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ADD5207ACPZ Datasheet(PDF) 12 Page - Analog Devices |
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ADD5207ACPZ Datasheet(HTML) 12 Page - Analog Devices |
12 / 16 page ADD5207 Data Sheet Rev. A | Page 12 of 16 the output voltage. When the OVP pin voltage reaches the OVP rising threshold, the boost converter stops switching, which causes the output voltage to drop. When the OVP pin voltage drops below the OVP falling threshold, the boot converter begins switching again, causing the output to rise. There is about 0.8 V hysteresis between the rising and falling thresholds. The OVP level is fixed at 39 V (typical). Open-Load Protection (OLP) The ADD5207 contains a headroom control circuit to minimize power loss at each current source. Therefore, the minimum feedback voltage is achieved by regulating the output voltage of the boost converter. If any LED string is open circuit during normal operation, the current source headroom voltage (VHR) is pulled to GND. In this condition, OLP is activated if VHR is less than 150 mV until the boost converter output voltage rises up to the OVP level. Undervoltage Lockout (UVLO) An undervoltage lockout circuit is included with built-in hysteresis. The ADD5207 turns on when VIN rises above 5.0 V (typical) and shuts down when VIN falls below 4.2 V (typical). Thermal Protection Thermal overload protection prevents excessive power dissipa- tion from overheating and damaging the ADD5207. When the junction temperature (TJ) exceeds 160°C, a thermal sensor immediately activates the fault protection, which shuts down the device and allows it to cool. The device self-starts when the junction temperature (TJ) of the die falls below 130°C. EXTERNAL COMPONENT SELECTION GUIDE Inductor Selection The inductor is an integral part of the step-up converter. It stores energy during the switch’s on time and transfers that energy to the output through the output diode during the switch’s off time. An inductor in the range of 4.7 µH to 22 µH is recommended. In general, lower inductance values result in higher saturation current and lower series resistance for a given physical size. However, lower inductance results in higher peak current, which can lead to reduced efficiency and greater input and/or output ripple and noise. Peak-to-peak inductor ripple current at close to 30% of the maximum dc input current typically yields an optimal compromise. The input (VIN) and output (VOUT) voltages determine the switch duty cycle (D), which, in turn, is used to determine the inductor ripple current. OUT IN OUT V V V D − = Use the duty cycle and switching frequency (fSW) to determine the on time. SW ON f D t = The inductor ripple current (ΔIL) in a steady state is: L t V I IN L ON × = ∆ Solve for the inductance value (L): L IN I t V L ∆ × = ON Make sure that the peak inductor current (that is, the maximum input current plus half of the inductor ripple current) is less than the rated saturation current of the inductor. In addition, ensure that the maximum rated rms current of the inductor is greater than the maximum dc input current to the regulator. For duty cycles greater than 50% that occur with input voltages greater than half the output voltage, slope compensation is required to maintain stability of the current mode regulator. The inherent open-loop stability causes subharmonic instability when the duty ratio is greater than 50%. To avoid subharmonic instability, the slope of the inductor current should be less than half of the compensation slope. Inductor manufacturers include: Coilcraft, Inc., Sumida Corporation, and Toko. Input and Output Capacitor Selection The ADD5207 requires input and output bypass capacitors to supply transient currents while maintaining a constant input and output voltage. Use a low effective series resistance (ESR) 10 μF or greater capacitor for the input capacitor to prevent noise at the ADD5207 input. Place the input between VIN and GND, as close as possible to the ADD5207. Ceramic capacitors are preferred because of their low ESR characteristics. Alternatively, use a high value, medium ESR capacitor in parallel with a 0.1 μF low ESR capacitor as close as possible to the ADD5207. The output capacitor maintains the output voltage and supplies current to the load while the ADD5207 switch is on. The value and characteristics of the output capacitor greatly affect the output voltage ripple and stability of the regulator. Use a low ESR output capacitor; ceramic dielectric capacitors are preferred. For very low ESR capacitors, such as ceramic capacitors, the ripple current due to the capacitance is calculated as follows. Because the capacitor discharges during the on time (tON), the charge removed from the capacitor (QC) is the load current multiplied by the on time. Therefore, the output voltage ripple (ΔVOUT) is OUT ON L OUT C OUT C t I C Q V × = = ∆ where: COUT is the output capacitance. IL is the average inductor current. |
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