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RT8070 Datasheet(PDF) 8 Page - Richtek Technology Corporation |
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RT8070 Datasheet(HTML) 8 Page - Richtek Technology Corporation |
8 / 12 page RT8070 8 DS8070-07 November 2012 www.richtek.com © Copyright 2012 Richtek Technology Corporation. All rights reserved. is a registered trademark of Richtek Technology Corporation. Application Information The basic IC application circuit is shown in Typical Application Circuit. External component selection is determined by the maximum load current and begins with the selection of the inductor value and operating frequency followed by CIN and COUT. Main Control Loop During normal operation, the internal upper power switch (P-MOSFET) is turned on at the beginning of each clock cycle. Current in the inductor increases until the peak inductor current reaches the value defined by the output voltage (VCOMP) of the error amplifier. The error amplifier adjusts its output voltage by comparing the feedback signal from a resistive voltage-divider on the FB pin with an internal 0.8V reference. When the load current increases, it causes a reduction in the feedback voltage relative to the reference. The error amplifier increases its output voltage until the average inductor current matches the new load current. When the upper power MOSFET shuts off, the lower synchronous power switch (N-MOSFET) turns on until the beginning of the next clock cycle. Output Voltage Setting The output voltage is set by an external resistive voltage- divider according to the following equation : × OUT REF R1 V = V (1+ ) R2 where VREF equals to 0.8V typical. The resistive voltage-divider allows the FB pin to sense a fraction of the output voltage as shown in Figure 1. Figure 1. Setting the Output Voltage Soft-Start The IC contains an external soft-start clamp that gradually raises the output voltage. The soft-start timing is programmed by the external capacitor between SS pin and GND. The chip provides an internal 10 μAcharge current for the external capacitor. If 10nF capacitor is used to set the soft-start, the period will be 800 μs (typ.). Power Good Output The power good output is an open-drain output and requires a pull up resistor. When the output voltage is 12.5% above or 12.5% below its set voltage, PGOOD will be pulled low. It is held low until the output voltage returns to within the allowed tolerances once more. During soft-start, PGOOD is actively held low and is only allowed to transition high when soft-start is over and the output voltage reaches 87.5% of its set voltage. Operating Frequency Selection of the operating frequency is a tradeoff between efficiency and component size. Higher frequency operation allows the use of smaller inductor and capacitor values. Lower frequency operation improves efficiency by reducing internal gate charge and switching losses but requires larger inductance and/or capacitance to maintain low output ripple voltage. The operating frequency of the IC is determined by an external resistor , ROSC, that is connected between the RT pin and ground. The value of the resistor sets the ramp current that is used to charge and discharge an internal timing capacitor within the oscillator. The practical switching frequency ranges from 200kHz to 2MHz. However, when the RT pin is floating, the internal frequency is set at 2MHz. Determine the RT resistor value by examining the curve below. Please notice the minimum on time is about 90ns. R1 GND RT8070 VOUT R2 FB |
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