SP6685ER-L/TR 데이터 시트보기 (PDF) - Exar Corporation

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SP6685ER-L/TR

700mA Buck/Boost Charge Pump LED Driver

Exar Corporation 

SP6685

700mA Buck/Boost Charge Pump LED Driver

APPLICATION INFORMATION

The SP6685 can be used with multiple LEDs in

parallel as shown in figure 34. For best

performance, the LEDs should be in a single

package, preferably from a single die to have

better matching for forward voltage Vf for a

given forward current If. In practice, if the Vf

of one LED is higher than the others, it will

consume a larger If, which will raise its

temperature which will then cause its Vf to

reduce, correcting the imbalance. The overall

current will be the sum of the individual

currents, for example Itotal = 4*ILED.

Fig. 34: Multiple LED Flash Circuit

THEORY OF OPERATION

The SP6685 is a charge pump regulator

designed for converting a Li-Ion battery

voltage of 2.7V to 4.2V to drive a white LED

used in digital still camera Flash and Torch

applications. The SP6685 has two modes of

operation which are pin selectable for either

Flash or Torch. Flash mode is usually used

with a pulse of about 200 to 300 millisecond to

generate a high intensity Flash. Torch can be

used continuously at a lower output current

than Flash and is often used for several

seconds in a digital still camera “movie” mode.

The SP6685 also has two modes of operation

to control the output current: the 1X mode

and 2X mode. Operation begins after the

enable pin EN receives a logic high, the

bandgap reference wakes up after about

200µsec, and then SP6685 goes through a

soft-start mode designed to reduce inrush

current. The SP6685 starts in the 1X mode,

which acts like a linear regulator to control the

output current by continuously monitoring the

feedback pin FB. In 1X mode, if the SP6685

auto detects a dropout condition, which is

when the FB pin is below the regulation point

for more than 32 cycles of the internal clock,

the SP6685 automatically switches to the 2X

mode. The SP6685 remains in the 2X mode

until one of four things happens: 1) the

enable pin EN has been toggled, 2) the Flash

pin has changed from High to Low, 3) VIN is

cycled. 4) a thermal fault occurs.

The 2X mode is the charge pump mode where

the output can be pumped as high as two

times the input voltage, provided the output

does not exceed the maximum voltage for the

SP6685, which is internally limited to about

5.5V. In the 2X mode, as in the 1X mode, the

output current is regulated by the voltage at

the FB pin.

© 2009 Exar Corporation

10/14

Rev. 2.0.0

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