ADM3312EARU-REEL7 Datasheet ADM3310EARUZ-REEL7, ADM3312EARUZ, ADM3312EACPZ | P13-P15

Data Sheet ADM3307E/ADM3310E/ADM3311E/ADM3312E/ADM3315E

Rev. J | Page 13 of 20

CIRCUIT DESCRIPTION

The internal circuitry consists mainly of four sections. These

include the following:

• A charge pump voltage converter

• 3.3 V logic to EIA-232 transmitters

• EIA-232 to 3.3 V logic receivers

• 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. 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.

V+ =

TERNA

LLA

02915-024

Figure 24. 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 V

. 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 2V

. This

voltage is used to charge C2. In the absence of any discharge

current, C2 charges up to 2V

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 3V

. This voltage is

then used to charge C3. The voltage inverter is illustrated in

Figure 25.

GND

S10

S11

V– = –(V+)

GND

V+

INTERNAL

OSCILLAT

FROM

VOLTAGE

TRIPLER

02915-025

Figure 25. Charge Pump Voltage Inverter

During the oscillator high phase, S10 and S11 are open, while

S8 and S9 are closed. C3 is charged to 3V

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 V

The V+ and V− supplies may also be used to power external

circuitry if the current requirements are small. See Figure 12 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, that is, the output voltage

is not regulated 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

maintaining 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 virtually zero, so the average current drain under light

load conditions is greatly reduced.

For ADM3310E and ADM3315E, replace with 6.5 V.

For ADM3310E and ADM3315E, replace with 6.25 V.

ADM3307E/ADM3310E/ADM3311E/ADM3312E/ADM3315E Data Sheet

Rev. J | Page 14 of 20

A block diagram of the Green Idle circuit is shown in Figure 26.

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

TRANSCEIVERS

ART/STOP

START/STOP

V+

V–

SHUTDOWN

CHARGE

PUMP

V– VOLTAG

COMPARATOR

WITH 250mV

HYS

TER

ESIS

BAND GAP

VOLTAGE

REFERENCE

V+ VOLTAG

COMPARATOR

WITH 250mV

HYSTERESIS

02915-026

Figure 26. Block Diagram of Green Idle Circuit

The operation of Green Idle for V+ under various load

conditions is illustrated in Figure 27. 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

7.25V

OSC

V+

7.25V

OSC

V+

7.25V

MEDIUM

LOAD

HEAVY

LOAD

FOR ADM3310E AND ADM3315E REPLACE WITH 6.5V.

FOR ADM3310E AND ADM3315E REPLACE WITH 6.25V.

02915-027

Figure 27. 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 Vs. Shutdown

Shutdown mode minimizes power consumption by shutting

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

the voltage tripler are configured so V+ is connected directly to

. 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 Green Idle 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 voltage; 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 accordingly. Like other Analog Devices

RS-232 transceiver products, the ADM3307E/ADM3310E/

ADM3311E/ADM3312E/ADM3315E devices feature slew rate

limiting and other techniques to minimize radiated and

conducted emissions.

For ADM3310E and ADM3315E, replace with 6.5 V.

For ADM3310E and ADM3315E, replace with 6.25 V.

Data Sheet ADM3307E/ADM3310E/ADM3311E/ADM3312E/ADM3315E

Rev. J | Page 15 of 20

Transmitter (Driver) Section

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

levels. With V

= 3.0 V and driving an EIA-232 load, the output

voltage swing is typically ±6.4 V (or ±5.5 V for ADM3310E and

ADM3315E).

Unused inputs may be left unconnected, because 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 grounded, so unused inputs should either be connected to

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 kΩ pull-down resistors (22 kΩ 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 kΩ (or 22 kΩ 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

impedance 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 Receiver R4 and

Receiver 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

because it reduces the power consumption to 66 nW. During

shutdown, the charge pump is also disabled. When exiting

shutdown, the charge pump is restarted and it takes approximately

100 µs for it to reach its steady-state operating conditions.

EN INPUT

– 0.1V

+ 0.1V

RECEIVER

OUTPUT

02915-028

Figure 28. Receiver Disable Timing

0.4V

EN INPUT

RECEIVER

OUTPUT

02915-029

Figure 29. Receiver Enable Timing

High Baud Rate

The ADM3307E/ADM3310E/ADM3311E/ADM3312E/

ADM3315E feature high slew rates, permitting data transmission

at rates well in excess of the EIA/RS-232E specifications. RS-232

voltage levels are maintained at data rates up to 230 kbps (460 kbps

for ADM3307E) under worst-case loading conditions. This allows

for high speed data links between two terminals.

LAYOUT AND SUPPLY DECOUPLING

Because of the high frequencies at which the ADM3307E/

ADM3310E/ADM3311E/ADM3312E/ADM3315E oscillator

operates, particular care should be taken with printed circuit

board layout, with all traces being as short as possible and C1 to

C3 being connected as close to the device as possible. The use of

a ground plane under and around the device is also highly

recommended.

When the oscillator starts up during Green Idle operation, large

current pulses are taken from V

. For this reason, V

should

be decoupled with a parallel combination of 10 µF tantalum and

0.1 µF ceramic capacitors, mounted as close to the V

pin as

possible.

Capacitor C1 to Capacitor C3 can have values between 0.1 µF and

1 µF. Larger values give lower ripple. These capacitors can be either

electrolytic capacitors chosen for low equivalent series resistance

(ESR) or nonpolarized types, but the use of ceramic types is highly

recommended. If polarized electrolytic capacitors are used, polarity

must be observed (as shown by C1+).

ESD/EFT TRANSIENT PROTECTION SCHEME

The ADM3307E/ADM3310E/ADM3311E/ADM3312E/

ADM3315E use protective clamping structures on all inputs and

outputs that clamp the voltage to a safe level and dissipate the

energy present in ESD (electrostatic) and EFT (electrical fast

transients) discharges. A simplified schematic of the protection

structure is shown in Figure 30 and Figure 31 (see Figure 32 and

Figure 33 for ADM3307E protection structure).

Each input and output contains two back-to-back high speed

clamping diodes. During normal operation with maximum RS-232

signal levels, the diodes have no effect as one or the other is reverse

biased depending on the polarity of the signal. If, however, the

voltage exceeds about ±50 V, reverse breakdown occurs and the

voltage is clamped at this level. The diodes are large p-n junctions

designed to handle the instantaneous current surge that can exceed

several amperes.

P1-P3 P4-P6 P7-P9 P10-P12 P13-P15 P16-P18 P19-P20

ADM3312EARU-REEL7

Mfr. #:

Buy ADM3312EARU-REEL7

Manufacturer:

Analog Devices Inc.

Description:

RS-232 Interface IC 15kV ESD, +2.7V TO 3.6V S/P TRANS I.C.

Lifecycle:

New from this manufacturer.

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ADM3312EARU-REEL7

ADM3312EARU-REEL7

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