Jul16-07 RevP

SP7686: 500mA LED Driver

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2007 Sipex Corporation

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The SP7686 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 SP7686 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 milliseconds 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 SP7686 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 200µsec, and then SP7686

goes through a softstart mode designed to reduce inrush

current. The SP7686 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 SP7686 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 SP7686

automatically switches to the 2X mode. The SP7686

remains in the 2X mode until one of four things happens:

1) the enable pin EN has been toggled, 2) the Flash pin

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 SP7686, 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.

SP7686A

The fixed current driver (SP7686A) uses the flash pin to

switch between two internally preset voltage levels, for

Flash and Torch mode. The part is enabled and disabled

by the separate ENABLE pin. The part has a 3.5s

maximum on time in Flash mode. After a 3.5s timeout

occurs the part shuts down and can be enabled by the

FLASH or ENABLE pins. This pin cannot be left open.

voltage is set to 160mV. When in Torch mode, (Flash =

GND), the FB regulation voltage is set to 60mV. The

output current is then set in either Flash or Torch mode

SP7686

The digital controlled current driver (SP7686) uses the

CTRL pin as a serial data input to a DAC to set the FB

voltage. The CTRL pin is used to control LED current.

Once enabled the adjustable driver counts positive going

edges at this pin and sequentially increases the FB

voltage from 60mV with 10mV steps to a maximum 31

steps or 360mV. When this pin is held LOW for longer

than 200µs the SP7686 enters shutdown. The part can

be enabled using this pin. The first positive edge resets

the part. A second positive edge sets the part to VFB=0.

A third positive going edge will increase FB voltage to

60mV and subsequent positive edges increment VFB in

10mV steps. The voltage level can increase only and

must be reset to zero by the 200µs shutdown timer in

order to go to a lower current value. The part will be

limited to a 3.5s on time when FB voltage is set to

120mV and above. The CTRL pin cannot be left open.

Flash Timeout Protection

Due to the high currents typically available in Flash

mode, it is necessary to protect the white LED from

damage if left on too long. The SP7686 has a timeout in

Flash mode of approximately 3.5 seconds after which it

will shutdown operation. Operation will not begin again in

Flash mode until the Enable pin has been set Low and

then High again.

Overtemperature Protection

When the temperature of the SP7686 Rises above 140

degrees

Celsius,

the

over

temperature

protection

circuitry turns off the output switches to prevent damage

to the device. If the temperature drops back down below

125 degrees Celsius, the part automatically recovers

and executes a soft start cycle.

Overvoltage Protection

The SP7686 has over voltage protection. If the output

voltage rises above the 5.5V threshold, the over voltage

protection shuts off all of the output switches to prevent

the output voltage from rising further. When the output

decreases below 5.5V, the device resumes normal

operation.

Overcurrent Protection

The overcurrent protection circuitry monitors the average

exceeds approximately 1.5Amp, then the overcurrent

protection circuitry shuts off the output switches to

protect the chip.

THEORY OF OPERATION