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ADM3311E 데이터시트(PDF) 7 Page - Analog Devices |
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ADM3311E 데이터시트(HTML) 7 Page - Analog Devices |
7 / 12 page ADM3311E –7– REV. A GREEN IDLE What Is Green Idle? Green Idle is a method of minimizing power consumption under idle (no transmit) conditions while still maintaining the ability to instantly transmit data. How Does it Work? Charge pump type dc-dc converters used in RS-232 line drivers normally operate open-loop, i.e., the output voltage is not regu- lated in any way. Under light load conditions the output voltage is close to twice the supply voltage for a doubler and three times the supply voltage for a tripler, with very little ripple. As the load current increases, the output voltage falls and the ripple voltage increases. Even under no-load conditions, the oscillator and charge pump are operating at a very high frequency with consequent switch- ing losses and current drain. Green Idle works by monitoring the output voltage and main- taining it at a constant value around 7 V. When the voltage rises above 7.25 V, the oscillator is turned off. When the supply volt- age falls below 7.00 V, the oscillator is turned on and a burst of charging pulses is sent to the reservoir capacitor. When the oscillator is turned off the power consumption of the charge pump is virtually zero, so the average current drain under light load conditions is greatly reduced. A block diagram of the Green Idle circuit is shown in Figure 17. Both V+ and V– are monitored and compared to a reference voltage derived from an on-chip bandgap device. If either V+ or V– fall below 7 V, the oscillator will start up until the voltage rises above 7.25 V. BANDGAP VOLTAGE REFERENCE V+ VOLTAGE COMPARATOR WITH 250mV HYSTERESIS TRANSCEIVERS CHARGE PUMP START/STOP START/STOP V+ V– SHUTDOWN V– VOLTAGE COMPARATOR WITH 250mV HYSTERESIS Figure 17. Block Diagram of Green Idle Circuit The operation of Green Idle for V+ under various load condi- tions is illustrated in Figure 18. Under light load conditions, C1 is maintained in a charged condition and only a single oscillator pulse will be required to charge up C2. Under these conditions V+ may actually overshoot 7.25 V slightly. Under medium load conditions it may take several cycles for C2 to charge up to 7.25 V. The average frequency of the oscillator will be higher because there are more pulses in each burst and the bursts of pulses are closer and more frequent. Under high load conditions, the oscillator will be on continu- ously if the charge pump output cannot reach 7.25 V. OSC 7V 7.25V OSC 7V 7.25V OVERSHOOT LIGHT LOAD MEDIUM LOAD HEAVY LOAD OSC 7V 7.25V V+ V+ V+ Figure 18. Operation of Green Idle Under Various Load Conditions Green Idle vs. Shutdown Shutdown mode minimizes power consumption by shutting down the charge pump altogether. In this condition the switches in the voltage tripler are configured so that V+ is connected directly to VCC. V– is zero because there is no charge pump operation to charge C5. This means there is a delay after com- ing out shutdown before V+ and V– achieve their normal operating voltages. Green Idle maintains the transmitter supply voltages under transmitter idle conditions, so this delay does not occur. Doesn’t It Increase Supply Voltage Ripple? The ripple on the output voltage of a charge pump operating open-loop depends on three factors: the oscillator frequency, the value of the reservoir capacitor and the load current. The value of the reservoir capacitor is fixed. Increasing the oscillator fre- quency will decrease the ripple voltage; decreasing the oscillator frequency will increase it. Increasing the load current will in- crease the ripple voltage; decreasing the load current will de- crease it. The ripple voltage at light loads will naturally be lower than that for high load currents. Using Green Idle, the ripple voltage is determined by the high and low thresholds of the Green Idle circuit. These are nomi- nally 7.00 V and 7.25 V, so the ripple will be 250 mV under most load conditions. With very light load conditions there may be some overshoot above 7.25 V, so the ripple will be slightly greater. Under heavy load conditions where the output never reaches 7.25 V, the Green Idle circuit will be inoperative and the ripple voltage will be determined by the load current, the same as in a normal charge pump. What About Electromagnetic Compatibility? Because Green Idle does not operate with a constant oscillator frequency, the frequency and spectrum of the oscillator signal will vary with load. Any radiated and conducted emissions will also vary accordingly. Like other Analog Devices RS-232 trans- ceiver products, the ADM3311E features slew rate limiting and other techniques to minimize radiated and conducted emissions. The device is characterized for EMC under all load conditions, and is well within the requirements of EN55022/CISPR22. |
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