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AIC3842 Datasheet(PDF) 6 Page - Analog Intergrations Corporation |
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AIC3842 Datasheet(HTML) 6 Page - Analog Intergrations Corporation |
6 / 7 page AIC3842 6 n APPLICATION INFORMATIONS l Undervoltage Lockout There are two separate undervoltage lockout comparators incorporated to make sure that the IC is fully functional before the output stage is enabled. One is for power supply voltage (VCC) and the other is for reference output voltage (VREF). Each has a built in hysteresis to prevent erratic output behavior when their respective thresholds are crossed. For VCC comparator the upper and lower thresholds are 16V and 10V, respectively. For VREF compactor the upper and lower threshold are 3.6V and 3.4V, respectively. The large hysteresis and low start up current (0.3mA) of the AIC3842 make it ideally suited in off-line converter applications where efficient bootstrap startup techniques are required. A 34V zener is connected as a shunt regulator from VCC to ground. Its purpose is to protect the IC from excessive voltage that can occur during system start-up. Reference Output The 5.0V reference output is trimmed to ±2.0% tolerance at T A=25°C. It supplies charging current to the oscillator timing capacitor and is capable of providing current in excess of 20mA for powering additional control system circuitry. In case of overload, the reference is short-circuit protected at about 85mA. Error Amplifier A fully compensated error amplifier is provided with inverting input and output externally accessible. The noninverting input is internally biased at 2.5V. The converter output voltage is usually divided down and connected to the inverting input. The output of the error amplifier is accessible for external loop compensation, with an offset at two diode drops ( ≅1.4V) and divided by three, before connected to the inverting input of the current sense comparator. This guarantees that no drive pulse appears at the output (pin 6). Oscillator The oscillator frequency can be programmed through the setting of timing components R T and C T. Capacitor C T is charged from the 5.0V reference output through R T to about 2.8V and discharged to about 1.2V by the internal discharge current. When C T is discharged the output (pin 6) must be in the low state, thus producing a controlled amount of output deadtime. Note that many values of R T and C T can produce the same frequency but only one combination will yield a specific output deadtime at a given frequency. Current Sense Comparator and PWM Latch The output switch of AIC3842 is initiated by the oscillator and terminated when the peak inductor current reaches the threshold level established by the error amplifier output (pin 1). The AIC3842 is operated at a current mode since the inductor current is monitored cycle-by-cycle and decides the duty cycle. The inductor current is converted to a voltage by inserting the ground referenced sense resistor R S in series with the source of output switch M1. This voltage is monitored by the current sense input (pin 3) and is compared to a level derived from the error amplifier output. In the normal operating conditions the peak inductor current is controlled by the voltage at pin 1 where I V pin 1 1 4V 3RS PK = − ( ) . PWM Latch is used to ensure that only a single pulse appears at the output during any given oscillator cycle. However, a narrow spike on the leading edge of the current waveform can usually be observed and may cause the power supply to exhibit an instability when the output is tightly loaded. Output Switch The AIC3842 contains a single totem-pole output stage that was specifically designed for direct drive of power MOSFET. If any undervoltage lockout is deetected, internal circuitry will keep the output switch in a sinking current mode, no external pull down resistor is needed. |
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