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CAT4106 Datasheet(PDF) 10 Page - ON Semiconductor |
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CAT4106 Datasheet(HTML) 10 Page - ON Semiconductor |
10 / 15 page CAT4106 http://onsemi.com 10 Device Operation In a typical application, the CAT4106 drives an LED array consisting of up to four separate strings. A single external resistor value is used to select the drive level in all output channels. Each channel provides a very well regulated and tightly matched bias current on all LED strings. An integrated DC/DC boost converter is used to generate the high voltage output required to power the LED strings. Up to a maximum of 6 W of LED power can be delivered when powered from a 12 V supply. To offer improved performance, the device also allows the user to set the operating voltage level on the output drive channels. A pair of external resistors is used to control the nominal channel voltage, during normal operation. The resulting output voltage (LED anodes) will be equal to the sum of the adjusted channel voltage plus the maximum forward voltage present on any string. The CAT4106 provides four tightly matched current sinks to accurately regulate LED current in each channel. Up to 175 mA per channel can be programmed by selecting a suitable value for the external R1 resistor (connected between RSET and GND pins). LED Current Setting The CAT4106 provides four tightly matched current sinks to accurately regulate LED current in each channel. The LED current is set by the external resistor R1 connected between the RSET pin and ground. The following formula gives the relationship between the resistor value and the LED current per channel. R1 [ 103 1.2 V LED current per channel Table 6. RESISTOR R1 AND LED CURRENT LED Current per Channel (mA) R1 ( W) 10 12.4 k 20 6.19 k 30 4.12 k 50 2.49 k 100 1.24 k 150 820 Setting the Channel Voltage Each LED channel typically requires less than 0.5 V of headroom for full load operation. The lowest channel operating voltage can be controlled and programmed via the external resistor network connected between, CTRL, FB and GND as shown in Figure 24. Lowest LED pin voltage + V FB R2 ) R3 R3 For most applications, a gain of 2x (R2 equals R3) is recommended. The CTRL output pin has optimal performance when loaded with around 15 mA, which equates to an R3 resistor value of 20 k W. LED Detection The CAT4106 detects failure conditions related to the LED load, such as if one of the LED strings has been disconnected (Open−LED) or if one LED string has a lower LED pin voltage than expected (Short−LED). Either condition is flagged by turning on the open−drain output on the FAULT pin. The Open−LED detection is active in normal operation. The Short−LED detection is only active during power−up, just after the EN/PWM input goes from low to high. If in normal operation (LEDs are in regulation) a short−LED condition occurs, it will not be detected. Figure 25. Cold Power−Up Short−LED Detection Figure 26. Normal Operation Open−LED Detection The test condition in Figure 25 is with one LED pin shorted to the output (top LED anode). The test condition in Figure 26 is with one LED pin floating or disconnected. In both cases, the CAT4106 is set to drive 50 mA per channel. Setting VFMIN Level (Short−LED) The VFMIN level represents the minimum level expected for the LED string voltage “window of operation”. This voltage setting is based on the number of series LEDs being used and the expected minimum forward voltage VF during normal operation. Example: Consider a string of 6 LEDs in series, with each LED having a VF range of 3.5 V ± 0.5 V. Since the minimum VF of each LED is 3.0 V the overall lowest expected string voltage would be 18 V. Any string voltage of less than 18 V is considered as a short−circuit fault in one or more of the series LEDs. |
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