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FAN5776UCX Datasheet(PDF) 11 Page - Fairchild Semiconductor |
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FAN5776UCX Datasheet(HTML) 11 Page - Fairchild Semiconductor |
11 / 15 page © 2010 Fairchild Semiconductor Corporation www.fairchildsemi.com FAN5776 • Rev. 1.1.0 11 Circuit Description Overview The FAN5776 is a 1.8 MHz synchronous step-up DC-DC converter with integrated constant-current high-side LED drivers capable of driving one to five LED strings up to 5 x 25 mA LED current. The device starts when at least one LED string is utilized and the appropriate EN pin is enabled. The device is disabled in 30 ms by setting both EN pins LOW. The VOUT voltage is internally set to 290 mV above the highest LED string voltage, and it is sampled at every falling LED PWM cycle. For 100% duty cycle, the LED-pin voltage is sampled and the VOUT voltage is refined every 10 ms. The LED strings can be disabled by connecting them to VOUT or shorting them to GND. They can also be left disconnected. If the LED string is temporarily disabled or shorted, the device must be re-enabled to enable the string again. The LED drivers work independently and allow multiple LED voltages, such that many types of LEDs can be driven at the same time and some strings can be used to drive a single LED while other channels are driving two LEDs in series. The VOUT voltage is defined by the highest LED voltage and the LED driver dropout voltage is increased to provide the LED string a specific voltage. If the voltage difference between the LED strings is large, the system efficiency may decrease. LED Current The LED string current is set by the resistor, RSET, between the ISET and GND pins. The same current is applied to across all strings such that total output current: IOUT = 5 x ILED = 5 x 20 mA = 100 mA if RSET = 25 kand all LED strings are used. In general, the LED string current can be calculated as follows: ���������������� = 500 ���������������� (1) Figure 26. LED Current vs. RSET Value Startup The three different startup functions depend on the system configuration: 1. All LED strings are utilized: Setting one or both EN pins HIGH enables the device and VOUT rises to 7.5 V. FAN5776 starts to step up or down to the appropriate regulated voltage. 2. At least one LED string in a group is shorted to GND. VOUT rises to 7.5 V while the shorted LED string is disabled and the device starts to step up or down to regulated voltage. 3. At least one LED string is floating or connected to VOUT. VOUT rises to 9.0 V, the floating LED string is disabled, and the device starts to step down to regulated voltage. These functions work for each group independently. If all five strings are utilized and EN13 is HIGH, VOUT rises to 7.5 V (case 1) and goes to the highest voltage required by LED1-3. Then EN45 is raised and VOUT is stepped up again to 7.5 V and regulates to highest voltage required by LED1-5. If VOUT cannot reach 7.5 V within 1.2 ms after an enable cycle, the device stays disabled and a new enable cycle is required. PWM Dimming A LED PWM signal of 100 Hz to 800 Hz can be applied to EN13 and EN45 pins to control LED1-3 and LED4-5 light intensity. The LED current is a linear function of the LED PWM duty cycle from 100% down to 0.4%. The FAN5776 can be started by a PWM signal with a low duty cycle to enable smooth startup. EN13 and EN45 pins can be operated either synchronously or asynchronously, which makes it possible to use the device to backlight two separate displays at the same time. Under-Voltage Lockout (UVLO) The Under-Voltage Lockout circuitry turns off all MOSFETs and the device remains in a very low quiescent current state until VIN has risen above the UVLO threshold. Short-Circuit Protection (SCP) The LED driver output current is limited to 0.5 µA or less when a LED number pin voltage is below 1.0 V. This limit shall be applied within one LED PWM cycle, or 10 ms, whichever elapses first. Over-Voltage Protection (OVP) When the regulator is active, it monitors the VOUT pin. If the VOUT voltage reaches 9.0 V, the regulator stops switching until the capacitor at VOUT discharges below 8.5 V. LED-Open Detection If VOUT is detected above >9.0 V, the LED voltages are scanned. All LED pins with voltage greater than VOUT - 0.5 V are disabled. If all LED pin s’ voltages exceed 8.5 V and VOUT is greater than 9.0 V, device is disabled and a new startup cycle is required. Over-Current Protection (OCP) The PWM converter is protected against overload through cycle-by cycle current limit using a fixed internal limit. Thermal Shutdown When the die temperature exceeds 150°C, reset occurs and remains in effect until the die cools to 125°C; at which time, the circuit enters the normal soft-start sequence. 0 5 10 15 20 25 20 40 60 80 100 120 140 160 180 200 Resistor RSET (k) |
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