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RT8206M Datasheet(PDF) 19 Page - Richtek Technology Corporation |
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RT8206M Datasheet(HTML) 19 Page - Richtek Technology Corporation |
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19 / 27 page  RT8206L/M 19 DS8206L/M-07 June 2012 www.richtek.com © Copyright 2012 Richtek Technology Corporation. All rights reserved. is a registered trademark of Richtek Technology Corporation. Application Information The RT8206L/M is a dual, high efficiency, Mach ResponseTM DRVTM dual ramp valley mode synchronous buck controller. The controller is designed for low voltage power supplies for notebook computers. Richtek Mach ResponseTM technology is specifically designed for providing 100ns “instant-on” response to load steps while maintaining a relatively constant operating frequency and inductor operating point over a wide range of input voltages. The DRVTM mode PWM modulator is specifically designed to have better noise immunity for such a dual output application. The RT8206L/M achieves high efficiency at a reduced cost by eliminating the current-sense resistor found in traditional current-mode PWMs. Efficiency is further enhanced by its ability to drive very large synchronous rectifier MOSFETs. The RT8206L/M includes 5V (LDO) linear regulator which can step down the battery voltage to supply both internal circuitry and gate drivers. When VOUT1 voltage is above 4.66V, an automatic circuit turns off the linear regulator and powers the device from VOUT1 through the BYP pin connected to VOUT1. PWM Operation The Mach ResponseTM DRVTM mode controller relies on the output filter capacitor's Effective Series Resistance (ESR) to act as a current-sense resistor, so the output ripple voltage provides the PWM ramp signal. Refer to the function block diagram, the UGATE driver will be turned on at the beginning of each cycle. After the internal one- shot timer expires, the UGATE driver will be turned off. The pulse width of this one shot is determined by the converter's input voltage and the output voltage to keep the frequency fairly constant over the input voltage range. Another one-shot sets a minimum off-time (300ns typ.). The on-time one-shot is triggered if the error comparator is high, the low-side switch current is below the current- limit threshold, and the minimum off-time one-shot has timed out. PWM Frequency and On-Time Control The Mach ResponseTM control architecture runs with pseudo-constant frequency by feed-forwarding the input and output voltage into the on-time one-shot timer. The high-side switch on-time is inversely proportional to the input voltage as measured by the VIN, and proportional to the output voltage. The on-time is given by : On-Time= K (VOUT / VIN) Where “K” is set by the TON pin-strap connector (Table 1). One-shot timing error increases for the shorter on- time setting due to fixed propagation delays that is approximately ±15% at high frequency and the ±10% at low frequency. The on-time guaranteed in the Electrical Characteristics tables is influenced by switching delays in the external high side power MOSFET. Two external factors that influence switching-frequency accuracy are resistive drops in the two conduction loops (including inductor and PC board resistance) and the dead-time effect. These effects are the largest contributors to the change of frequency with changing load current. The dead-time effect increases the effective on-time, reducing the switching frequency as one or both dead times. It occurs only in PWM mode (SKIP = high) when the inductor current reverses at light or negative load currents. With reversed inductor current, the inductor's EMF causes PHASEX to go high earlier than normal, extending the on- time by a period equal to the low-to-high dead time. For loads above the critical conduction point, the actual switching frequency is : fS = (VOUT +VDROP1) / tON x (VIN + VDROP1 − VDROP2 ) VDROP1 is the sum of the parasitic voltage drops in the inductor discharge path, including synchronous rectifier, inductor, and PC board resistances; VDROP2 is the sum of the resistances in the charging path; and tON is the on- time calculated by the RT8206L/M. Table 1. TON Setting and PWM Frequency Table TON TON = VCC TON = REF TON = GND VOUT1 K-Factor 5 μs 3.33 μs 2.5 μs VOUT1 Frequency 200kHz 300kHz 400kHz VOUT2 K-Factor 4 μs 2.67 μs 2 μs VOUT2 Frequency 300kHz 400kHz 500kHz Approximate K-Factor Error ±10% ±12.5% ±15% |
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