LINEAR LTC3593 User's Manual

L DESIGN IDEAS

White LED Driver with Output Disconnect and 1-Wire

Current Programming by Ahmed Hashim

Introduction The LT3593 is designed to drive up to The LT3593 is available in a 6-lead programmed current. To shut down ten white LEDs in series from a single (2mm × 2mm) DFN as well as 6-lead the part, the CTRL pin is held low; lithium-ion cell, ensuring matched SOT-23. 128µs after the falling edge of CTRL, LED current and eliminating the need the part shuts down. Figure 2 shows

for ballast resistors. The LT3593 in- 1-Wire Current Programming current programming and shutdown

ternally compensated step-up DC/DC The LED current can be programmed timing. converter switches at 1MHz so that linearly to 32 unique values by strobing If the LED current needs to be

EF FI IE NC

it can be used with tiny, low profile the CTRL pin. A 5-bit internal counter reprogrammed, there is no need to external components. is decremented on each rising edge shutdown and then reprogram. The The LT3593 features an internal on the CTRL pin, reducing the pro- LT3593 can be reprogrammed from 5-bit DAC, allowing the LED current grammed current by 625µA from the one LED current to another by simply to be programmed using only one full-scale current of 20m A with each strobing the CTRL pin and, 128µs after pin. The device features true output step. The programmed LED current the last rising edge, the part starts disconnect in shutdown as well as a can be calculated using the following regulating the newly programmed unique high side current sense allow- equation: current (also shown in Figure 2). ing it to function as a “1-wire” current

ILED = 20m A – (N – 1) • 625µA

source where the low side of the LEDs Buck-Boost Mode

can be returned to ground anywhere. where N is the number of rising edges If a low number of LEDs is needed, A typical application schematic along on the CTRL pin. Strobing the CTRL there could be a case where the re- with expected efficiency can be seen pin more than 32 times results in quired output voltage is lower than the in Figure 1. The functionality and the minimum current of 625µA. The input voltage. In this case, the LT3593 feature set available in the LT3593 CTRL pin must stay high after the last can be used in buck-boost mode where make it ideal for portable electronics CTRL strobe and 128µs later the part the LED string is returned to the input display back-lighting applications. will turn on and start to regulate the supply instead of ground. Figure 3

VIN 3V TO 5V VIN = 3.6V C2

10 LEDs 1µF

VIN SW

LT3593

VIN SHUTDOWN AND 3V TO 5V

CTRL LED C1

CURRENT CONTROL

1µF VIN SW

LT3593

SHUTDOWN AND

CTRL LED

CURRENT CONTROL

C1: TAIYO YUDEN EMK107BJ105MA

C2: MURATA GRM31MR71H105KA88 L1: MURATA LQH43CN100 D1: CENTRAL CMDSH05-4 LED CURRENT (m A)

C1: TAIYO YUDEN EMK107BJ105MA C2: TAIYO YUDEN GMK316BJ105ML L1: MURATA LQH43CN100

Figure 1. LED driver for ten white LEDs

D1: CENTRAL CMDSH05-4

Figure 3. LED driver in Buck-Boost

mode driving one LED

CURRENT SHDN

Figure 2. Current programming and shutdown timing

Linear Technology Magazine • June 2008