SP507 datasheet(20/30 Pages) SIPEX

20 / 30 page

SP505/6/7APN/03

SP505, SP506, SP507 Application Note

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 22

µF pump

capacitors. However, the SP505, SP506, and SP507

can use 10

µF 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

and V

OL levels, and output ripple.

Figure 32 shows the typical driver output (TxD in

this case) in an unloaded condition using 10

µF charge

pump capacitors. Figure 33 shows the same driver

Figure 32. Unloaded Driver Output Using 10

µF Pump

Capacitors

but loaded with 3k

Ω 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 22

µF charge pump capacitors where as

Figure 35 uses 10

µF 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 33. Driver Output Loaded w/ 3k

Ω // 2,500pF

Using 10

µF Pump Capacitors

Figure 34. Charge Pump Ripple of Driver Output

w/ 22

µF Pump Capacitors

Figure 35. Charge Pump Ripple of Driver Output

w/ 10

µF Pump Capacitors

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