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ACT6358 Datasheet(PDF) 5 Page - Active-Semi, Inc |
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ACT6358 Datasheet(HTML) 5 Page - Active-Semi, Inc |
5 / 11 page ACT6357/ACT6358 Rev 1, 15-Nov-12 Innovative PowerTM - 5 - www.active-semi.com Copyright © 2012 Active-Semi, Inc. FUNCTIONAL BLOCK DIAGRAM Control Scheme The ACT6357 and ACT6358 use a minimum off- time, current-mode control scheme to achieve ex- cellent performance under high duty-cycle operating conditions. This control scheme initiates a switching cycle only when needed to maintain output voltage regulation, resulting in very high efficiency opera- tion. During each switching cycle, the N-channel power MOSFET turns on, increasing the inductor current. The switching cycle terminates when either the in- ductor current reaches the current limit (500mA for the ACT6357, 1A for the ACT6358) or when the cycle lasts longer than the maximum on-time of 4µs. Once the MOSFET turns off, it remains off for at least the minimum off-time of 320ns, then an- other switching begins when the error comparator detects that the output is falling out of regulation again. Soft-Start The ACT6357 and ACT6358 include a programma- ble soft-start function, which can be used to opti- mize an application between start-up time and start-up inrush current. Soft start is achieved by connecting a capacitor CSS between the SS pin and G. The soft start duration can be calculated from the following equation: where tSS is the required soft start duration. In a typical application, use 0.1µF to generate 20ms soft start time. Over Voltage Protection Both the ACT6357 and ACT6358 include internal over-voltage protection circuitry that monitors the OV pin voltage. Over-voltage protection is critical when one of the LEDs in the LED string fails as an open circuit. When this happens the feedback volt- age drops to zero, and the control switches at maxi- mum on time causing the output voltage to keep rising until it exceeds the maximum voltage rating of the power MOSFET. The ACT6357 and ACT6358's over-voltage protection detects this condition and switching ceases if the voltage at the OV pin reaches 1.21V. To set the maximum output voltage, connect a re- sistor divider from the output node to G, with center tap at OV, and select the two resistors with the fol- lowing equation: where VOV is the over voltage detection threshold, ROV1 is the resistor between OV and G, and ROV2 is the resistor from the output to the OV pin. As a first estimate, the OV threshold can often be set to 4V times the number of LEDs in the string. ⎥ ⎦ ⎤ ⎢ ⎣ ⎡ − ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ × = 1 V 21 . 1 V R R OV 1 OV 2 OV s F μ 5 t C SS SS × = |
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