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ISL8563IB-T Datasheet(PDF) 6 Page - Intersil Corporation |
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ISL8563IB-T Datasheet(HTML) 6 Page - Intersil Corporation |
6 / 10 page 6 Capacitor Selection The charge pumps operate with 0.1 µF (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 µF 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. 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 Ω in parallel with 2500pF. Note that the transmitters enable only when the magnitude of the supplies exceed approximately 3V. High Data Rates The ISL8563 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 ISL8563 TRANSITION RX TX R2OUT T1IN VCC VCC TO R2IN T1OUT VOUT =HI-Z POWERED SHDN = GND, EN =VCC DETECTOR DOWN UART WAKE-UP LOGIC FIGURE 7. TRANSMITTER LOOPBACK TEST CIRCUIT TIME (20 µs/DIV.) T1 T2 2V/DIV. 5V/DIV. VCC = +3.3V SHDN FIGURE 6. TRANSMITTER OUTPUTS WHEN EXITING POWERDOWN C1 - C4 = 0.1 µF ISL8563 VCC C1 C2 C4 C3 + + + + 1000pF V+ V- 5K TIN ROUT C1+ C1- C2+ C2- RIN TOUT + VCC 0.1 µF VCC EN SHDN 400k Ω VCC ISL8563 |
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