5
Feb 9-07 Rev E Constant Current LED Driver © 2007 Sipex Corporation
THEORY OF OPERATION
Introduction
The SP7619 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-current converter, 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 dened by internal
circuitry. The input to this circuitry is the CTRL
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
Sleep Mode sets the device into Torch Mode.
The LED output current will reach its nal
value 50µs after the CTRL pin transition from
LOW to HIGH or from HIGH to LOW.
Output Driver
The SP7619 features a low-side regulated
output current driver. A large MOSFET device
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
the package. Even though there are two LED
and GNDP pins, there is actually only one
output intended to drive one LED.
The light emitting diode connects from VCC
to LED pins. The Iled current regulation is
guaranteed at dropout at the LED pins to be
less than 100mV. The regulation attempts to
maintain the output current to within ve 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 dened current.
Thermal Protection
The SP7619 has built-in thermal protec-
tion. An internal p-n junction is compared
to the internal temperature-compensated
reference. When the p-n junction reaches a
temperature specic 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 dened to be
160°C typical.
The comparator circuit has built-in hysteresis
corresponding to 25
°
C. The output stage will
resume operation and start sinking the cur-
rent that was dened prior to shutdown when
the internal temperature drops by 25
°
C. If the
part remains in a high ambient temperature
environment or if a high power dissipation
condition exists, the output stage will cycle on
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.
