SP7121 Datasheet, PDF (5/11 Pages) Sipex Corporation – 2-Channel Adjustable Low Dropout High Side Linear LED Driver

Example
For a 2kHz signal, the minimum duty cycle
for the PWM signal is
%Minimum Duty cycle =3%
Fault Operation
If one of the LEDs is shorted, the LED
voltage at this pin will be VCC but that
channel will provide the nominal current
value, thereby increasing power dissipation.
If all channels are shorted, excessive power
dissipation may damage the device. If an
LED is open, the LED pin voltage will be
pulled up to Vcc, and the LED current will
be reduced to 3mA for the other channels.
The SP7121 doesn’t have protection from
the ISET pin being shorted to ground or the
RSET value being too low. With an
operational maximum current of 30mA per
channel, the device may be damaged if the
ISET pin is shorted to the ground or the RSET
value is below 8kΩ.
Although the SP7121 does not require an
input capacitor for normal operation, a 1nF
capacitor can be used to obtain better
overall circuit performance. To obtain the
best performance from the SP7121, a
printed circuit board with ground plane is
required. High quality, low series resistance
APPLICATION INFORMATION
Thermal Considerations
The SP7121 does not have
overtemperature protection. Special care
needs to be taken to ensure that the driver’s
junction temperature stays below 125˚C.
The thermal resistance of the package is
160˚C per Watt. Under normal operation
conditions with about 30mA of current per
LED and a voltage drop across the part of
1V, the part will still only dissipate .06 Watts
-- giving a temperature rise of only 11.4˚C
Example of thermal calculations.
Where P is total power dissipation
Vd is the voltage drop across the SP7121
IOUT is the total output current
Where P is the total power dissipation
ΘJA is the thermal resistance of the package
per Watt.
Above example
Temperature Rise = 11.4˚C
The part will be able will operate up to 85˚C
under these conditions.
BOARD LAYOUT AND GROUNDING
ceramic 1nF bypass capacitors should be
used at the Vcc and GND pins (pins 1 and
2). This capacitor must be located as close
to the pins as possible. The traces
connecting the pins and the capacitor must
be kept short and should be made as wide
as possible.
Sept 11 - 07 Rev E
SP7121 Low Dropout LED Driver
Page 5
© 2007 Sipex Corporation