SP505AN Datasheet, PDF (20/30 Pages) Sipex Corporation – Multi-Protocol Serial Transceivers

Using Smaller Charge Pump
Capacitors with the SP50x
The charge pump of the SP505, SP506, and SP507
have been designed to drive the RS-232 voltage
levels through the drivers using 22µF pump
capacitors. However, the SP505, SP506, and SP507
can use 10µF capacitors for operation while still
maintaining the critical specifications.
There are two issues involved with lowering the
charge pump capacitors; RS-232 driver output V_OH
and V_OL levels, and output ripple.
Figure 32 shows the typical driver output (TxD in
this case) in an unloaded condition using 10µF charge
pump capacitors. Figure 33 shows the same driver
but loaded with 3kΩ and 2,500pF to ground.
Running at a worse case speed of 120kHz, the driver
output voltages shown in Figure 34 clearly comply
with the RS-232 and ITU-T V.28 specifications
under these conditions.
Figures 34 and 35 show the driver output's ripple
when a DC input is asserted. The ripple in Figure 34
uses 22µF charge pump capacitors where as
Figure 35 uses 10µF capacitors. The ripple amplitude
is increased from approximately 60mV to 400mV.
Although the RS-232 voltages are within the
specifications and the ripple amplitude is negligible
compared to the RS-232 signal amplitude, the
designer should examine the EMC consequences of
reducing the charge pump capacitors.
Figure 32. Unloaded Driver Output Using 10µF Pump
Capacitors
Figure 33. Driver Output Loaded w/ 3kΩ // 2,500pF
Using 10µF Pump Capacitors
Figure 34. Charge Pump Ripple of Driver Output
w/ 22µF Pump Capacitors
Figure 35. Charge Pump Ripple of Driver Output
w/ 10µF Pump Capacitors
SP505/6/7APN/03
SP505, SP506, SP507 Application Note
20
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