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RT8206M Datasheet(PDF) 20 Page - Richtek Technology Corporation |
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RT8206M Datasheet(HTML) 20 Page - Richtek Technology Corporation |
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20 / 27 page  RT8206L/M 20 DS8206L/M-07 June 2012 www.richtek.com © Copyright 2012 Richtek Technology Corporation. All rights reserved. is a registered trademark of Richtek Technology Corporation. Operation Mode Selection The RT8206L/M supports three operation modes: Diode- Emulation Mode, Ultrasonic Mode, and Forced-CCM Mode. Users can set operation mode by SKIP pin. All of the three operation modes will be introduced as follows. Diode-Emulation Mode (SKIP = GND) In Diode-Emulation mode, the RT8206L/M automatically reduces switching frequency at light-load conditions to maintain high efficiency. This reduction of frequency is achieved smoothly and without the increase of VOUTx ripple or load regulation. As the output current decreases from heavy-load condition, the inductor current is also reduced, and eventually comes to the point that its valley touches zero current, which is the boundary between continuous conduction and discontinuous conduction modes. By emulating the behavior of diodes, the low side MOSFET allows only partial negative current when the inductor free- wheeling current becomes negative. As the load current further decreases, it takes longer and longer to discharge the output capacitor to the level that requires for the next “ON” cycle. The on-time is kept the same as that in the heavy-load condition. In reverse, when the output current increases from light load to heavy load, the switching frequency increases to the preset value as the inductor current reaches the continuous conduction. The transition load point to the light-load operation can be calculated as the following equation. IN OUT LOAD ON (V V ) I t 2L − ≈× IL t 0 tON Slope = (VIN -VOUT) / L iL, peak iLoad = iL, peak / 2 Figure 3. Boundary Condition of CCM/DEM where tON is the given On-time. The switching waveforms may appear noisy and asynchronous when light loading causes Diode-Emulation operation, but this is a normal operating condition that results in high light-load efficiency. Trade-offs in PFM noise vs. light-load efficiency is made by varying the inductor value. Generally, low inductor values produce a broader efficiency vs. load curve, while higher values result in higher full-load efficiency (assuming that the coil resistance remains fixed) and less output voltage ripple. Penalties for using higher inductor values include larger physical size and degraded load-transient response (especially at low input-voltage levels). Ultrasonic Mode (SKIP = REF) Connecting SKIP to REF activates a unique Diode- Emulation mode with a minimum switching frequency above 25kHz. This ultrasonic mode eliminates audio- frequency modulation that would otherwise be present when a lightly loaded controller automatically skips pulses. In ultrasonic mode, the low side switch gate-driver signal is ORed with an internal oscillator (>25kHz). Once the internal oscillator is triggered, the ultrasonic controller forces the LGATEx high, turning on the low side MOSFET to induce a negative inductor current. At the point that the output voltage is higher than that of REF, the controller turns off the low side MOSFET (LGATEx pulled low) and triggers a constant on-time (UGATEx driven high). When the on-time has expired, the controller re-enables the low- side MOSFET until the controller detects that the inductor current has dropped below the zero-crossing threshold. Forced-CCM Mode (SKIP = VCC) The low noise, forced-CCM mode (SKIP = VCC) disables the zero-crossing comparator, which controls the low side switch on-time. This causes the low side gate-driver waveform to become the complement of the high side gate-driver waveform. This in turn causes the inductor current to reverse at light loads as the PWM loop strives to maintain a duty ratio of VOUT/VIN. The benefit of the forced-CCM mode is to keep the switching frequency fairly constant, but it comes at a cost : The no-load battery current can be from 10mA to 40mA, depending on the external MOSFETs. |
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