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RT8509GQW Datasheet(PDF) 6 Page - Richtek Technology Corporation |
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RT8509GQW Datasheet(HTML) 6 Page - Richtek Technology Corporation |
6 / 9 page ![]() RT8509 6 DS8509-01 March 2012 www.richtek.com © Copyright 2012 Richtek Technology Corporation. All rights reserved. is a registered trademark of Richtek Technology Corporation. Application Information The RT8509 is a high performance step-up DC/DC converter that provides a regulated supply voltage for panel source driver ICs. The RT8509 incorporates current mode, fixed frequency, Pulse Width Modulation (PWM) circuitry with a built in N-MOSFET to achieve high efficiency and fast transient response. The following content contains detailed description and information for component selection. Boost Regulator The RT8509 is a current mode boost converter integrated with a 24V/3.5A power switch, covering a wide VIN range from 2.8V to 14V. It performs fast transient responses to generate source driver supplies for TFT LCD display. The high operation frequency allows use of smaller components to minimize the thickness of the LCD panel. The output voltage can be adjusted by setting the resistive voltage-divider sensing at the FB pin. The error amplifier varies the COMP voltage by sensing the FB pin to regulate the output voltage. For better stability, the slope compensation signal summed with the current sense signal will be compared with the COMP voltage to determine the current trip point and duty cycle. Soft-Start The RT8509 provides soft-start function to minimize the inrush current. When powered on, an internal constant current charges an external capacitor. The rising voltage rate on the COMP pin is limited from VSS = 0V to 1.24V and the inductor peak current will also be limited at the same time. When powered off, the external capacitor will be discharged until the next soft-start time. The soft-start function is implemented by the external capacitor with a 5 μA constant current charging to the soft- start capacitor. Therefore, the capacitor should be large enough for output voltage regulation. A typical value for soft-start capacitor is 33nF. The available soft-start capacitor range is from 10nF to 100nF. If CSS < 220pF, the internal soft-start function will be turned on and period time is approximately 1ms. OUT REF REF R1 V = V x 1 , where V = 1.25V (typ.) R2 ⎛⎞ + ⎜⎟ ⎝⎠ The recommended value for R2 should be at least 10k Ω without some sacrificing. Place the resistive voltage divider as close as possible to the chip to reduce noise sensitivity. Loop Compensation The voltage feedback loop can be compensated with an external compensation network consisting of R3. Choose R3 to set high frequency integrator gain for fast transient response and C1 to set the integrator zero to maintain loop stability. For typical application, VIN = 5V, VOUT = 13.6V, COUT = 4.7 μF x 3, L1 = 4.7μH, while the recommended value for compensation is as follows : R3 = 56k Ω, C1 = 1nF. Over Current Protection The RT8509 boost converter has over current protection to limit the peak inductor current. It prevents large current from damaging the inductor and diode. During the On-time, once the inductor current exceeds the current limit, the internal LX switch turns off immediately and shortens the duty cycle. Therefore, the output voltage drops if the over current condition occurs. The current limit is also affected by the input voltage, duty cycle, and inductor value. Over Temperature Protection The RT8509 boost converter has thermal protection function to prevent the chip from overheating. When the junction temperature exceeds 155 °C, the function shuts down the device. Once the device cools down by approximately 10 °C, it will automatically restart to normal operation. To guarantee continuous operation, do not operate over the maximum junction temperature rating of 125 °C. Inductor Selection The inductance depends on the maximum input current. As a general rule, the inductor ripple current range is 20% to 40% of the maximum input current. If 40% is selected Output Voltage Setting The regulated output voltage is shown as the following equation : |
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