
6
Capacitor Selection
The charge pumps operate with 0.1
F (or greater)
capacitors for 3.0V
≤ VCC ≤ 5.5V. Increasing the capacitor
values (by a factor of 2) reduces ripple on the transmitter
outputs and slightly reduces power consumption. C2, C3, and
C4 can be increased without increasing C1’s value, however,
do not increase C1 without also increasing C2, C3, and C4 to
maintain the proper ratios (C1 to the other capacitors).
When using minimum required capacitor values, make sure
that capacitor values do not degrade excessively with
temperature. If in doubt, use capacitors with a larger nominal
value. The capacitor’s equivalent series resistance (ESR)
usually rises at low temperatures and it influences the
amount of ripple on V+ and V-.
Power Supply Decoupling
In most circumstances a 0.1
F bypass capacitor is
adequate. In applications that are particularly sensitive to
power supply noise, decouple VCC to ground with a
capacitor of the same value as the charge-pump capacitor C1.
Connect the bypass capacitor as close as possible to the IC.
Operation Down to 2.7V
ISL8563E transmitter outputs meet RS-562 levels (±3.7V), at
the full data rate, with VCC as low as 2.7V. RS-562 levels
typically ensure interoperability with RS-232 devices.
Transmitter Outputs When Exiting
Powerdown
Figure 6 shows the response of two transmitter outputs
when exiting powerdown mode. As they activate, the two
transmitter outputs properly go to opposite RS-562/232
levels, with no glitching, ringing, nor undesirable transients.
Each transmitter is loaded with 3k
in parallel with 2500pF.
Note that the transmitters enable only when the magnitude
of the supplies exceed approximately 3V.
High Data Rates
The ISL8563E maintains the RS-232 ±5V minimum
transmitter output voltages even at high data rates. Figure 7
details a transmitter loopback test circuit, and Figure 8
illustrates the loopback test result at 120kbps. For this test,
all transmitters were simultaneously driving RS-232 loads in
parallel with 1000pF, at 120kbps. Figure 9 shows the
loopback results for a single transmitter driving 1000pF and
an RS-232 load at 250kbps. The static transmitters were
also loaded with an RS-232 receiver.
FIGURE 4. POWER DRAIN THROUGH POWERED DOWN
PERIPHERAL
OLD
VCC
POWERED
GND
SHDN = GND
VCC
Rx
Tx
VCC
CURRENT
VOUT = VCC
FLOW
RS-562/232 CHIP
DOWN
UART
FIGURE 5. DISABLED RECEIVERS PREVENT POWER DRAIN
ISL8563E
TRANSITION
RX
TX
R2OUT
T1IN
VCC
TO
R2IN
T1OUT
VOUT = HI-Z
POWERED
SHDN = GND, EN = VCC
DETECTOR
DOWN
UART
WAKE-UP
LOGIC
TIME (20
s/DIV)
T1
T2
2V/DIV
5V/DIV
VCC = +3.3V
SHDN
FIGURE 6. TRANSMITTER OUTPUTS WHEN EXITING
POWERDOWN
C1 - C4 = 0.1
F
ISL8563E