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MIC2182-5.0BSM Datasheet(PDF) 11 Page - Micrel Semiconductor |
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MIC2182-5.0BSM Datasheet(HTML) 11 Page - Micrel Semiconductor |
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11 / 28 page  April 22, 2004 11 M9999-042204 MIC2182 Micrel A block diagram of the MIC2182 PWM current-mode control loop is shown in Figure 3 and the PWM mode voltage and current waveforms are shown in figure 5A. The inductor current is sensed by measuring the voltage across the resistor, R CS. A ramp is added to the amplified current-sense signal to provide slope compensation, which is required to prevent unstable operation at duty cycles greater than 50%. A transconductance amplifier is used for the error amplifier, which compares an attenuated sample of the output voltage with a reference voltage. The output of the error amplifier is the COMP (compensation) pin, which is compared to the current-sense waveform in the PWM block. When the current signal becomes greater than the error signal, the comparator turns off the high-side drive. The COMP pin (pin 3) provides access to the output of the error amplifier and allows the use of external components to stabilize the voltage loop. Skip-Mode Control Loop This control method is used to improve efficiency at light output loads. At light output currents, the power drawn by the MIC2182 is equal to the input voltage times the IC supply current (I Q). At light output currents, the power dissipated by the IC can be a significant portion of the total output power, which lowers the efficiency of the power supply. The MIC2182 draws less supply current in skip mode by disabling portions of the control and drive circuitry when the IC is not switching. The disadvantage of this method is greater output voltage ripple and variable switching frequency. A block diagram of the MIC2182 skip mode is shown in Figure 4. Skip mode voltage and current waveforms are shown in figure 5B. Low Comp Skip-Mode Current Limit 0.07V LOW FORCES PWM MODE –2%VBG VIN VDD Reference 11 14 10 16 15 13 12 8 9 7 MIC2182 [adj.] Skip Mode FB R2 R1 VOUT CSH VDD VBST CIN VIN HSD Q2 Q1 D2 CBST COUT VIN VOUT L1 VSW LSD PGND 4.7 µF V 1.245V R1 R2 OUT =+ 1 Current Sense Amp S R Q CONTROL LOGIC AND SKIP-MODE LOGIC RCS AV = 2 ONE SHOT LOW-SIDE DRIVER ONE SHOT Hysteresis Comp ±1% Figure 4. Skip-Mode Operation |
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