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C2012X5R0J106M Datasheet(PDF) 9 Page - Richtek Technology Corporation |
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C2012X5R0J106M Datasheet(HTML) 9 Page - Richtek Technology Corporation |
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9 / 14 page  RT8055 9 DS8055-05 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 RT8055 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. Output Voltage Setting The output voltage is set by an external resistive divider according to the following equation : Figure 1. Setting the Output Voltage ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ + × = R2 R1 1 V V REF OUT Soft-Start The RT8055 contains an internal soft-start clamp that gradually raises the clamp on the COMP pin. Operating Frequency Selection of the operating frequency is a tradeoff between efficiency and component size. High frequency operation allows the use of smaller inductor and capacitor values. Operation at lower frequency 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 RT8055 is determined by an external resistor that is connected between the SHDN/ RT pin and GND. The value of the resistor sets the ramp current that is used to charge and discharge an internal timing capacitor within the oscillator. The RT resistor value can be determined by examining the frequency vs. RRT curve. Although frequencies as high as 2MHz are possible, the minimum on-time of the RT8055 imposes a minimum limit on the operating duty cycle. The minimum on-time is typically 110ns. Therefore, the minimum duty cycle is equal to 100 x 110ns x f (Hz). Figure 2 RT8055 FB GND VOUT R1 R2 100% Duty Cycle Operation When the input supply voltage decreases toward the output voltage, the duty cycle increases toward the maximum on-time. Further reduction of the supply voltage forces the main switch to remain on for more than one cycle eventually reaching 100% duty cycle. The output voltage will then be determined by the input voltage minus the voltage drop across the internal P-MOSFET and the inductor. Low Supply Operation The RT8055 is designed to operate down to an input supply voltage of 2.6V. One important consideration at low input supply voltages is that the RDS(ON) of the P-Channel and N-Channel power switches increases. The user should calculate the power dissipation when the RT8055 is used at 100% duty cycle with low input voltages to ensure that thermal limits are not exceeded. Slope Compensation and Inductor Peak Current Slope compensation provides stability in constant frequency architectures by preventing sub-harmonic oscillations at duty cycles greater than 50%. It is accomplished internally by adding a compensating ramp to the inductor current signal. Normally, the maximum inductor peak current is reduced when slope compensation is added. In the RT8055, however, separated inductor current signals are used to monitor over current condition. 0.0 0.5 1.0 1.5 2.0 2.5 3.0 0 200 400 600 800 1000 ROSC (K ) ROSC (k Ω) RRT = 180k for 1.8MHz where VREF equals to 0.8V typical. The resistive divider allows the FB pin to sense a fraction of the output voltage as shown in Figure 1. |
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