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SG2526N Datasheet(PDF) 6 Page - Microsemi Corporation |
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SG2526N Datasheet(HTML) 6 Page - Microsemi Corporation |
6 / 9 page Rev 1.1a Copyright © 1994 11861 Western Avenue ∞ ∞ ∞ ∞ ∞ Garden Grove, CA 92841 6 (714) 898-8121 ∞ ∞ ∞ ∞ ∞ FAX: (714) 893-2570 SG1526/SG2526/SG3526 APPLICATION INFORMATION VOLTAGE REFERENCE The reference regulator of the SG1526 is based on a temperature compensated zener diode. The circuitry is fully active at supply voltages above +8 volts., and provides up to 20mA of load current to external circuitry at +5.0 volts. In systems where additional current is required, an external PNP transistor can be used to boost the available current. A rugged low frequency audio-type transistor should be used, and lead lengths between the PWM and transistor should be as short as possible to minimize the risk of oscillations. Even so, some types of transistors may require collector-base capacitance for stability. Up to 1amp of load current can be obtained with excellent regulation if the device selected maintains high current gain. UNDERVOLTAGE LOCKOUT The undervoltage lockout circuit protects the SG1526 and the power devices it controls from inadequate supply voltage. If +V IN is too low, the circuit disables the output drivers and holds the RESET pin LOW. This prevents spurious output pulses while the control circuitry is stabilizing, and holds the soft-start timing capacitor in a discharged state. The circuit consists of a +1.2 volt bandgap reference and comparator circuit which is active when the reference voltage has risen to 3V BE’or 1.8 volts at 25°C. When the reference voltage rises to approximately +4.4 volts, the circuit enables the output drivers and releases the RESET pin, allowing a normal soft-start. The comparator has 200mV of hysteresis to minimize oscillation at the trip point. When +V IN to the PWM is removed and the reference drops to +4.2 volts, the undervolt- age circuit pulls RESET LOW again. The soft-start capacitor is immediately discharged, and the PWM is ready for another soft-start cycle. The SG1526 can operate from a +5 volt supply by connecting the V REF pin to the +V IN pin and maintaining the supply between +4.8 and +5.2 volts. FIGURE 18. SIMPLIFIED UNDERVOLTAGE LOCKOUT SOFT-START CIRCUIT The soft-start circuit protects the power transistors and rectifier diodes from high current surges during power supply turn-on. When supply voltage is first applied to the SG1526, the undervoltage lockout circuit holds RESET LOW with Q3. Q1 is turned on, which holds the soft-start capacitor voltage at zero. The second collector of Q1 clamps the output error amplifier to ground, guaranteeing zero duty cycle at the driver outputs. When the supply voltage reaches normal operating range, RESET will go HIGH. Q1 turns off, allowing the internal 100µA current source to charge C S. Q2 clamps the error amplifier output to 1V BE above the voltage on C S. As the soft-start voltage ramps up to +5 volts, the duty cycle of the PWM linearly increases to whatever value the voltage regulation loop requires for an error null. Figure 10 gives the timing relationship between C S and ramp time to 100% duty cycle. FIGURE 19. SOFT-START CIRCUIT SCHEMATIC FIGURE 20. DIGITAL CONTROL PORT SCHEMATIC DIGITAL CONTROL PORTS The three digital control ports of the SG1526 are bi-directional. Each pin can drive TTL and 5 volt CMOS logic directly, up to a fan-out of 10 low-power Schottky gates. Each pin can also be directly driven by open-collector voltage comparators; fan- in is equivalent to 1 low-power Schottky gate. Each port is normally HIGH; the pin is pulled LOW to activate the particular function. Driving SYNC LOW initiates a discharge cycle in the oscillator. Pulling SHUTDOWN LOW immediately inhibits all PWM output pulses. Holding RESET LOW discharges the soft-start capacitor. The logic threshold is +1.1 volts at 25°C. Noise immunity can be gained at the expense of fan-out with an external 2K pullup resistor to +5 volts. FIGURE 17. EXTENDING REFERENCE OUTPUT CURRENT |
Similar Part No. - SG2526N |
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Similar Description - SG2526N |
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