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AOZ1014 Datasheet(PDF) 9 Page - Alpha & Omega Semiconductors |
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AOZ1014 Datasheet(HTML) 9 Page - Alpha & Omega Semiconductors |
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9 / 21 page  ^lwNMNQ February 2006 www.aosmd.com Page 9 of 21 Detailed Description The AOZ1014 is a current-mode step down regulator with integrated high side PMOS switch and a low side freewheeling Schottky diode. It operates from a 4.5V to 16V input voltage range and supplies up to 5A of load current. The duty cycle can be adjusted from 6% to 100% allowing a wide range of output voltage. Features include enable control, Power-On Reset, input under voltage lockout, fixed internal soft-start and thermal shut down. The AOZ1014 is available in SO-8 and thermally enhanced DFN-8 package. Enable and Soft Start The AOZ1014 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 4.0V and voltage on EN pin is HIGH. In soft start process, the output voltage is ramped to regulation voltage in typically 4ms. The 4ms soft start time is set internally. The EN pin of the AOZ1014 is active high. Connect the EN pin to VIN if enable function is not used. Pull it to ground will disable the AOZ1014. Do not leave it open. The voltage on EN pin must be above 2.0 V to enable the AOZ1014. When voltage on EN pin falls below 0.6 V, the AOZ1014 is disabled. If an application circuit requires the AOZ1014 to be disabled, an open drain or open collector circuit should be used to interface to EN pin. Steady-State Operation Under steady-state conditions, the converter operates in fixed frequency and Continuous-Conduction Mode (CCM). The AOZ1014 integrates an internal P-MOSFET 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, which shows on the COMP pin, 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. The AOZ1014 uses a P-Channel MOSFET as the high side switch. It saves the bootstrap capacitor normally seen in a circuit which is using an NMOS switch. It allows 100% turn-on of the upper switch to achieve linear regulation mode of operation. The minimum voltage drop from V IN to VO is the load current times DC resistance of MOSFET plus DC resistance of buck inductor. It can be calculated by equation below: ) ( ) ( _ inductor ON DS O IN MAX O R R I V V + × − = Where V O_MAX is the maximum output voltage; V IN is the input voltage from 4.5V to 16V; I O is the output current from 0A to 5A; R DS(ON) is the on resistance of internal MOSFET, the value is between 25m Ω and 55m Ω depending on input voltage and junction temperature; R inductor is the inductor DC resistance; Switching Frequency The AOZ1014 switching frequency is fixed and set by an internal oscillator. The practical switching frequency could range from 350kHz to 600kHz due to device variation. Output Voltage Programming Output voltage can be set by feeding back the output to the FB pin with a resistor divider network. In the application circuit shown in Figure 1. The resistor divider network includes R 1 and R2. Usually, a design is started by picking a fixed R 2 value and calculating the required R1 with equation below. ) 1 ( 8 . 0 2 1 R R V O + × = Some standard value of R 1, R2 for most commonly used output voltage values are listed in Table 1. Table 1. Vo (V) R1 (k Ω) R2 (k Ω) 0.8 1.0 open 1.2 4.99 10 1.5 10 11.5 1.8 12.7 10.2 2.5 21.5 10 3.3 31.6 10 5.0 52.3 10 Combination of R1 and R2 should be large enough to avoid drawing excessive current from the output, which will cause power loss. |
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