MAX3222CUP+T中文资料(8)

Transceivers Using Four 0.1µF External Capacitors

MAX3222/MAX3232/MAX3237/MAX3241

RS-232 Receivers

The receivers convert RS-232 signals to CMOS-logic out-put levels. The MAX3222/MAX3237/MAX3241 receivershave inverting three-state outputs. In shutdown, thereceivers can be active or inactive (Table 1).

The complementary outputs on the MAX3237 (R1OUTB)and the MAX3241 (R1OUTB, R2OUTB) are always active,regardless of the state of ENor SHDN. This allows forRing Indicator applications without forward biasing otherdevices connected to the receiver outputs. This is idealfor systems where VCCis set to 0V in shutdown toaccommodate peripherals, such as UARTs (Figure 2).

MAX3222/MAX3237/MAX3241

Shutdown Mode

Supply current falls to less than 1µA in shutdown mode(SHDN= low). When shut down, the device’s chargepumps are turned off, V+ is pulled down to VCC, V- ispulled to ground, and the transmitter outputs are dis-abled (high impedance). The time required to exit shut-down is typically 100µs, as shown in Figure 3. ConnectSHDN to VCCif the shutdown mode is not used. SHDNhas no effect on R_OUT or R_OUTB.

MAX3222/MAX3237/MAX3241

Enable Control

The inverting receiver outputs (R_OUT) are put into ahigh-impedance state when ENis high. The complemen-tary outputs R1OUTB and R2OUTB are always active,regardless of the state of ENand SHDN(Table 1). ENhas no effect on T_OUT.

__________Applications Information

Capacitor Selection

The capacitor type used for C1–C4 is not critical forproper operation; polarized or nonpolarized capacitorscan be used. The charge pump requires 0.1µF capaci-tors for 3.3V operation. For other supply voltages, refer toTable 2 for required capacitor values. Do not use valueslower than those listed in Table 2. Increasing the capaci-tor values (e.g., by a factor of 2) reduces ripple on thetransmitter outputs and slightly reduces power consump-tion. C2, C3, and C4 can be increased without changingC1’s value. However, do not increase C1 without alsoincreasing the values of C2, C3, and C4, to maintain theproper ratios (C1 to the other capacitors).

When using the minimum required capacitor values,make sure the capacitor value does not degrade exces-sively with temperature. If in doubt, use capacitors with ahigher nominal value. The capacitor’s equivalent seriesresistance (ESR), which usually rises at low tempera-tures, influences the amount of ripple on V+ and V-.

Figure 2. Detection of RS-232 Activity when the UART andInterface are Shut Down; Comparison of MAX3237/MAX3241(b) with Previous Transceivers (a).8

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