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RTQ2533W Datasheet(PDF) 14 Page - Richtek Technology Corporation |
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RTQ2533W Datasheet(HTML) 14 Page - Richtek Technology Corporation |
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14 / 19 page  RTQ2533W 14 DSQ2533W-00 April 2019 www.richtek.com © Copyright 2019 Richtek Technology Corporation. All rights reserved. is a registered trademark of Richtek Technology Corporation. operating voltage and temperature and the design engineer must be aware of these characteristics. Ceramic capacitors are usually recommended to be derated by 50%. A 47 μF or greater output ceramic capacitor (or 22μF effective capacitance) is suggested to ensure stability. Input capacitance is selected to minimize transient input drop during load current steps. For general applications, an input capacitor of at least 47 μF is highly recommended for minimal input impedance. If the trace inductance between the RTQ2533W input supply is high, a fast load transient can cause VIN voltage level ringing above the absolute maximum voltage rating which damages the device. Adding more input capacitors is available to restrict the ringing and keep it below the device absolute maximum ratings. Generally, a 47 μF 0805-sized ceramic capacitor in parallel with two 10 μF 0805-sized ceramic capacitor ensures the minimum effective capacitance at high input voltage and high output voltage requirement. Place these capacitors as close to the pins as possible for optimum performance and to ensure stability. Feed-Forward Capacitor (CFF) The RTQ2533W is designed to be stable without the external feed-forward capacitor (CFF). However, a 10nF external feed-forward capacitor optimizes the transient, noise, and PSRR performances. A higher capacitance CFF can be also used, but the start-up time is longer and the power-good signal can incorrectly indicate that the output voltage is settled. Soft-Start and Noise Reduction (CNR/SS) The RTQ2533W is designed for a programmable, monotonic soft-start time during the output rising, which can be achieved via an external capacitor (CNR/SS) on NR/ SS pin. Using an external CNR/SS is recommended for general application, not only for the in-rush current minimization but also helps reduce the noise component from the internal reference. During the monotonic start-up procedure, the error amplifier of the RTQ2533W tracks the voltage ramp of the external soft-start capacitor (CNR/SS) until the voltage approaches the internal reference 0.8V. The soft-start ramp time can be calculated with Equation a1, which depends on the REF NR/SS SS NR/SS VC t = a1 I For noise-reduction, CNR/SS in conjunction with an internal noise-reduction resistor forms a low-pass filter (LPF) and filters out the noise from the internal bandgap reference before being amplified via the error amplifier, thus reducing the total device noise floor. Input Inrush Current During start-up, the input Inrush current into VIN pin consists of the sum of load current and the charging current of the output capacitor. The inrush current is difficult to measure because the input capacitor must be removed, which is not recommended. Generally, the soft-start inrush current can be estimated by Equation b1, where VOUT(t) is the instantaneous output voltage of the power-on ramp, dVOUT(t) / dt is the slope of the VOUT ramp and RLOAD is the resistive load impedance. OUT OUT OUT OUT LOAD CdV t Vt I t = + b1 dt R Under-Voltage Lockout (UVLO) The Under-Voltage Lockout (UVLO) threshold is the minimum input operational voltage range that ensures the device stays disabled. Figure 5 explain the UVLO circuits be triggered between three different input voltage events(duration a, b and c), assuming VEN ≥ VEN_H all the time. For duration “a”, the input voltage starts rising and when VIN is over the UVLO rising threshold, VOUT starts the power-on process, then when Vout reaches the target level it is under regulation. During “b”, although the power line has a voltage drop, but does not cross UVLO low threshold (falling threshold), the device maintains normal operation, and VOUT is still regulated. At duration “c” VIN drops below the UVLO falling threshold, the control loop is disabled and there is no regulation; meanwhile VOUT drops. For general application, instant power line transient with long power trace at the VIN pin may have VIN level unstable and force a trap as shown in duration “c” which makes VOUT collapse. In this case, adding more input capacitance or improving input trace layout on PCB are effective to improve input power stabilization. soft-start charging current (INR/SS), the soft-start capacitance (CNR/SS), and the internal reference 0.8V (VREF). |
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