ADM3311 AD [Analog Devices], ADM3311 Datasheet - Page 7

• Manufacturer Part Number: ADM3311
• Manufacturer: AD [Analog Devices]

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.
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.
SHUTDOWN
START/STOP
START/STOP
COMPARATOR
COMPARATOR
V+ VOLTAGE
HYSTERESIS
V– VOLTAGE
WITH 250mV
HYSTERESIS
WITH 250mV
CHARGE
PUMP
V+
V–
REFERENCE
TRANSCEIVERS
BANDGAP
VOLTAGE
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 V
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.
V+
OSC
V+
OSC
V+
OSC
7.25V
7.25V
7.25V
7V
7V
7V
CC
OVERSHOOT
. V– is zero because there is no charge pump
MEDIUM LOAD
HEAVY LOAD
LIGHT LOAD
ADM3311E