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UC3842ADR2 Scheda tecnica(PDF) 10 Page - ON Semiconductor |
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UC3842ADR2 Scheda tecnica(HTML) 10 Page - ON Semiconductor |
10 / 18 page UC3842A, UC3843A, UC2842A, UC2843A http://onsemi.com 10 PIN FUNCTION DESCRIPTION Pin Function Description 8−Pin 14−Pin 1 1 Compensation This pin is Error Amplifier output and is made available for loop compensation. 2 3 Voltage Feedback This is the inverting input of the Error Amplifier. It is normally connected to the switching pow- er supply output through a resistor divider. 3 5 Current Sense A voltage proportional to inductor current is connected to this input. The PWM uses this infor- mation to terminate the output switch conduction. 4 7 RT/CT The Oscillator frequency and maximum Output duty cycle are programmed by connecting resistor RT to Vref and capacitor CT to ground. Operation to 500 kHz is possible. 5 − GND This pin is the combined control circuitry and power ground (8−pin package only). 6 10 Output This output directly drives the gate of a power MOSFET. Peak currents up to 1.0 A are sourced and sunk by this pin. 7 12 VCC This pin is the positive supply of the control IC. 8 14 Vref This is the reference output. It provides charging current for capacitor CT through resistor RT. − 8 Power Ground This pin is a separate power ground return (14−pin package only) that is connected back to the power source. It is used to reduce the effects of switching transient noise on the control circuitry. − 11 VC The Output high state (VOH) is set by the voltage applied to this pin (14−pin package only). With a separate power source connection, it can reduce the effects of switching transient noise on the control circuitry. − 9 GND This pin is the control circuitry ground return (14−pin package only) and is connected back to the power source ground. − 2,4,6,13 NC No connection (14−pin package only). These pins are not internally connected. Undervoltage Lockout Two undervoltage lockout comparators have been incorporated to guarantee that the IC is fully functional before the output stage is enabled. The positive power supply terminal (VCC) and the reference output (Vref) are each monitored by separate comparators. Each has built−in hysteresis to prevent erratic output behavior as their respective thresholds are crossed. The VCC comparator upper and lower thresholds are 16 V/10 V for the UCX842A, and 8.4 V/7.6 V for the UCX843A. The Vref comparator upper and lower thresholds are 3.6V/3.4 V. The large hysteresis and low startup current of the UCX842A makes it ideally suited in off−line converter applications where efficient bootstrap startup techniques are required (Figure 34). The UCX843A is intended for lower voltage dc to dc converter applications. A 36 V zener is connected as a shunt regulator form VCC to ground. Its purpose is to protect the IC from excessive voltage that can occur during system startup. The minimum operating voltage for the UCX842A is 11 V and 8.2 V for the UCX843A. Output These devices contain a single totem pole output stage that was specifically designed for direct drive of power MOSFETs. It is capable of up to ±1.0 A peak drive current and has a typical rise and fall time of 50 ns with a 1.0 nF load. Additional internal circuitry has been added to keep the Output in a sinking mode whenever an undervoltage lockout is active. This characteristic eliminates the need for an external pull−down resistor. The SOIC−14 surface mount package provides separate pins for VC (output supply) and Power Ground. Proper implementation will significantly reduce the level of switching transient noise imposed on the control circuitry. This becomes particularly useful when reducing the Ipk(max) clamp level. The separate VC supply input allows the designer added flexibility in tailoring the drive voltage independent of VCC. A zener clamp is typically connected to this input when driving power MOSFETs in systems where VCC is greater than 20 V. Figure 26 shows proper power and control ground connections in a current sensing power MOSFET application. Reference The 5.0 V bandgap reference is trimmed to ±1.0% tolerance at TJ = 25°C on the UC284XA, and ± 2.0% on the UC384XA. Its primary purpose is to supply charging current to the oscillator timing capacitor. The reference has short circuit protection and is capable of providing in excess of 20 mA for powering additional control system circuitry. |
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