L DESIGN IDEAS White LED Driver with Output Disconnect and 1-Wire by Ahmed Hashim 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 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: 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. 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). ILED = 20mA – (N – 1) • 625µA Buck-Boost Mode 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 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 1-Wire Current Programming 10µH 90 1µF VIN SW CAP LT3593 SHUTDOWN AND CURRENT CONTROL VIN = 3.6V 10 LEDs 1µF CTRL LED GND 80 EFFI CIENCY (%) VIN 3V TO 5V The LT3593 is available in a 6-lead (2mm × 2mm) DFN as well as 6-lead SOT-23. C2 1µF 70 VIN 3V TO 5V 60 L1 10µH C1 1µF VIN D1 SW CAP 50 LT3593 40 C1: TAIYO YUDEN EMK107BJ105MA C2: MURATA GRM31MR71H105KA88 L1: MURATA LQH43CN100 D1: CENTRAL CMDSH05-4 30 SHUTDOWN AND CURRENT CONTROL 0 5 10 15 LED CURRENT (mA) 128µs GND 20 C1: TAIYO YUDEN EMK107BJ105MA C2: TAIYO YUDEN GMK316BJ105ML L1: MURATA LQH43CN100 D1: CENTRAL CMDSH05-4 Figure 1. LED driver for ten white LEDs 128µs LED CTRL Figure 3. LED driver in Buck-Boost mode driving one LED 128µs CTRL 20mA LED CURRENT 20mA 17.5mA 17.5mA SHDN Figure 2. Current programming and shutdown timing 36 Linear Technology Magazine • June 2008 DESIGN IDEAS L shows an application where a single LED is driven from a 5V supply. 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. LTC3100, continued from page 32 power while maximizing battery life. The diode on the USB input prevents any reverse current from the 3.3V output (while operating on batteries) back to the USB input when it is open or grounded. Figure 2 shows the converter efficiency versus load with various input sources, illustrating the high efficiency over a wide load range. The LDO in the LTC3100 (with its input internally tied to the Boost output) provides a second regulated output, in this case programmed to 1.8V. 3.3µH VBATT 0.9V TO 1.5V + 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 Because the buck converter input can come from the boost output, the LTC3100 can function as an ultra-low voltage buck-boost converter, providing a regulated 1.2V output from a single alkaline or NiMH cell. This is shown in Figure 3, where the LTC3100 generates two regulated outputs from a single cell input (whose voltage may be above or below 1.2V) by boosting VIN up to 3.5V and then regulating down to 1.2V and 3.3V using the buck and the LDO. In this example, the Power Good outputs and the LDO are used 3.5V SWBST VINBK CIN 2.2µF A waveform showing the LT3593’s response to an open LED fault can be seen in Figure 4. 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. VINBST 1M VBST 10µF FBBST 523k LTC3100 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. L IL 200mA/DIV VCAP 20V/DIV VSW 20V/DIV VIN = 3.6V 400µs/DIV FIGURE 1 LEDs DISCONNECTED APPLICATION CIRCUIT AT THIS INSTANT Figure 4. Open LED fault protection to provide voltage sequencing, so that the 1.2V core supply comes up before the 3.3V I/O supply, as shown in the scope photo of Figure 4. The LDO also provides additional noise filtering and ripple rejection for the 3.3V output, guaranteeing a low noise output for sensitive analog circuitry, even when the converters are in Burst Mode operation. Conclusion The LTC3100 is a high efficiency, multichannel converter that can operate from a wide range of voltage sources. Independent input voltages for each converter, Power Good outputs and an LDO make the LTC3100 a small, highly integrated and flexible solution for many demanding applications. L 3.3V AT 50mA V_I/O VLDO 115k 2.2µF FBLDO FF EN_BURST OFF ON 25.5k MODE 3.3µH V_CORE = 1.2V SWBK RUNBST 1M FBBK RUNLDO RUNBK 120mA AT VBATT = 0.9V 220mA AT VBATT = 1.2V GND PGBST PGBK 1M 1M 1M VCORE, 1V/DIV BOOST_GOOD BUCK_GOOD Figure 3. Single-cell dual output converter with voltage sequencing Linear Technology Magazine • June 2008 VBST, 1V/DIV VI/O, 1V/DIV 10µF 1ms/DIV Figure 4. Voltage sequencing of the output voltages for the circuit of Figure 3 37