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FAN73402MX Datasheet(PDF) 11 Page - Fairchild Semiconductor |
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FAN73402MX Datasheet(HTML) 11 Page - Fairchild Semiconductor |
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11 / 16 page  © 2012 Fairchild Semiconductor Corporation www.fairchildsemi.com FAN73402 • 1.0.0 11 Functional Description The FAN73402 operates as a constant-current source for driving high-current LEDs. It uses Current-Mode control with programmable slope compensation to prevent subharmonic oscillation. The IC provides open-LED protection, over-voltage protection, and over-current protection for improved system reliability. The IC internally generates a FAULT OUT signal with a delay in case an abnormal LED string condition occurs. PWM dimming and analog dimming functions can be implemented independently. Internal soft-start prevents inrush current flowing into output capacitor at startup. VCC Under-Voltage Lockout (UVLO) An internal regulator provides the regulated 5V used to power the IC. The Under-Voltage Lockout (UVLO) turns off the IC in the event of the voltage dropping below the specific threshold level. The UVLO circuit inhibits powering the IC until a voltage reference is established, up to predetermined threshold level. Enable Applying voltage higher than 1.22V (typical) to the ENA pin enables the IC. Applying voltage lower than 1.15V (typical) to the ENA pin disables the IC. If ENA pin voltage is higher than 1.22V (typical) and VCC is higher than 9.0V (typical), the IC starts to supply the 5V reference voltage from VCC. Oscillator (Boost Switching Frequency) Boost switching frequency is programmed by the value of the resistor connected from the RT pin to ground. RT pin voltage is set to 2V. The current through the RT pin resistor determines the boost switching frequency according to formula: f . kHz (1) Soft-Start Function at Startup During initial startup, the switching device can be damaged due to the over-current coming from the input line by the negative control. This can result in the initial overshoot of the LED current. Therefore, during initial startup, the soft-start control gradually increases the duty cycle so that the output voltage can rise smoothly to control inrush current and overshoot. FAN73402 adapts the soft-start function in the boost converter stage. During soft-start period, boost switch turn-on duty is limited by the clamped CMP voltage. The soft-start period is dependent on the boost switching frequency, which is determined by the RT resistor (Equation 1). Soft-start period is set to be cumulative time when the BDIM (PWM dimming) signal is HIGH: t 600/f seconds (2) Figure 26. Soft-Start Waveforms LED Current Setting During the boost converter operating periods, the output LED current can be set by equation: I ADIM V R 60mΩ (3) where ADIM(V) is ADIM pin applied voltage and, RSENSE is the sensing resistor value. Note: 5. An additional 60m Ω comes from an internal wire bonding resistor. To calculate LED current precisely, consider the wire bonding resistor. Analog Dimming and PWM Dimming Analog dimming (ADIM) is achieved by varying the voltage level at the ADIM pin. This can be implemented either with a potentiometer from the VREF pin or from an external voltage source and a resistor divider circuit. The ADIM voltage level is adjusted to be the same as the feedback level (VSENSE). A VADIM range from 0.3V to 3V is recommended. PWM dimming (BDIM) helps achieve a fast PWM dimming response in spite of the shortcomings of the boost converter. The PWM dimming signal controls three nodes in the IC: gate signal to the switching FET, gate signal to the dimming FET, and output connection of the transconductance amplifier. When the PWM dimming signal is HIGH, the gates of the switching FET and dimming FET are enabled. At the same time, the output of the transconductance op-amp is connected to the compensation network. This allows the boost converter to operate normally. Dynamic Contrast Ratio The Dynamic Contrast Ratio (DCR) means the maximum contrast ratio achievable by adjusting the amount of light (dimming) of the screen using the backlight during an extremely short period of time. FAN73402 can normally drive the LED backlight under 0.1% dimming duty cycle at 200Hz dimming frequency. Even operating at 5µs dimming MOSFET turn-on time and extremely low dimming duty, FAN73402 can operate LEDs with normal peak current level. |
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