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RT8577 Datasheet(PDF) 9 Page - Richtek Technology Corporation |
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RT8577 Datasheet(HTML) 9 Page - Richtek Technology Corporation |
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9 / 12 page  9 DS8577-00 November 2011 www.richtek.com RT8577 × ≅ 9 SW RT 21.6 10 f100k + R SENSE 0.5V Current Limit (A) R ≅ Current Limit Protection The RT8577 can sense the RSENSE voltage between the SEN pin and GND to achieve over current protection. The boost converter senses the inductor current during the on period. The duty cycle depends on the current signal and internal slope compensation compared with the error signal. The external switch will be turned off when the current signal is larger than the internal slope compensation. In the off period, the inductor current will decrease until the internal switch is turned on by the oscillator. The current limit value can be calculated by the following equation : Brightness Control The RT8577 features a digital dimming control scheme. A very high contrast ratio true digital PWM dimming is achieved by driving the PWM pin with a PWM signal. The recommended PWM frequency is 200Hz to 10kHz, but the LED current cannot be 100% proportional to duty cycle, especially for high frequency and low duty ratio. Over Temperature Protection The RT8577 has over temperature protection function to prevent the IC from overheating due to excessive power dissipation. The IC will shut down and the STATUS pin will be pulled high when junction temperature exceeds 150 °C. Main converter starts switching after junction temperature cools down by approximately 20 °C. Inductor Selection The value of the inductance, L, can be approximated by the following equation, where the transition is from Discontinuous Conduction Mode (DCM) to Continuous Conduction Mode (CCM) : The duty cycle can be calculated as the following equation : where VOUT is the maximum output voltage, VIN is the minimum input voltage, f is the operating frequency, and IOUT is the sum of current from all LED strings. The boost converter operates in DCM over the entire input voltage range when the inductor value is less than this value, L. With an inductance greater than L, the converter operates in CCM at the minimum input voltage and may be discontinuous at higher voltages. The inductor must be selected with a saturated current rating that is greater than the peak current as provided by the following equation : where η is the efficiency of the power converter. Diode Selection Schottky diodes are recommended for most applications because of their fast recovery time and low forward voltage. Power dissipation, reverse voltage rating, and pulsating peak current are important parameters for consideration when making a Schottky diode selection. Make sure that the diode's peak current rating exceeds IPEAK and reverse voltage rating exceeds the maximum output voltage. Capacitor Selection The input capacitor reduces current spikes from the input supply and minimizes noise injection to the converter. For general applications, six 4.7 μF ceramic capacitors are sufficient. A value higher or lower may be used depending on the noise level from the input supply and the input current to the converter. It is recommended to choose a ceramic capacitor based on the output voltage ripple requirements. The minimum value of the output capacitor, COUT, can be calculated by the following equation : () ×− × ×× 2 OUT OUT D1 D V L = 2f I − OUT IN OUT VV D = V × ×× + ×× OUT OUT IN PEAK IN VI VD T I = η V2 L Setting the Switching Frequency The RT8577 switching frequency is programmable from 100kHz to 1MHz by adjusting the oscillator resistor, RRT. The switching frequency can be calculated by the following equation : × Δ× OUT OUT OUT ID C = Vf where ΔVOUT is the peak-to-peak ripple voltage at the output. |
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