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поискавой системы для электроныых деталей |
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поискавой системы для электроныых деталей |
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SP6887 датащи(PDF) 7 Page - Exar Corporation |
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SP6887 датащи(HTML) 7 Page - Exar Corporation |
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7 / 10 page  ELECTRICAL SPECIFICATIONS Rev. C – 10/31/2007 SP6887 4 Channel WLED Driver © 2007 Exar Corporation 7/10 13. Output Ripple THEORY OF OPERATION Detailed Description As shown in the block diagram on page 2, the main components within the SP6887 include a fractional charge pump, mode selection circuit, output selection logic, LED current setting detection circuit, and 4 current sense circuits. The fractional charge pump multiplies the input voltage a multiple of 1X and 1.5X times the input voltage. The charge pump switches at a fixed 1MHz when the mode is 1.5X. The charge pump does not switch during 1X mode, saving power and improving efficiency. The mode selection circuit automatically selects the mode as 1X or 1.5X based on circuit conditions such as LED voltage, input voltage and load current. 1X is the more efficient mode than 1.5X mode. Table 1 on page 2 shows the output selection logic control over the LED outputs for on and off functions with 8 different output states. The current set and detection circuit uses an external resistor and a 1.20V reference to program the LED current. 4 current regulating circuits sink matched currents from the LEDs. LEDs with matched forward voltage will produce the best possible matched currents. For best matching performance it is recommended that the Vf between LEDs be under 250mV. The unused LED channels can be turned off by CTR0, CTR1 and CTR2, and connecting the respective LED pins to VOUT pin, in which case, the corresponding LED driver sink current is only about 20µA. Methods for Setting LED Current There are 4 methods for setting and adjusting the LED current outlined here. The methods are: 1) RSET only 2) Analog Reference VSET 3) PWM Input at CTR0 4) PWM Input at EN Method 1: LED Current Setting Using the External Resistor RSET The most basic means of setting the LED current is with a resistor connected from RSET to GND, as shown in the application circuit on Page 1. The resistor RSET establishes the reference current needed for a constant LED current. Values of RSET for a fixed LED current are given in Table 2, “RSET |
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