LDS8681

© 2009 IXYS Corp.

5

Doc. No. 8681_DS, Rev. N1.0

Characteristics subject to change without notice

BLOCK DIAGRAM

Figure 2. LDS8681 Functional Block Diagram

BASIC OPERATION

LDS8681 is a charge pump designed for converting

at Li-Ion battery voltage to drive a white LED used in

photo flash applications. The LEDs current is factory

programmed at 400 mA for both channels. Consult

factory about other current values. Every LED may

be factory programmed separately in the range from

25 to 800 mA in 25 mA steps for LEDA and in the

range from 25 to 400 mA for LEDB.

voltage until logic HIGH level is applied to EN pin.

The device requires approximately 100 µs to wake

up.

Device starts operating with soft-start at 1-x mode to

minimize inrush current

(less any internal voltage losses). If the output

voltage is sufficient to regulate all LED currents, the

device remains in 1-x operating mode.

The charge pump has two modes of operation: 1-X,

and 2-x. The device changes modes if the output

voltage drops below the minimum needed to support

. The low dropout PowerLite

™ Current regulator

(PCR) performs well at input voltages up to 150 mV

driver’s efficiency. The LDS8681 monitors voltage

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

this voltage falls below 150 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.

is a Charge Pump Output Resistance at given mode,

pump’ multiplication ratio.

If the input voltage is insufficient or falls to a level

where Vd

≤150 mV, and the regulated currents

cannot be maintained, the low dropout PowerLite

™

Current Regulator switches the charge pump into 2-x

mode (after a fixed delay time of about 800

μs). In 2-x

mode,

the

charge

pump’

output

voltage

is

approximately equal to 2 times the input supply

voltage (less any internal voltage losses).

LED Current Setting

The current in each of the two LED channels is

factory programmed; however, average current value

may be decreased using PWM pulses. Check LED

Data sheet to avoid LED thermal damage at high

current.