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TPS7510X Datasheet(PDF) 9 Page - Texas Instruments |
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TPS7510X Datasheet(HTML) 9 Page - Texas Instruments |
9 / 22 page 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 • VISET is the internal reference voltage Figure 6). • ILED 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 51.1k Ω is required. This value sets current source employs NMOS MOSFETs, and a up a reference current of 23.9 μA (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 (23.9 μA × 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 ISET pin to GND, or leaving ISET unconnected to employ the factory-programmed RSET 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 ISET pin. Current levels in excess of 3μA signify Unused outputs may be left unconnected or tied to the presence of an external resistor and the device the VIN supply. While open outputs are acceptable, uses the external resistor to set the reference current. tying unused outputs to the VIN supply increases ESD If the current from ISET 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. Copyright © 2006–2013, Texas Instruments Incorporated Submit Documentation Feedback 9 |
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