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AOZ1022DI Datasheet(PDF) 7 Page - Alpha & Omega Semiconductors |
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AOZ1022DI Datasheet(HTML) 7 Page - Alpha & Omega Semiconductors |
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7 / 17 page  AOZ1022 Rev. 1.6 December 2010 www.aosmd.com Page 7 of 17 Detailed Description The AOZ1022 is a current-mode step down regulator with integrated high-side PMOS switch and a low-side NMOS switch. It operates from a 4.5V to 16V input voltage range and supplies up to 3A 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, output over voltage protection, active high power good state, fixed internal soft-start and thermal shut down. Enable and Soft Start The AOZ1022 has an 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 4.1V and voltage on EN pin is HIGH. In the soft start process, the output voltage is typically ramped to regulation voltage in 4ms. The 4ms soft start time is set internally. The EN pin of the AOZ1022 is active HIGH. Connect the EN pin to VIN if the enable function is not used. Pulling EN to ground will disable the AOZ1022. Do not leave it open. The voltage on the EN pin must be above 2V to enable the AOZ1022. When voltage on the EN pin falls below 0.6V, the AOZ1022 is disabled. If an appli- cation circuit requires the AOZ1022 to be disabled, an open drain or open collector circuit should be used to interface to the EN pin. Power Good The output of Power-Good is an open drain N-channel MOSFET which supplies an active high power good stage. A pull-up resistor (R3) should connect this pin to a DC power trail with maximum voltage of 6V. The AOZ1022 monitors the FB voltage. When FB voltage is lower than 90% of the normal voltage, N-channel MOSFET turns on and the Power-Good pin is pulled low. This indicates the power is abnormal. Steady-State Operation Under steady-state conditions, the converter operates in fixed frequency and Continuous-Conduction Mode (CCM). The AOZ1022 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 the 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 internal low-side N-MOSFET switch to output. The internal adaptive FET driver guarantees no turn on overlap of both high-side and low-side switch. Comparing with regulators using freewheeling Schottky diodes, the AOZ1022 uses freewheeling NMOSFET to realize synchronous rectification. It greatly improves the converter efficiency and reduces power loss in the low-side switch. The AOZ1022 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 high-side switch to achieve linear regulation mode of operation. The minimum voltage drop from VIN to VO is the load current x DC resistance of MOSFET + DC resistance of buck inductor. It can be calculated by the equation below: where; VO_MAX is the maximum output voltage, VIN is the input voltage from 4.5V to 16V, IO is the output current from 0A to 3A, and RDS(ON) is the on resistance of internal MOSFET, the value is between 97m Ω and 200mΩ depending on input voltage and junction temperature. Switching Frequency The AOZ1022 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 by using a resistor divider network. See the application circuit shown in Figure 1. The resistor divider network includes R1 and R2. Usually, a design is started by picking a fixed R2 value and calculating the required R1 with equation on the next page: VO_MAX VIN IO RDS ON () × – = VO 0.8 1 R1 R2 ------- + ⎝⎠ ⎜⎟ ⎛⎞ × = |
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