SP7121 View Datasheet(PDF) - Exar Corporation
Part Name
Description
MFG CO.
'SP7121' PDF : 10 Pages View PDF
SP7121
2-Channel Adjustable Low Dropout High Side Linear
LED Driver
THEORY OF OPERATION
The SP7121 allows the LED current level to be
adjusted by an external resistor connected to
the ISET pin. This option lets costumer select
the 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:
200
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.
1
Where PWMf is the PWM frequency.
Step 2: Obtain the minimum Enable ON time,
this is 15µs for the SP7121 device.
15μ
%
100
Where ILED = required LED current 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.
ENABLE
The SP7121 driver has 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 tristated or left
floating. There are no predefined internal
states, so leaving this pin open will cause the
part to operate incorrectly.
Example:
For a 2kHz signal, the minimum duty cycle for
the PWM signal is
1
2000
500μ
15μ
%
100 3%
500μ
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.
© 2010 Exar Corporation
7/10
Rev. 2.0.0
Share Link: 
