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MIC2291YD5 Datasheet(PDF) 5 Page - Micrel Semiconductor |
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MIC2291YD5 Datasheet(HTML) 5 Page - Micrel Semiconductor |
5 / 9 page August, 2004 5 M9999-081104 MIC2291 Micrel Functional Description The MIC2291 is a constant frequency, PWM current mode boost regulator. The block diagram is shown above. The MIC2291 is composed of an oscillator, slope compensation ramp generator, current amplifier, g m error amplifier, PWM generator, and a 500mA bipolar output transistor. The oscillator generates a 1.2MHz clock. The clock’s two func- tions are to trigger the PWM generator that turns on the output transistor and to reset the slope compensation ramp genera- tor. The current amplifier is used to measure the switch current by amplifying the voltage signal from the internal sense resistor. The output of the current amplifier is summed with the output of the slope compensation ramp generator. This summed current-loop signal is fed to one of the inputs of the PWM generator. Functional Diagram GND VREF PWM Generator Ramp Generator 1.2MHz Oscillator SW EN FB OVP* VIN 95mV *OVP available on MLFTM package option only gm OVP* Σ Figure 1. MIC2291 Block Diagram The g m error amplifier measures the LED current through the external sense resistor and amplifies the error between the detected signal and the 95mV reference voltage. The output of the g m error amplifier provides the voltage-loop signal that is fed to the other input of the PWM generator. When the current-loop signal exceeds the voltage-loop signal, the PWM generator turns off the bipolar output transistor. The next clock period initiates the next switching cycle, maintain- ing the constant frequency current-mode PWM control. The LED is set by the feedback resistor: I LED = 95mv R FB The Enable pin shuts down the output switching and disables control circuitry to reduce input current-to-leakage levels. Enable pin input current is zero at zero volts. |
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