SP334CT-L-TR Datasheet, PDF (9/14 Pages) Exar Corporation – Programmable RS-232/RS-485 Transceiver

to ground and transfers the generated l0V
across C₂ to C₄, the V_DD storage capacitor.
Again, simultaneously with this, the positive
side of capacitor C₁ is switched to +5V and
the negative side is connected to ground,
and the cycle begins again.
Since both V+ and V^– are separately gen-
erated from
and V^– will
V_CC in a no–load
be symmetrical.
condition, V+
Older charge
pump approaches that generate V^– from
V+ will show a decrease in the magnitude
of V^– compared to V+ due to the inherent
inefficiencies in the design.
The clock rate for the charge pump typically
operates at 15kHz. The external capacitors
must be a minimum of 0.1µF with a 16V
breakdown rating.
External Power Supplies
For applications that do not require +5V only,
external supplies can be applied at the V+
and V^– pins. The value of the external sup-
ply voltages must be no greater than ±l0V.
The current drain for the ±10V supplies is
used for RS-232. For the RS-232 driver the
current requirement will be 3.5mA per driver.
The external power supplies should provide
a power supply sequence of :+l0V, then +5V,
followed by –l0V.
V_CC = +5V
+
C_{1 –}
–5V
+5V
+
C_{2 –}
–5V
C₄
+ – _{VDD Storage Capacitor}
– + _{VSS Storage Capacitor}
C₃
Figure 13. Charge Pump Phase 1.
V_CC = +5V
C₄
+
+
+ – _{VDD Storage Capacitor}
C_{1 –}
C_{2 –}
– + _{VSS Storage Capacitor}
–10V
C₃
Figure 14. Charge Pump Phase 2.
V_CC = +5V
+
C_{1 –}
–5V
+5V
+
C_{2 –}
–5V
C₄
+ – _{VDD Storage Capacitor}
– + _{VSS Storage Capacitor}
C₃
Figure 15. Charge Pump Phase 3.
V_CC = +5V
+10V
C₄
+
+
+ – _{VDD Storage Capacitor}
C_{1 –}
C_{2 –}
– + _{VSS Storage Capacitor}
C₃
Figure 16. Charge Pump Phase 4.
+10V
a) C2+
GND
GND
b) C2-
-10V
Figure 17. Charge Pump Waveforms
Exar Corporation 48720 Kato Road, Fremont CA, 94538 • 510-668-7017 • www.exar.com

SP334_100_090909