Datasheet

R =
ISET
K V´
ISET
I
LED
TPS7510x
www.ti.com SBVS080I –SEPTEMBER 2006–REVISED NOVEMBER 2013
APPLICATIONS INFORMATION
LIMITATIONS ON LED FORWARD
VOLTAGES
SETTING THE OUTPUT CURRENT LEVEL
The TPS7510x is a linear current source
The TPS7510x is a quad matched current source.
implementing LDO regulator building blocks.
Each of the four current source output levels is set by
Therefore, there are some limitations to the forward
a single reference current. An internal voltage
(output) voltages that can be used while maintaining
reference of 1.225V (nominal) in combination with a
accurate operation. The first limitation is the
resistor sets the reference current level. This
maximum LED forward voltage. Because LDO
reference current is then mirrored onto each of the
technology is employed, there is the dropout voltage
four outputs with a ratio of typically 420:1. The
to consider. The TPS7510x is an ultra-low dropout
resistor required to set the LED current is calculated
device with typical dropouts in the range of 30mV at
using Equation 1:
5mA. Care must be taken in the design to ensure that
the difference between the lowest possible input
voltage (for example, battery cut-off) and the highest
possible forward voltage yields at least 100mV of
where:
headroom. Headroom levels less than dropout
• K is the current ratio
decrease the accuracy of the current source (see
• V
ISET
is the internal reference voltage
Figure 6).
• I
LED
is the desired LED current (1)
The other limitation to consider is the minimum output
For example, to set the LED current level to 10mA, a
voltage required to yield accurate operation. The
resistor value of 51.1kΩ is required. This value sets
current source employs NMOS MOSFETs, and a
up a reference current of 23.9μA (1.22V/51.1kΩ). In
minimum forward LED voltage of approximately 1.5V
turn, this reference current is mirrored to each output
on the output is required to maintain highest
current source, resulting in an output current of 10mA
accuracy. The TPS7510x is ideal for white LEDs and
(23.9μA × 420).
color LEDs with forward voltages greater than 1.5V.
This range includes red LEDs that have typical
The TPS7510x offers two methods for setting the
forward voltages of 1.7V.
output current levels. The LED current is set either by
connecting a resistor (calculated using Equation 1)
USE OF EXTERNAL CAPACITORS
from the I
SET
pin to GND, or leaving I
SET
unconnected
to employ the factory-programmed R
SET
resistance.
The TPS7510x does not require the use of any
The internal programmed resistance is implemented
external capacitors for stable operation. Nominal
using high-precision processing and yields a
stray and/or power-supply decoupling capacitance on
reference current accuracy of 0.5%, nominal.
the input is adequate for stable operation. Capacitors
Accuracy using external resistors is subject to the
are not recommended on the outputs because they
tolerance of the external resistor and the accuracy of
are not needed for stability.
the internal reference voltage.
USE OF UNUSED OUTPUTS OR TYING
The TPS7510x automatically detects the presence of
OUTPUTS TOGETHER
an external resistor by monitoring the current out of
the I
SET
pin. Current levels in excess of 3 μA signify
Unused outputs may be left unconnected or tied to
the presence of an external resistor and the device
the V
IN
supply. While open outputs are acceptable,
uses the external resistor to set the reference current.
tying unused outputs to the V
IN
supply increases ESD
If the current from I
SET
is less than 3μA, the device
protection. Connecting unused output to ground
defaults to the preset internal reference set resistor.
violates the minimum recommended output voltage,
The TPS7510x is available with eight preset current
results in current levels that potentially exceed the
levels, from 3mA to 10mA (per output) in 1mA
set/preset LED current and should be avoided.
increments. Solutions using the preset internal
Connecting outputs in parallel is an acceptable way
current level eliminate an external component,
of increasing the amount of LED current drive. This
thereby increasing accuracy and reducing cost.
configuration is a useful trick when the higher current
level is a multiple of the preset value.
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