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ACT6359 Datasheet(PDF) 5 Page - Active-Semi, Inc |
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ACT6359 Datasheet(HTML) 5 Page - Active-Semi, Inc |
5 / 8 page ACT6359 Rev 1, 14-Feb-11 Innovative PowerTM - 5 - www.active-semi.com Copyright © 2011 Active-Semi, Inc. FUNCTIONAL BLOCK DIAGRAM Control Scheme The ACT6359 uses a minimum off-time, current- mode control scheme to achieve excellent perform- ance 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 operation. 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) 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 another switching begins when the error compara- tor detects that the output is falling out of regulation again. Over Voltage Protection The ACT6359 includes internal over-voltage pro- tection 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 voltage drops to zero, and the control switches at maximum on time causing the output voltage to keep rising until it ex- ceeds the maximum voltage rating of the power MOSFET. The over-voltage protection circuit de- tects this condition and switching ceases if the volt- age 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. Setting the LED Current The LED current is programmed by appropriate selection of the feedback resistor RFB connected between FB and G. To set the LED current, choose the resistor according to the equation: where VFB is the FB feedback voltage (typically 290mV at VEN = 3.3V) and ILED is the desired maxi- mum LED current. DRIVER ENABLE CIRCUIT UVLO CONTROL LOGIC THERMAL SHUTDOWN BANDGAP REFERENCE MAXIMUM ON TIME MINIMUM OFF TIME 1.21V OV COMPARATOR + - ERROR COMPARATOR + - EN G FB IN OV SW ACT6359 0.29V ⎥ ⎦ ⎤ ⎢ ⎣ ⎡ − ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ × = 1 V 21 . 1 V R R OV 1 OV 2 OV LED FB FB I V R = |
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