SP385A Datasheet, PDF (6/9 Pages) Sipex Corporation – +3V to +5V RS-232 Line Driver/Receiver

S1
V_CC
S2
GND
INTERNAL
OSCILLATOR
S3
+
C1
S4
+
C3
V+ = 2V_CC
V_CC
S1
V+
FROM
VOLTAGE DOUBLER _S2
GND
INTERNAL
OSCILLATOR
S3
+
C2
S4
+
C4
GND
V- = -(V+)
Figure 1. Charge Pump Voltage Doubler
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 C₁ equal to V_cc, referenced to
ground. During the next transition of the oscillator
this charge is boot-strapped to transfer charge to
Vca_c^p_c^atcoitVor+.C^{3. The voltage across C3 is now from}
In the inverter section (Figure 2), the voltage
ttaorcarnVoss⁺_fseaCrcc³rhoisastrsrgaCen_{2ts.ofeBCror4}^{eodtto-tsogtreCanp2}_{rpfaotienrcgVino-}^g.f
a range of 0V
C₂ will then
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 0.1µF. Because
the charge pump runs at a higher frequency, the
0.1µF 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
to ground, other driver outputs, and V⁺ or V^-. If
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
Figure 2. Charge Pump Voltage Inverter
voltages exceeding ±15V, two back-to-back zener
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.
SP385ADS/05
SP385A Line Driver/Receiver
6
© Copyright 2000 Sipex Corporation