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MIC3839 Datasheet(PDF) 8 Page - Micrel Semiconductor |
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MIC3839 Datasheet(HTML) 8 Page - Micrel Semiconductor |
8 / 11 page MIC3838/3839 Micrel MIC3838/3839 8 April 2005 Functional Description The MIC3838/9 is a high-speed power supply controller with push-pull output drive capability. MIC3838 has a higher VDD turn-on threshold and more hysteresis between VDD turn-on and turn-off than the MIC3839. The outputs of the controller operate in a push-pull fashion with a guaranteed dead time between them. A block diagram of the MIC3838/9 controller is shown in Figure 1. Functional Diagram T Q /Q Q S R Q S R Q S R Oscillator VDD OK VDD VDD Ð1V Peak Current Comparator Overcurrent Comparator PWM Comparator PWM Latch Slope = 1V/ms Soft Start 2.2V 2.0V 0.5V R 0.8V 14V VDD OUTA OUTB GND 0.5V 1.2R 0.75V Error Amplifier RC VREF FB COMP RAMP ILIM Voltage Reference 3.65V Figure 1. MIC3838 Block Diagram MIC3838 330pF 330pF VIN = 36V to 75V 50k 150k 13k 4.5k VCOMP 0.5V 0.75V Peak Current Comparator Over Current Comparator PWM Comparator Oscillator VMAX at Ramp Input = 1.5V VMAX at ILIM Input = 0.5V Figure 2. Volt Second Clamp and Voltage Feed Forward Circuit VDD and Turn-on Sequence The oscillator and output gate drive signals are disabled when VDD is lower than the turn on threshold. Circuitry in the output drivers eliminates glitching or random pulsing during the start-up sequence. The oscillator is enabled when VDD is applied and reaches the turn-on threshold. The VDD com- parator also turns off the internal soft-start discharge FET, slowly bringing up the COMP pin voltage. The VDD pin is internally clamped. As VDD approaches this clamp voltage, the VDD current will increase over the normal current draw of the IC. IDD currents greater than 20mA may cause excessive power dissipation in the MIC3838/9. |
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