5

Feb 9-07 Rev E

Constant Current LED Driver

© 2007 Sipex Corporation

THEORY OF OPERATION

Introduction

The SP769 is a linear low-side constant

-current driver designed to drive high power

LEDs from an input voltage rail. The design

consists of a regulator reference voltage

source,voltage-to-currentconverter,thermal

shutdown, timeout shutdown, and output

driver. The precision reference voltage en-

sures good performance over voltage and

temperature. The built-in thermal protection

automatically stops LED current to prevent

overheating. A timeout function serves as a

failsafe, shutting down the output at Flash

Mode after 4 seconds of continuous opera-

tion if the enable input is not cycled.

Current Setting

The LED output current is defined by internal

circuitry.TheinputtothiscircuitryistheCTRL

pin. LOW logic level determines Torch Mode

while HIGH logic level determines Flash

Mode with appropriate output currents. The

ENABLE pin and CTRL pin logic HIGH level

may be applied with Vcc voltage. At Flash

Mode the maximum Time Duration is 4s.

After that, the device is automatically sent

into Sleep Mode. This function is intended

to protect the LED if, for example, the device

was left on in a high current condition for a

long period of time.

To return the device to the Flash Mode use

Shutdown (EN = HIGH to LOW/LOW to

HIGH). Switching the CTRL pin LOW during

SleepModesetsthedeviceintoTorchMode.

The LED output current will reach its final

value 50µs after the CTRLpin transition from

LOW to HIGH or from HIGH to LOW.

Output Driver

The SP769 features a low-side regulated

outputcurrentdriver. AlargeMOSFETdevice

is connected between the LED and GNDP

pins. The MOSFET drain is connected to

the LED pins and the MOSFET source is

connected to the GNDP pins. There are

two pins on each terminal of the MOSFET

in order to minimize power dissipation inside

thepackage.EventhoughtherearetwoLED

and GNDP pins, there is actually only one

output intended to drive one LED.

The light emitting diode connects from VCC

guaranteed at dropout at the LED pins to be

less than 00mV. The regulation attempts to

maintain the output current to within five per-

cent of nominal even if the voltage between

the LED and GNDP pins varies over a wide

range. The nominal output is guaranteed

within ±10% of the defined current.

Thermal Protection

The SP769 has built-in thermal protec-

to the internal temperature-compensated

temperature specific voltage it trips a com-

parator which shuts down the driver stage of

the part, placing the device into sleep mode

while current setting circuitry remains active.

The shutdown temperature is defined to be

60°C typical.

Thecomparatorcircuithasbuilt-inhysteresis

resume operation and start sinking the cur-

rent that was defined prior to shutdown when

part remains in a high ambient temperature

environment or if a high power dissipation

conditionexists,theoutputstagewillcycleon

and off to maintain a preset internal junction

temperature to avoid catastrophic damage.

In that case, the LED will appear to be blink-

ing. Lower the LED voltage to avoid excess

power dissipation in the IC.