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FAN5608DMPX Datasheet(PDF) 6 Page - Fairchild Semiconductor |
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FAN5608DMPX Datasheet(HTML) 6 Page - Fairchild Semiconductor |
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6 / 17 page  PRODUCT SPECIFICATION FAN5608 6 REV. 1.0.3 6/28/04 Digital Control The FAN5608’s digital decoder allows selection of the fol- lowing modes of operation: OFF, 5mA, 10mA, 20mA per branch. Analog Control Inputs A1 and A2 are used to control the LED currents. Inputs B1 and B2 should be connected to GND (logic level “0”). An external resistor (R) is connected from A1 and/or A2 to a stable voltage source (VExternal) to control the LED current, ILED. The ILED can be determined using the formula and the graph below: Where VRef = 1.22V, VRef < VExternal < (VIN – 0.7V) PWM Control in Digital Mode The logic level HIGH, VH and logic level LOW, VL of the PWM signal should be: (VIN – 0.7V) < VH < VIN and 0 < VL < 0.6V The frequency of the PWM signal should be within 50Hz to 1kHz range; it can go up to 30kHz at any input if the other input is kept HIGH. In the case of FAN5608MPX and FAN5608DMPX, the B1 and B2 inputs are internally con- nected to GND and the PWM signal can be applied to A1 and A2 inputs only. Consequently, the maximum LED current, for 100% duty cycle, is 5mA on each channel. PWM Control in Analog Mode The logic level HIGH, VH and logic level LOW, VL of the PWM signal should be: VRef < VH < (VIN – 0.7V) and 0 < VL < 0.6V The frequency of the PWM signal should be within 50Hz to 1kHz range. The VH sets the maximum LED current while the duty cycle sets the average current between 0 and ILEDmax. If the analog inputs A1 and/or A2 are driven in digital mode by an open drain output, it is important to choose the appro- priate value of the pull-up resistor. Its resistance should be low enough to ensure less than 0.7V dropout, hence VA > (VIN – 0.7V) as required for HIGH logic level: Open-Circuit Protection A built-in over voltage protection circuit prevents the device from being damaged when it is powered up with no load. This circuit reduces the boost converter duty cycle, to a minimum thus limiting the output voltage to a safe value when no load condition is detected. If one of the two enable branches is accidentally disconnected, the converter contin- ues the operation, however, the current in the remaining branch is no longer regulated and the actual branch current will be determined by the input voltage, the inductor value and the switching frequency. However, the FAN5608 can be damaged when a full load (more than six LEDs, driven by 20mA) is suddenly discon- nected from VOUT. To protect the FAN5608 against this unlikely event, an external 24V Zener diode can be con- nected between VOUT and GND. Shutdown Mode Each branch can be independently disabled by applying LOW logic level voltage to the A and B inputs. When both branches are disabled, the FAN5608 enters Shutdown mode and the supply current is reduced to less than 1 µA. A 0101 B 0011 ILED OFF 5mA 10mA 20mA I LED V External V Ref – R ----------------------------------------- Multiplication Ratio × = 0 5 10 15 20 25 1.25 1.5 1.75 2 2.25 2.5 2.75 3 3.25 3.5 VExternal (V) R=10Kohm R=100kohm R pull up – 700mV 60 µA ------------------ < 11.66k Ω = |
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