8

Mar16-06 Rev B

SP7648 Ultra-low Quiescent Current, High Efficiency Boost Regulator

© 2006 Sipex Corporation

current to avoid saturating the inductor,

which would result in a loss in efficiency and

could damage the inductor. The SP7648

=4.2V.Othervaluescouldbeselectedusing

the above relationships.

Using the FLASH Control Pin

The SP7648 will regulate the output by the

equations above depending on the state of

the FLASH pin. When the FLASH pin is

low (<0.4V), the internal reference voltage

is defined as 0.8V. When the FLASH pin is

high (>0.4V), the internal reference voltage

is defined as 0.288V. This allows the use of

smaller values for the sense resistor for

current regulation mode. This improves ef-

ficiency and reduces the physical size of the

sense resistor. An external MOSFET switch

can be used to change the sense resistor

when changing to the Flash Mode.

High Brightness White LED

For the high brightness LumiLED white LED

application, the SP7648 is generally pro-

grammed as a current source. The bias

resistors R1 and R2 are used to set the

operating current of the white LED with the

equation:

THeORY OF OPeRaTiOn

the LED. To set the operating current to be

about 200mA in torch mode, the flash pin is

forced low, R2 is selected as 0.8V/ 0.2 = 4 ,

asshowninthetypicalapplicationcircuit.To

set the operating current to 700mA in flash

mode, the flash pin is forced high, R is

selected as 0.288V/0.41 = 700mA. In real-

ity R in Flash includes the series MOSFET

4 shown by the formula:

R in Flash =

If the SP7648 is powered up before the LED

is plugged in, the circuit will bring the feed-

back pin to ZeroV and the SP7648 has a

feature to set the output voltage to be 5V.

Once the LED is plugged in, the feedback

pinwillgoupto0.8Vintorchmodeor0.288V

in flash mode and begin to regulate. The

When the LED is open, the feedback pin

voltage will go to ZeroV and the output

voltage will go to 5V which will protect the

part from overvoltage at the output.

One approach to control LED brightness is

to apply a PWM signal to the SHDN input of

the SP7648. In this case, the output current

the average duty cycle at the SHDN pin. An

optional 10K potentiometer may also be

usedfordimmingtheLEDcurrentbyvarying

the potentiometer between low brightness

and full brightness.

If the FB pin is pulled below 150mV the

output will default to 5V defined by an inter-

nal resistor divider.