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RT8457A Scheda tecnica(PDF) 9 Page - Richtek Technology Corporation |
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RT8457A Scheda tecnica(HTML) 9 Page - Richtek Technology Corporation |
9 / 11 page RT8457A 9 DS8457A-02 September 2013 www.richtek.com © Copyright 2013 Richtek Technology Corporation. All rights reserved. is a registered trademark of Richtek Technology Corporation. Application Information Output Voltage Setting The voltage control loop is controlled via the first trans- conductance operational amplifier. An opto-coupler is directly connected to the output and an external resistor bridge is connected between the output positive line and the ground reference. The relationship between R2 and R1 is shown below : ( ) × OUT OVP R1 + R2 V = V R2 where VOUT is the desired maximum output voltage. To avoid discharge of the load, the resistor bridge, R1 and R2, should be highly resistive. For this type of application, a total value of 100k Ω (or more) would be appropriate for the resistors, R1 and R2. Output Current Setting The current control loop is controlled via the second trans- conductance operational amplifier. An opto-coupler and sense resistor, RS, are placed in series on the output negative line. The sense voltage threshold is achieved externally via a resistor bridge tied to the reference voltage, VREF. The output current that flows through the LEDs is set by an external resistor, RS, which is connected between the LED and GND terminal. The relationship between output current, IOUT, and RS is shown in below equation : OUT S 170mV I = R where IOUT is the desired maximum output current and the sense voltage threshold for the current control loop. Note that the sense resistor, RS, should be chosen after taking into account its maximum power dissipation (PLIMIT) during full load operation. Analog Dimming Control The ACTL terminal is driven by an external voltage, VACTL, to adjust the output current to an average value set by RS. The voltage range for VACTL to adjust the output current is from 0.09V to 1.1V. When VACTL is larger than 0.09V, the output current value will just be set by the external resistor (RS). () () ACTL OUTavg S V0.09 I = 170mV / R 1.01 − × Compensation Both the voltage control trans-conductance amplifier and the current control trans-conductance amplifier are fully compensated. The output and negative inputs are directly accessible for external compensation components, as shown in the Typical Application Circuit. The typical component values for the compensation network of voltage control loop is CIC2 = 2.2nF and RIC2 = 22k Ω. The typical component value for the compensation network of current control loop is CIC1 = 2.2nF, RIC1 = 22k Ω and RIC2 = 1kΩ. However, in many applications, the current control loop can be stable without compensation network. When the voltage control loop is used as the voltage limit protection or the current control loop is used as the current limit protection, no compensation network is needed for the protecting control loop. A resistor, RLED, must be connected in series with the opto-coupler since it is part of the compensation network. Although the value of RLED is not critical, it's recommended to be in the range from 0.33k Ω to (VOUT − VOPTO − 1) / IOUTH Ω, where IOUTH is recommended to be less than 10mA. Thermal Considerations For continuous operation, do not exceed absolute maximum junction temperature. The maximum power dissipation depends on the thermal resistance of the IC package, PCB layout, rate of surrounding airflow, and difference between junction and ambient temperature. The maximum power dissipation can be calculated by the following formula : PD(MAX) = (TJ(MAX) − TA) / θJA where TJ(MAX) is the maximum junction temperature, TA is the ambient temperature, and θJA is the junction to ambient thermal resistance. For recommended operating condition specifications, the maximum junction temperature is 125 °C. The junction to ambient thermal resistance, θJA, is layout dependent. For SOP-8 package, the thermal resistance, θJA, is 188°C/W |
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