MAX3224E Maxim, MAX3224E Datasheet - Page 13

• Manufacturer Part Number: MAX3224E
• Description: 15kV ESD-Protected / 1A / 1Mbps / 3.0V to 5.5V / RS-232 Transceivers with AutoShutdown Plus
• Manufacturer: Maxim

lower in the IEC1000-4-2 model. Hence, the ESD with-
stand voltage measured to IEC1000-4-2 is generally
lower than that measured using the Human Body
Model. Figure 7a shows the IEC1000-4-2 model and
Figure 7b shows the current waveform for the 8kV,
IEC1000-4-2, Level 4, ESD contact-discharge test.
The air-gap test involves approaching the device with a
charged probe. The contact-discharge method connects
the probe to the device before the probe is energized.
The Machine Model for ESD tests all pins using a
200pF storage capacitor and zero discharge resis-
tance. Its objective is to emulate the stress caused by
contact that occurs with handling and assembly during
manufacturing. Of course, all pins require this protec-
tion during manufacturing, not just RS-232 inputs and
outputs. Therefore, after PC board assembly, the
Machine Model is less relevant to I/O ports.
The capacitor type used for C1–C4 is not critical for
proper operation; polarized or nonpolarized capacitors
can be used. The charge pump requires 0.1µF capaci-
tors for 3.3V operation. For other supply voltages, see
Table 3 for required capacitor values. Do not use val-
ues smaller than those listed in Table 3. Increasing the
capacitor values (e.g., by a factor of 2) reduces ripple
on the transmitter outputs and slightly reduces power
consumption. C2, C3, and C4 can be increased without
changing C1’s value. However, do not increase C1
without also increasing the values of C2, C3, C4,
and C
the other capacitors).
Table 3. Required Minimum Capacitance
Values
±15kV ESD-Protected, 1µA, 1Mbps, 3.0V to 5.5V,
__________Applications Information
3.15 to 3.6
3.0 to 3.6
4.5 to 5.5
3.0 to 5.5
RS-232 Transceivers with AutoShutdown Plus
BYPASS
V
(V)
CC
, to maintain the proper ratios (C1 to
______________________________________________________________________________________
C1, C
0.047
(µF)
0.22
0.22
0.1
BYPASS
Capacitor Selection
Machine Model
C2, C3, C4
0.22
0.33
(µF)
0.1
1
When using the minimum required capacitor values,
make sure the capacitor value does not degrade
excessively with temperature. If in doubt, use capaci-
tors with a larger nominal value. The capacitor’s equiv-
alent series resistance (ESR), which usually rises at low
temperatures, influences the amount of ripple on V+
and V-.
In most circumstances, a 0.1µF V
is adequate. In applications that are sensitive to power-
supply noise, use a capacitor of the same value as
charge-pump capacitor C1. Connect bypass capaci-
tors as close to the IC as possible.
Figure 8 shows two transmitter outputs when exiting
shutdown mode. As they become active, the two trans-
mitter outputs are shown going to opposite RS-232 lev-
els (one transmitter input is high, the other is low). Each
transmitter is loaded with 3kΩ in parallel with 1000pF.
The transmitter outputs display no ringing or undesir-
able transients as they come out of shutdown. Note that
the transmitters are enabled only when the magnitude
of V- exceeds approximately -3V.
Figure 8. Transmitter Outputs when Exiting Shutdown or
Powering Up
5V/div
2V/div
5V/div
0
0
0
V
C1–C4 = 0.1 F
CC
= 3.3V
5 s/div
Power-Supply Decoupling
when Exiting Shutdown
Transmitter Outputs
CC
bypass capacitor
FORCEON = FORCEOFF
T1OUT
T2OUT
READY
13