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AOZ1084DI Datasheet(PDF) 6 Page - Alpha & Omega Semiconductors |
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AOZ1084DI Datasheet(HTML) 6 Page - Alpha & Omega Semiconductors |
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6 / 12 page  AOZ1084 Rev. 0.3 April 2012 www.aosmd.com Page 6 of 12 Detailed Description The AOZ1084 is a high efficiency, simple to use, 1.2A buck HB LED driver optimized for general lighting applications. Features include enable control, under voltage lock-out, internal soft-start, output over-voltage protection, over-current protection and thermal shut down. The AOZ1084 is available in a DFN2x2-8L package. Soft Start and PWM Dimming The AOZ1084 has internal soft start feature to limit in-rush current and ensure the output voltage ramps up smoothly to regulation voltage. A soft start process begins when the input voltage rises to the voltage higher than UVLO and voltage on Dim pin is HIGH. In soft start process, the output voltage is ramped to regulation voltage in typically 600 s. The 600s soft start time is set internally. The DIM pin of the AOZ1084 is active high. Connect the DIM pin to VIN if enable function is not used. Pull it to ground will disable the AOZ1084. Do not leave it open. The voltage on DIM pin must be above 1.2V to enable the AOZ1084. When voltage on DIM pin falls below 0.4V, the AOZ1084 is disabled. Steady-State Operation Under steady-state conditions, the converter operates in fixed frequency and Continuous-Conduction Mode (CCM). The AOZ1084 integrates an internal NMOS as the high- side switch. Inductor current is sensed by amplifying the voltage drop across the drain to source of the high side power MOSFET. Output voltage is divided down by the external voltage divider at the FB pin. The difference of the FB pin voltage and reference is amplified by the internal transconductance error amplifier. The error voltage, is compared against the current signal, which is sum of inductor current signal and ramp compensation signal, at PWM comparator input. If the current signal is less than the error voltage, the internal high-side switch is on. The inductor current flows from the input through the inductor to the output. When the current signal exceeds the error voltage, the high-side switch is off. The inductor current is freewheeling through the external Schottky diode to output. Switching Frequency The AOZ1084 switching frequency is fixed and set by an internal oscillator. The switching frequency is set internally 450kHz. LED Current Programming LED current can be set by feeding back the output to the FB pin with the sense resistor RS shown in Figure 1. The LED current can be programmed as: Protection Features The AOZ1084 has multiple protection features to prevent system circuit damage under abnormal conditions. Over Current Protection (OCP) The sensed inductor current signal is also used for over current protection. The cycle by cycle current limit threshold is set normal value of 2A. When the load current reaches the current limit threshold, the cycle by cycle current limit circuit turns off the high side switch immediately to terminate the current duty cycle. The inductor current stop rising. The cycle by cycle current limit protection directly limits inductor peak current. The average inductor current is also limited due to the limitation on peak inductor current. When cycle by cycle current limit circuit is triggered, the output voltage drops as the duty cycle decreasing. The AOZ1084 has internal short circuit protection to protect itself from catastrophic failure under output short circuit conditions. As a result, the converter is shut down and hiccups. The converter will start up via a soft start once the short circuit condition disappears. In short circuit protection mode, the inductor average current is greatly reduced. UVLO An UVLO circuit monitors the input voltage. When the input voltage exceeds 2.9V, the converter starts operation. When input voltage falls below 2.2V, the converter will stop switching. Thermal Protection An internal temperature sensor monitors the junction temperature. It shuts down the internal control circuit and high side NMOS if the junction temperature exceeds 150ºC. The regulator will restart automatically under the control of soft-start circuit when the junction temperature decreases to 110°C. I LED 0.16 R S ----------- = |
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