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RT7281 Datasheet(PDF) 6 Page - Richtek Technology Corporation |
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RT7281 Datasheet(HTML) 6 Page - Richtek Technology Corporation |
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6 / 22 page  RT7274/79/80/81 6 DS7274/79/80/81-01 February 2013 www.richtek.com © Copyright 2013 Richtek Technology Corporation. All rights reserved. is a registered trademark of Richtek Technology Corporation. changes, the switch voltage drops change causing a switching frequency variation with load current. Also, at light loads if the inductor current goes negative, the switch dead-time between the synchronous rectifier turn-off and the high-side switch turn-on allows the switching node to rise to the input voltage. This increases the effective on- time and causes the switching frequency to drop noticeably. One way to reduce these effects is to measure the actual switching frequency and compare it to the desired range. This has the added benefit eliminating the need to sense the actual output voltage, potentially saving one pin connection. ACOTTM uses this method, measuring the actual switching frequency and modifying the on-time with a feedback loop to keep the average switching frequency in the desired range. To achieve good stability with low-ESR ceramic capacitors, ACOTTM uses a virtual inductor current ramp generated inside the IC. This internal ramp signal replaces the ESR ramp normally provided by the output capacitor's ESR. The ramp signal and other internal compensations are optimized for low-ESR ceramic output capacitors. ACOTTM One-shot Operation The RT7274/79/80/81 control algorithm is simple to understand. The feedback voltage, with the virtual inductor current ramp added, is compared to the reference voltage. When the combined signal is less than the reference the on-time one-shot is triggered, as long as the minimum off-time one-shot is clear and the measured inductor current (through the synchronous rectifier) is below the current limit. The on-time one-shot turns on the high-side switch and the inductor current ramps up linearly. After the on-time, the high-side switch is turned off and the synchronous rectifier is turned on and the inductor current ramps down linearly. At the same time, the minimum off- time one-shot is triggered to prevent another immediate on-time during the noisy switching time and allow the feedback voltage and current sense signals to settle. The minimum off-time is kept short (230ns typical) so that rapidly-repeated on-times can raise the inductor current quickly when needed. Discontinuous Operating Mode (RT7274/80 Only) After soft start, the RT7279/81 operates in fixed frequency mode to minimize interference and noise problems. The RT7274/80 uses variable-frequency discontinuous switching at light loads to improve efficiency. During discontinuous switching, the on-time is immediately increased to add “hysteresis” to discourage the IC from switching back to continuous switching unless the load increases substantially. The IC returns to continuous switching as soon as an on- time is generated before the inductor current reaches zero. The on-time is reduced back to the length needed for 700kHz switching and encouraging the circuit to remain in continuous conduction, preventing repetitive mode transitions between continuous switching and discontinuous switching. Current Limit The RT7274/79/80/81 current limit is a cycle-by-cycle “valley” type, measuring the inductor current through the synchronous rectifier during the off-time while the inductor current ramps down. The current is determined by measuring the voltage between source and drain of the synchronous rectifier, adding temperature compensation for greater accuracy. If the current exceeds the upper current limit, the on-time one-shot is inhibited until the inductor current ramps down below the upper current limit plus a wide hysteresis band of about 1A and drops below the lower current limit level. Thus, only when the inductor current is well below the upper current limit is another on- time permitted. This arrangement prevents the average output current from greatly exceeding the guaranteed upper current limit value, as typically occurs with other valley-type current limits. If the output current exceeds the available inductor current (controlled by the current limit mechanism), the output voltage will drop. If it drops below the output under-voltage protection level (see next section) the IC will stop switching to avoid excessive heat. The RT7279/81 also includes a negative current limit to protect the IC against sinking excessive current and possibly damaging the IC. If the voltage across the synchronous rectifier indicates the negative current is too |
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Similar Description - RT7281 |
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