SP232ACT-L-TR Datasheet, PDF (5/11 Pages) Exar Corporation – Enhanced RS-232 Line Drivers/Receivers

Phase 4
Vdd transfer. The fourth phase connects the
negative terminal of C2 to ground and the posi-
tive terminal of C2 to the Vdd storage capacitor .
This transfers the doubled (V+) voltage onto C3.
Meanwhile, capacitor C1 is charged from Vcc to
prepare it for its next phase.
Under lightly loaded conditions, the intelligent
pump oscillator maximizes efficiency by running
only as needed to maintain V+ and V- voltage
levels. Since interface transceivers spend most
of their time at idle, this power-efficient innova-
tion can greatly reduce total power consump-
tion. This improvement is made possible by
the independent phase sequences of the Exar
charge pump design.
DESCRIPTION
The clock rate of the charge pump typically op-
erates greater than 70kHz allowing the pump to
operate efficiently with small 0.1uF capacitors.
Efficient operation depends on rapid charging
and discharging of C1 and C2. Therefore, the
capacitors should be mounted as close as pos-
sible to the IC and have a low ESR (equivalent
series resistance). Inexpensive, surface mount
ceramic capacitors are ideal for use on charge
pump. If polarized capacitors are used the posi-
tive and negative terminals should be connect-
ed as shown on the typical operating circuit. A
diagram of the 4 individual phases is shown in
Figure 1.
+
V
CC
- _{Phase 2 – Vss transfer from C2 to C4.}
Meanwhile C1 is charged to Vcc
+
V
CC
Phases 1 and 3: Store/Double.
- _{Double charge from C1 onto C2.}
C2 is now charged to -2xVcc
C+
1
+
C
2
e^-
e^-
+
V+
C
3
V- ₊
C
4
C+
1
e^-
+
C+
2
V+
C
3
e^-
e^-
+
C
4
Vss
Patented 5,306,954
C+
1
e^-
+
V
CC
-
Phase 4 VDD transfer from C2 to C3.
Meanwhile C1 is charged to Vcc
V
DD
e⁺
e⁺
+
C+
2
V+
C
3
V- +
C
4
Figure 1. Charge pump phases
Exar Corporation 48720 Kato Road, Fremont CA, 94538 • [phone] • Fax [phone] • www.exar.com

SP232A_100_051308