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MIC4723YML Datasheet(PDF) 10 Page - Micrel Semiconductor |
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MIC4723YML Datasheet(HTML) 10 Page - Micrel Semiconductor |
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10 / 20 page  Micrel, Inc. MIC4723 June 2008 10 M9999-060308-E () L V V D OUT + − The total off time can be calculated as; 2.0MHz D 1 TOFF − = Figure 4. Off-Time Discontinuous Operation Discontinuous operation is when the inductor current discharges to zero during the off cycle. Figure 5 demonstrates the switch voltage and inductor currents during discontinuous operation. Figure 5. Discontinuous Operation When the inductor current (IL) has completely discharged, the voltage on the switch node rings at the frequency determined by the parasitic capacitance and the inductor value. In Figure 5, it is drawn as a DC voltage, but to see actual operation (with ringing) refer to the functional characteristics. Discontinuous mode of operation has the advantage over full PWM in that at light loads, the MIC4723 will skip pulses as nessasary, reducing gate drive losses, drastically improving light load efficiency. Efficiency Considerations Calculating the efficiency is as simple as measuring power out and dividing it by the power in; 100 P P Efficiency IN OUT × = Where input power (PIN) is; IN IN IN I V P × = and output power (POUT) is calculated as; OUT OUT OUT I V P × = The Efficiency of the MIC4723 is determined by several factors. • Rdson (Internal P-channel Resistance) • Diode conduction losses • Inductor Conduction losses • Switching losses Rdson losses are caused by the current flowing through the high side P-channel MOSFET. The amount of power loss can be approximated by; D I R P 2 OUT DSON SW × × = Where D is the duty cycle. Since the MIC4723 uses an internal P-channel MOSFET, Rdson losses are inversely proportional to supply voltage. Higher supply voltage yields a higher gate to source voltage, reducing the Rdson, reducing the MOSFET conduction losses. A graph showing typical Rdson vs input supply voltage can be found in the typical characteristics section of this datasheet. Diode conduction losses occur due to the forward voltage drop (VF) and the output current. Diode power losses can be approximated as follows; () D 1 I V P OUT F D − × × = For this reason, the Schottky diode is the rectifier of choice. Using the lowest forward voltage drop will help reduce diode conduction losses, and improve efficiency. Duty cycle, or the ratio of output voltage to input voltage, determines whether the dominant factor in conduction losses will be the internal MOSFET or the Schottky diode. Higher duty cycles place the power losses on the high side switch, and lower duty cycles place the power |
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