SP3508 Datasheet, PDF (25/33 Pages) Sipex Corporation – Rugged 3.3V, 20Mbps, 8 Channel Multiprotocol Transceiver with Programmable DCE/DTE and Termination Resistors

FEATURES
Phase 2
-V_SS1 transfer -Phase two of the clock connects the negative terminal of C2 to the V_SS1 storage capacitor
and the positive terminal of C2 to ground, and transfers the negative generated voltage to C_VSS1. This
generated voltage is regulated to -5.5V. Simultaneously, the positive side of the capacitor C1 is switched
to V_CC and the negative side is connected to ground.
V_CC = +3V
+
C_{1 –}
+
C_{2 –}
–6V
C_VDD
+ – _{VDD Storage Capacitor}
– + _{VSS Storage Capacitor}
C_VSS1
Figure 46. Charge Pump - Phase 2.
Phase 3
-V_DD charge storage -The third phase of the clock is identical to the first phase-the charge transferred in
C1 produces -V_CC in the negative terminal of C1 which is applied to the negative side of the capacitor C2.
Since C2+ is at V_CC, the voltage potential across C2 is 2xV_CC.
V_CC = +3V
+
C_{1 –}
–3V
+3V
+
C_{2 –}
–3V
C_VDD
+ – _{VDD Storage Capacitor}
– + _{VSS1 Storage Capacitor}
C_VSS1
Figure 47.Charge Pump - Phase 3.
Phase 4
-V_DD transfer -The fourth phase of the clock connects the negative terminal of C2 to ground, and transfers
the generated 5.5V across C2 to C_VDD, the V_DD storage capacitor. This voltage is regulated to +5.5V. At
the regulated voltage, the internal oscillator is disabled and simultaneously with this, the positive side of
capacitor C1 is switched to V_CC and the negative side is connected to ground, and the cycle begins again.
The charge pump cycle will continue as long as the operational conditions for the internal oscillator are
present. Since both V+ and V- are separately generated from V_CC; in a no-load condition V+ and V- will
be symmetrical. 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 250kHz. The external capacitors can be as low as 1µF with a 16V breakdown voltage
rating.
Date: 06/14/04
SP3508 Enhanced WAN Multi–Mode Serial Transceiver
25
© Copyright 2004 Sipex Corporation