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CAT4008Y-GT2 Datasheet(PDF) 9 Page - ON Semiconductor |
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CAT4008Y-GT2 Datasheet(HTML) 9 Page - ON Semiconductor |
9 / 13 page CAT4008 http://onsemi.com 9 Block Diagram Figure 17. CAT4008 Functional Block Diagram Current Setting + CURRENT SINKS LATCHES SHIFT REGISTER 1.2 V Ref GND RSET BLANK VIN VDD LED1 LED2 LED3 LED8 CLK SIN LATCH L1 L2 L3 L8 S1 S2 S3 S8 SOUT Basic Operation The CAT4008 uses 8 tightly matched current sinks to accurately regulate the LED current in each channel. The external resistor, RSET, is used to set the LED channel current to about 51 times the current in RSET. LED current ^ 51 1.2 RSET Tight current regulation for all channels is possible over a wide range of input and LED voltages due to independent current sensing circuitry on each channel. The LED channels have a maximum dropout of 0.4 V for most current and supply voltage conditions. This helps improve the heat dissipation and efficiency of the LED driver. Upon power−up, an under−voltage lockout circuit clears all latches and shift registers and sets all outputs to off. Once the under−voltage lockout threshold has been reached the device can be programmed. The driver delays the activation of each consecutive LED output channel by 17 ns (typical). Relative to LED1, LED2 is delayed by 17 ns, LED3 by 34 ns and LED8 by 120 ns typical. The delay is introduced when LATCH is activated. The delay minimizes the inrush current on the LED supply by staggering the turn on and off current spikes over a period of time and therefore allowing usage of smaller bypass capacitors. Pull−up and pull−down resistors are internally provided to set the state of the BLANK and LATCH pins to the off−state when not externally driven. Serial Interface A high−speed serial 4−wire interface is provided to program the state of each LED on or off. The interface contains an 8−bit serial to parallel shift register (S1−S8) and an 8−bit latch (L1−L8). Programming the serial to parallel register is accomplished via SIN and CLK input pins. On each rising edge of the CLK signal, the data from SIN is moved through the shift register serially. Data is also moved out of SOUT which can be connected to a next device if programming more than one device on the same interface. On the rising edge of LATCH, the data contents of the serial to parallel shift register is reflected in the latches. On the falling edge of LATCH, the state of the serial to parallel register at that particular time is saved in the latches and does not change irrespective of the contents of the serial to parallel register. BLANK is used to disable all LEDs (turn off) simultaneously while maintaining the same data in the latch register. When low, the LED outputs reflect the data in the latches. When high, all outputs are high impedance (zero current). |
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