SP7121 Datasheet, PDF (4/11 Pages) Sipex Corporation – 2-Channel Adjustable Low Dropout High Side Linear LED Driver

The SP7121 allows the LED current level to
be adjusted by an external resistor
connected to the ISET pin. This option lets
costumer select LED current amplitude by
selecting appropriate external RSET resistor
value. The SP7121 circuit consists of
enable, bandgap reference, and current
amplifier circuit blocks. The Enable circuit
block provides the enable and PWM
function for the IC. The Bandgap reference
provides a stable voltage source from which
the output current is derived.
Setting the Current in the SP7121
The internal current amplifier provides a
current gain function where the RSET
resistor is connected from the ISET pin (pin
4) to GND. The current, produced by the
RSET resistor is then amplified and delivered
to the LED output. The external RSET
resistor value for SP7121 is determined by
following equation:
RSET = 200 x VSET/ILED
Where ILED = required LED current value in
mA per channel.
VSET = 1.22V
200 is the typical gain
The LED current level can be set anywhere
from 10mA to 25mA per each output.
Specifications are not guaranteed for
current levels that are set outside this
range.
The SP7121 doesn’t have protection from
ISET pin being shorted to ground or the RSET
value being too low. With an operational
maximum current of 30mA per channel
device may be damaged if the ISET pin is
shorted to the ground or the RSET value is
below 8kΩ.
APPLICATION INFORMATION
ENABLE
The SP712X drivers have a low current
shutdown function. In shutdown mode, the
part draws less than 1µA current maximum.
The part can be shut down using the EN pin
(EN = LOW). The enable pin cannot be tri-
stated or left floating. There are no
predefined internal states, so leaving this
pin open will cause the part to operate
incorrectly.
PWM Dimming
The SP7121 driver allows LED-dimming
control by applying a PWM signal to the EN
pin. The acceptable frequency range of this
signal is 100Hz to 20 kHz. The minimum
ON time that is required for the ISET pin to
set is 15µs from the time the Enable pin
goes high. Thus, for a 2kHz PWM signal,
the acceptable duty cycle range is 3% to
100%. To find the minimum PWM duty cycle
the following steps need to be followed.
Step 1: Determine the time period of the
PWM frequency.
Where PWMf is the PWM frequency
Step 2: Obtain the minimum Enable ON
time; this is 15µS for the SP7121 part.
Sept 11 - 07 Rev E
SP7121 Low Dropout LED Driver
Page 4
© 2007 Sipex Corporation