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RT6203B Datasheet(PDF) 17 Page  Richtek Technology Corporation 

17 page RT6203B 17 DS6203B01 July 2017 www.richtek.com © Copyright 2017 Richtek Technology Corporation. All rights reserved. is a registered trademark of Richtek Technology Corporation. Inductor Selection The consideration of inductor selection includes inductance, RMS current rating and, saturation current rating. The inductance selection is generally flexible and is optimized for the low cost, low physical size, and high system performance. Choosing lower inductance to reduce physical size and cost, and it is useful to improve the transient response. However, it causes the higher inductor peak current and output ripple voltage to decrease system efficiency. Conversely, higher inductance increase system efficiency, but the physical size of inductor will become larger and transient response will be slow because more transient time is required to change current (up or down) by inductor. A good compromise between size, efficiency, and transient response is to set a inductor ripple current ( ΔIL) about 20% to 50% of the desired full output load current. Calculate the approximate inductance by the input voltage, output voltage, switching frequency (fSW), maximum rated output current (IOUT(MAX)) and inductor ripple current ( ΔIL). OUT IN OUT IN SW L VV V L = Vf I It will be safe to choose inductor saturation current larger than 8.1A. Input Capacitor Selection The input filter capacitors are needed to smooth out the RMS input ripple current drawn from the input power source and ripple voltage seen at the input of the converter. The voltage rating of the input filter capacitors must be greater than the maximum input voltage. It's also important to consider the ripple current capabilities of capacitors. The RMS input ripple current (IRMS) is a function of the input voltage (VIN), output voltage (VOUT), and rated output current (IOUT) : Once the inductance is chosen, the inductor ripple current ( ΔIL) and peak inductor current can be calculated. OUT IN OUT L IN SW L(PEAK) OUT(MAX) L L(VALLY) OUT(MAX) L VV V I= Vf L 1 I = I I 2 1 I = I I 2 The typical operating circuit design for the RT6203B, the output voltage is 1.2V, maximum rated output current is 5A, input voltage is 12V, and inductor ripple current is 1A which is 20% of the maximum rated output current, the calculated inductance value is : 3 1.2 12 1.2 L = = 1.53μH 12 700 10 1 The inductor ripple current can be set larger than 1A and so we select 1.5 μH inductance. The actual inductor ripple current and required peak current is shown as below : L 36 1.2 12 1.2 I = = 1.02A 12 700 10 1.5 10 L(PEAK) OUT(MAX) L 11 I = I I = 5 + = 5.5A 22 Inductor saturation current should be chosen over IC’s current limit. Set valley current limit of the RT6203B is 7A by I2C. When touching current limit of the RT6203B, the peak inductor current is : L(PEAK) L(VALLEY) L I = I + I = 7 + 1 = 8A OUT IN RMS OUT IN OUT V V I = I 1 VV The maximum RMS input ripple current occurs at maximum output load and it needs to be concerned about the ripple current capabilities of capacitors at maximum output load. Ceramic capacitors are most often used because of their low cost, small size, high RMS current ratings, and robust surge current capabilities. It should pay attention that value of capacitors change as temperature, bias voltage, and operating frequency change. For example the capacitance value of a capacitor decreases as the dc bias across the capacitor increases. However, take care when these capacitors are used at the input of circuits supplied by a wall adapter or other supply connected through long and thin wires. Current surges through the inductive wires can induce ringing at the IC's power input which could potentially cause large, damaging voltage spikes at VIN pin. If this phenomenon is observed, some bulk input capacitance may be required. Ceramic capacitors can be placed in parallel with other types such as tantalum, electrolytic, or polymer to 
