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8

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© 2011 Exar Corporation

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Rev. 1.0.0

a logic low to the converter via the SDB signal.

Furthermore, the SDB signal is synchronous to

the PWM signal when PWM dimming is used.

The XRP7618 provides a built-in protection for

open LED channel. Upon detection of an open

LED channel event, the FB signal is driven low

to allow the step-up converter to operate open

loop which in turn raises the voltage on the

remaining

CHx

pins.

Upon

reaching

an

internally

pre-set

threshold

of

3V,

the

XRP7618 identifies the faulty channel and

removes it from the control and reporting

loop. At this point the FB signal resumes its

normal function of reporting the lowest LED

channel voltage and hence restores the proper

operations of the converter. Note that the

open LED channel is not disabled; however, its

CHx voltage is no longer reported. Upon

reconnection of the open LED channel, current

will be driven at the programmed level.

Multiple XRP7618s can be placed in parallel to

drive over 8 strings of LEDs; however, unused

channels should be tied to ground.

While the initial XRP7618 is connected to the

step-up converter via its FB and SDB pins, the

daisy chained XRP7618s are connected to each

other through the respective FBX and SDBX

pins. In this configuration, the initial XRP7618

reports the lowest LED voltage and SDB signal

of all the XRP7618s in parallel. Refer to figure

12.

TYPICAL APPLICATION SCHEMATICS

Fig. 11: Single XRP7618 Application