REV. G

–12–

ADM3307E/ADM3310E/ADM3311E/ADM3312E/ADM3315E

The operation of Green Idle for V+ under various load conditions

is illustrated in Figure 4. Under light load conditions, C1 is

maintained in a charged condition, and only a single oscillator

pulse is required to charge up C2. Under these conditions, V+

may actually overshoot 7.25 V

** slightly.

OSC

V+

OVERSHOOT

LIGHT

LOAD

MEDIUM

LOAD

HEAVY

LOAD

OSC

V+

OSC

V+

NOTES

1FOR ADM3310E AND ADM3315E REPLACE WITH 6.5V.

2FOR ADM3310E AND ADM3315E REPLACE WITH 6.25V.

Figure 4. Operation of Green Idle under Various Load

Conditions

Under medium load conditions, it may take several cycles for C2

to charge up to 7.25 V

**. The average frequency of the oscillator

is higher because there are more pulses in each burst and the

bursts of pulses are closer together and more frequent.

Under high load conditions, the oscillator is on continuously if

the charge pump output cannot reach 7.25 V

**.

Green Idle Versus Shutdown

Shutdown mode minimizes power consumption by shutting down

the charge pump altogether. In this mode, the switches in the

V– is zero because there is no charge pump operation to charge C5.

This means there is a delay when coming out of Shutdown mode

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 in

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 frequency

decreases the ripple voltage; decreasing the oscillator frequency

increases it. Increasing the load current increases the ripple volt-

age; decreasing the load current decreases it. The ripple voltage

at light loads is naturally 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 nominally 7 V

*

and 7.25 V

**, so the ripple is 250 mV under most load conditions.

With very light load conditions, there may be some overshoot

above 7.25 V

**, so the ripple is slightly greater. Under heavy load

conditions where the output never reaches 7.25 V

**, the Green Idle

circuit is inoperative and the ripple voltage is determined by the load

current, the same as in a normal charge pump.

What about Electromagnetic Compatibility?

Green Idle does not operate with a constant oscillator frequency.

As a result, the frequency and spectrum of the oscillator signal vary

with load. Any radiated and conducted emissions also vary accord-

ingly. Like other Analog Devices RS-232 transceiver products, the

ADM33xxE devices feature slew rate limiting and other techniques

to minimize radiated and conducted emissions.

Transmitter (Driver) Section

The drivers convert 3.3 V logic input levels into EIA-232 output

voltage swing is typically

±6.4 V (or ±5.5 V for ADM3310E

and ADM3315E).

Unused inputs may be left unconnected, as an internal 400 kV

pull-up resistor pulls them high forcing the outputs into a low

state. The input pull-up resistors typically source 8 mA when

or left unconnected in order to minimize power consumption.

Receiver Section

The receivers are inverting level shifters that accept RS-232 input

levels and translate them into 3.3 V logic output levels. The inputs

have internal 5 k

W pull-down resistors (22 kW for the ADM3310E)

to ground and are also protected against overvoltages of up to

±30 V.

Unconnected inputs are pulled to 0 V by the internal 5 k

W (or

22 k

W for the ADM3315E) pull-down resistor. This, therefore,

results in a Logic 1 output level for unconnected inputs or for

inputs connected to GND.

The receivers have Schmitt trigger inputs with a hysteresis level

of 0.14 V. This ensures error-free reception for both noisy inputs

and for inputs with slow transition times.

ENABLE AND SHUTDOWN

The enable function is intended to facilitate data bus connections

where it is desirable to three-state the receiver outputs. In the

disabled mode, all receiver outputs are placed in a high imped-

ance state. The shutdown function is intended to shut the device

down, thereby minimizing the quiescent current. In shutdown,

all transmitters are disabled. All receivers are shut down, except

for receiver R3 (ADM3307E, ADM3312E, and ADM3315E),

receiver R5 (ADM3311E), and receivers R4 and R5 (ADM3310E).

Note that disabled transmitters are not three-stated in shutdown,

so it is not permitted to connect multiple (RS-232) driver outputs

together.

The shutdown feature is very useful in battery-operated systems

since it reduces the power consumption to 66 nW. During shut-

down, the charge pump is also disabled. When exiting shutdown,

the charge pump is restarted and it takes approximately 100

ms

for it to reach its steady state operating conditions.

NOTES

*For ADM3310E and ADM3315E, replace with 6.25 V.

**For ADM3310E and ADM3315E, replace with 6.5 V.