ADM3311EARS datasheet(13/24 Pages) AD

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Data Sheet

ADM3307E/ADM3310E/ADM3311E/ADM3312E/ADM3315E

Rev. I | Page 13 of 24

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.

GND

VCC

V+ = 3VCC

VCC

INTERNAL

OSCILLATOR

VCC

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 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 discharge

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 25.

GND

S10

S11

V– = –(V+)

GND

V+

INTERNAL

OSCILLATOR

FROM

VOLTAGE

TRIPLER

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 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. 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 V1. When the

voltage rises above 7.25 V2 the oscillator is turned off. When the

voltage falls below 7 V1, 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.

1 For ADM3310E and ADM3315E, replace with 6.5 V.

2 For ADM3310E and ADM3315E, replace with 6.25 V.

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