SP385ADS/05

SP385A Line Driver/Receiver

© Copyright 2000 Sipex Corporation

6

Charge Pump

The charge pump section of the these devices

allows the circuit to operate from a single +5V

±10% power supply by generating the required

operating voltages internal to the devices. The

charge pump consists of two sections — 1) a

voltage doubler and 2) a voltage inverter.

As shown in Figure 1, an internal oscillator trig-

gers the charge accumulation and voltage inver-

sion. The voltage doubler momentarily stores a

charge on capacitor C

ground. During the next transition of the oscillator

this charge is boot-strapped to transfer charge to

capacitor C

V

In the inverter section (Figure 2), the voltage

across C

transfer charge to C

One of the significant enhancements over previ-

ous products of this type is that the values of the

capacitors are no longer critical and have been

decreased in size considerably to 0.1

µF. Because

the charge pump runs at a higher frequency, the

0.1

µF capacitors are sufficient to transfer and

sustain charges to the two transmitters.

APPLICATION HINTS

Protection From Shorts to

±15V

The driver outputs are protected against shorts

the possibility exists that the outputs could be

inadvertently connected to voltages higher than

±15V, then it is recommended that external

protection be provided. For protection against

voltages exceeding

±15V,twoback-to-backzener

diodes connected from each output to ground will

clamp the outputs to an acceptable voltage level.

Shutdown (ON/OFF)

The SP385A has a shut-down/standby mode to

conserve power in battery-powered systems. To

activate the shutdown mode, which stops the op-

eration of the charge pump, a logic “0” is applied

to the appropriate control line. The shutdown

mode is controlled on the SP385A by a logic “0”

on the ON/OFF control line (pin 18 for the SOIC

and pin 20 for the SSOP packages); this puts the

transmitter outputs in a tri–state mode.

Figure 1. Charge Pump Voltage Doubler

Figure 2. Charge Pump Voltage Inverter

S1

GND

V

CC

+

V+ = 2V

CC

+

V

CC

S2

S3

S4

C1

C3

INTERNAL

OSCILLATOR

S1

FROM

VOLTAGE DOUBLER

V+

+

V- = -(V+)

+

S2

S3

S4

C2

C4

INTERNAL

OSCILLATOR

GND

GND