White LED Driver with Output Disconnect and 1-Wire Current Programming

Introduction

The LT3593 is designed to drive up to ten white LEDs in series from a single lithium-ion cell, ensuring matched LED current and eliminating the need for ballast resistors. The LT3593 internally compensated step-up DC/DC converter switches at 1MHz so that it can be used with tiny, low profile external components.

The LT3593 features an internal 5-bit DAC, allowing the LED current to be programmed using only one pin. The device features true output disconnect in shutdown as well as a unique high side current sense allowing it to function as a “1-wire” current source where the low side of the LEDs can be returned to ground anywhere. A typical application schematic along with expected efficiency can be seen in Figure 1. The functionality and feature set available in the LT3593 make it ideal for portable electronics display back-lighting applications. The LT3593 is available in a 6-lead (2mm × 2mm) DFN as well as 6-lead SOT-23.

Figure 1. LED driver for ten white LEDs.

1-Wire Current Programming

The LED current can be programmed linearly to 32 unique values by strobing the CTRL pin. A 5-bit internal counter is decremented on each rising edge on the CTRL pin, reducing the programmed current by 625µA from the full-scale current of 20mA with each step. The programmed LED current can be calculated using the following equation:

ILED = 20mA – (N – 1) • 625µA

where N is the number of rising edges on the CTRL pin. Strobing the CTRL pin more than 32 times results in the minimum current of 625µA. The CTRL pin must stay high after the last CTRL strobe and 128µs later the part will turn on and start to regulate the programmed current. To shut down the part, the CTRL pin is held low; 128µs after the falling edge of CTRL, the part shuts down. Figure 2 shows current programming and shutdown timing.

Figure 2. Current programming and shutdown timing.

If the LED current needs to be reprogrammed, there is no need to shutdown and then reprogram. The LT3593 can be reprogrammed from one LED current to another by simply strobing the CTRL pin and, 128µs after the last rising edge, the part starts regulating the newly programmed current (also shown in Figure 2).

Buck-Boost Mode

If a low number of LEDs is needed, there could be a case where the required output voltage is lower than the input voltage. In this case, the LT3593 can be used in buck-boost mode where the LED string is returned to the input supply instead of ground. Figure 3 shows an application where a single LED is driven from a 5V supply.

Figure 3. LED driver in Buck-Boost mode driving one LED.

Output Disconnect

The LT3593 has an internal disconnect switch that is used to sense the LED current during normal operation. This internal switch also serves to provide output disconnect during shutdown so that the LEDs are truly disconnected from the output of the regulator.

Fault Protection

The LT3593 protects against both open and shorted LED faults. In the case of an open LED fault, the output voltage VCAP continues to rise. Once VCAP reaches 38V, an open fault is triggered and the part goes into a low frequency mode clamping the output to 38V and minimizing input current. A waveform showing the LT3593’s response to an open LED fault can be seen in Figure 4.

Figure 4. Open LED fault protection.

In a shorted LED fault, the LED pin can be shorted to ground, running excessive current from VCAP. To protect from such a fault, the LT3593 limits the maximum current out of the LED pin to approximately 45mA.

Conclusion

The LT3593 is a step-up LED driver that can drive up to ten white LEDs from a single lithium-ion cell. It can easily be programmed through a single pin interface and combines many desirable features as well as fault protection against open or shorted LEDs.

The feature-rich LT3593 is available in the 6-lead (2mm × 2mm) DFN as well as the 6-lead SOT-23. These two small, low profile packages, together with internal compensation and an output disconnect device, are ideal for a complete small board area LED driver solution, especially in portable device display backlighting applications.

Author

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Ahmed Hashim