICs prevents them from driving multiple LEDs in series, but you can circumvent this limitation by adding a common-base-transistor level shifter as shown in Figure 1.

Figure 1: This level-shifting transistor allows the IC to drive series LEDs from a high-voltage supply.
(Click on image to enlarge)

NPN transistor Q₁ mirrors more than 99% of its emitter current through the collector, allowing the circuit to drive dozens of LEDs in series. The optional series-resistor R₁ reduces power dissipation in the driver IC.

An unfortunate side-effect of this level shifter is loss of the fault-detection circuit, whose purpose is to indicate open-circuit conditions. The fault detector senses invalid (low) output voltages, such as <200mV for the MAX6974 24-port driver IC. But, if LEDs in the level-shifter circuit are open-circuited, the IC never sees an output fault. The transistor simply supplies the 20 mA (required by the IC) via its base-emitter diode. Adding a single 150 Ω resistor (R₂ in Figure 2) restores the fault detection.

Figure 2: The addition of R₂ in Figure 1 restores the fault-detection function.
(Click on image to enlarge)

Because Q₁ has a high minimum beta (β) of 100, R₂ drops less than 150 Ω (20 mA/β) = 30 mV.

The oscilloscope traces of Figure 3 and Figure 4 compare the original Figure 1 circuit with the improved version by monitoring test points TP₁ (top trace) and TP₂ (bottom trace). Both signals are 1 V/div, and referenced to ground potential.

Figure 3: As TP₁ in Figure 1 (top trace) decreases, indicating an open in the LED chain, the bottom trace (TP₂) is hardly affected, and therefore produces no fault alarm.
(Click on image to enlarge)

Figure 4: As TP₁ in Figure 2 (top trace) decreases, indicating an open in the LED chain, the bottom trace (TP₂) also decreases, until the IC senses the low voltage that indicates an LED fault.
(Click on image to enlarge)

As an open LED (or a lower supply voltage) causes the top trace to decrease, the transistor eventually saturates, as indicated by a flattening of this trace around 2.7 V. The bottom trace in Figure 3 is hardly affected because the base-emitter diode of Q₁ supplies the 20 mA load current. Fault detection is not active.

In Figure 4, the bottom waveform also decreases as Q₁ hits saturation, thereby activating the fault circuit as the waveform falls below the 200 mV threshold.

The component values in Figure 2, selected for a 20 mA load current, 3.3V supply voltage, and 200 mV fault-detection threshold, allow LED currents in the range 12 mA to 24 mA. You can adjust the values to obtain other current ranges (R₁ = 0 gives the widest operational range).

where:

• V_SUPPLY is the logic power supply voltage,
• V_BE1 (0.7 V) is the typical base-emitter voltage,
• V_SAT is the driver IC saturation voltage (about 1 V for the MAX6974),
• I_MAX is the maximum programmed current range.

where:

• V_BE2 is the worst-case base-emitter voltage (0.5 V at high temperature),
• V_FDT is the fault detection threshold of the driver IC (about 200 mV for the MAX6974),
• I_MIN is the minimum programmed current range.

About the author
John Guy is an applications engineer manager at Maxim Integrated Products, Sunnyvale, CA. He has been with Maxim for 9 years. Prior to joining Maxim, he worked at a now-defunct start up for two years, and twelve years with Precision Monolithics (now part of Analog Devices, Inc). He graduated from San Jose State University in 1992 with a BSEE.