LDS8621

© 2009 IXYS Corp.

4

Doc. No. 8621DS, Rev. N1.0

Characteristics subject to change without notice

BLOCK DIAGRAM

Figure 2. LDS8621 Functional Block Diagram

BASIC OPERATION

At power-up, PWM1 and PWM2 pins should be logic

LOW. During power-up device performs internal

circuit reset that requires less than 10 µs. To start

device, either PWM1 or PWM2 pin should be set

logic HIGH. The LDS8621 starts operating in 1x

mode, at which the output is approximately equal to

the input supply voltage (less any internal voltage

losses). If the output voltage is sufficient to regulate

all LED currents, the device remains in 1x operating

mode.

The low dropout PowerLite

™ Current regulator

(PCR) performs well at input voltages up to 50 mV

above LED forward voltage significantly increasing

driver’s efficiency. The LDS8621 monitors voltage

drop Vd across PCR at every channel in ON state. If

this voltage falls below 50 mV (typical) at any one

channel, (channel with LED with highest forward

voltage), the Mode Control Block changes charge

pump mode to the next multiplication ratio.

Charge Pump Output Resistance at given mode, Iout

is sum of all LED currents, and M is a charge pump’

multiplication ratio.

If the input voltage is insufficient or falls to a level

where Vd

≤50 mV, and the regulated currents cannot

be maintained, the low dropout PowerLite

™ Current

Regulator switches the charge pump into 1.33x mode

(after a fixed delay time of about 800

μs). In 1.33x

mode,

the

charge

pump’

output

voltage

is

approximately equal to 1.33 times the input supply

voltage (less any internal voltage losses).

This sequence repeats at every mode until driver

enters the 2x mode.

If the device detects a sufficient input voltage is

present to drive all LED currents in 1x mode, it will

change automatically back to 1x mode. This only

applies for changing back to the 1x mode. The

difference between the input voltage when exiting 1x

mode and returning to 1x mode is called the 1x mode

transition hysteresis (about 600 mV).

LED Current Setting

The maximum current value in each of the LED is

factory preset. Each LED has independendet current

setting that allows use one of the LEDs for flash,

while other for a torch mode. If both LEDs should be

used in flash mode, or both cannels are connected in