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ADM3312EARU-REEL データシート(PDF) 11 Page - Analog Devices |
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ADM3312EARU-REEL データシート(HTML) 11 Page - Analog Devices |
11 / 16 page REV. G ADM3307E/ADM3310E/ADM3311E/ADM3312E/ADM3315E –11– CIRCUIT DESCRIPTION The internal circuitry consists mainly of four sections. These include the following: 1. A charge pump voltage converter 2. 3.3 V logic to EIA-232 transmitters 3. EIA-232 to 3.3 V logic receivers 4. Transient protection circuit on all I/O lines Charge Pump DC-to-DC Voltage Converter The charge pump voltage converter consists of a 250 kHz (300 kHz for ADM3307E) oscillator and a switching matrix. The converter generates a ±9 V supply from the input 3.0 V level. This is done in two stages using a switched capacitor technique, as illustrated. First, the 3.0 V input supply is tripled to 9.0 V using capacitor C4 as the charge storage element. The +9.0 V level is then inverted to generate –9.0 V using C5 as the storage element. However, it should be noted that, unlike other charge pump dc-to- dc converters, the charge pump on the ADM3307E does not run open-loop. The output voltage is regulated to ±7.25 V (or ±6.5 V for the ADM3310E and ADM3315E) by the Green Idle circuit and never reaches ±9 V in practice. This saves power as well as maintains a more constant output voltage. + + INTERNAL OSCILLATOR GND C2 C1 S1 S2 S3 S4 VCC + C4 V+ = 3VCC VCC VCC S5 S6 S7 Figure 1. Charge Pump Voltage Tripler The tripler operates in two phases. During the oscillator low phase, S1 and S2 are closed and C1 charges rapidly to VCC. S3, S4, and S5 are open, and S6 and S7 are closed. During the oscillator high phase, S1 and S2 are open, and S3 and S4 are closed, so the voltage at the output of S3 is 2VCC. This voltage is used to charge C2. In the absence of any dis- charge current, C2 charges up to 2VCC after several cycles. During the oscillator high phase, as previously mentioned, S6 and S7 are closed, so the voltage at the output of S6 is 3VCC. This voltage is then used to charge C3. The voltage inverter is illustrated in Figure 2. FROM VOLTAGE TRIPLER + + INTERNAL OSCILLATOR GND C5 C3 S8 S9 S10 S11 V+ GND V– = – (V+) Figure 2. Charge Pump Voltage Inverter During the oscillator high phase, S10 and S11 are open, while S8 and S9 are closed. C3 is charged to 3VCC from the output of the voltage tripler over several cycles. During the oscillator low phase, S8 and S9 are open, while S10 and S11 are closed. C3 is connected across C5, whose positive terminal is grounded and whose negative terminal is the V– output. Over several cycles, C5 charges to –3 VCC. The V+ and V– supplies may also be used to power external circuitry if the current requirements are small. Please refer to TPC 3 in the Typical Performance Characteristics section. What Is Green Idle? Green Idle is a method of minimizing power consumption under idle (no transmit) conditions while still maintaining the ability to transmit data instantly. How Does it Work? Charge pump type dc-to-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 operate at a very high frequency with consequent switching losses and current drain. Green Idle works by monitoring the output voltage and maintain- ing it at a constant value of around 7 V *. When the voltage rises above 7.25 V ** the oscillator is turned off. When the voltage falls below 7 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 virtu- ally 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 3. Both V+ and V– are monitored and compared to a reference voltage derived from an on-chip band gap device. If either V+ or V– fall below 7 V *, the oscillator starts up until the voltage rises above 7.25 V **. V– VOLTAGE COMPARATOR WITH 250mV HYSTERESIS BAND GAP VOLTAGE REFERENCE V+ VOLTAGE COMPARATOR WITH 250mV HYSTERESIS TRANSCEIVERS CHARGE PUMP START/STOP START/STOP V+ V– SHUTDOWN Figure 3. Block Diagram of Green Idle Circuit NOTES *For ADM3310E and ADM3315E, replace with 6.25 V. **For ADM3310E and ADM3315E, replace with 6.5 V. |
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