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PCA9545ABS Datasheet(PDF) 8 Page - NXP Semiconductors |
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PCA9545ABS Datasheet(HTML) 8 Page - NXP Semiconductors |
8 / 27 page 9397 750 14311 © Koninklijke Philips Electronics N.V. 2005. All rights reserved. Product data sheet Rev. 03 — 3 March 2005 8 of 27 Philips Semiconductors PCA9545A 4-channel I2C switch with interrupt logic and reset 7.4 Power-On Reset When power is applied to VDD, an internal Power-On Reset (POR) holds the PCA9545A in a reset condition until VDD has reached VPOR. At this point, the reset condition is released and the PCA9545A registers and I2C-bus state machine are initialized to their default states—all zeroes—causing all the channels to be deselected. Thereafter, VDD must be lowered below 0.2 V to reset the device. 7.5 Voltage translation The pass gate transistors of the PCA9545A are constructed such that the VDD voltage can be used to limit the maximum voltage that will be passed from one I2C-bus to another. Figure 7 shows the voltage characteristics of the pass gate transistors (note that the graph was generated using the data specified in Section 11 “Static characteristics” of this data sheet). In order for the PCA9545A to act as a voltage translator, the Vo(sw) voltage should be equal to, or lower than the lowest bus voltage. For example, if the main bus was running at 5 V, and the downstream buses were 3.3 V and 2.7 V, then Vo(sw) should be equal to or below 2.7 V to effectively clamp the downstream bus voltages. Looking at Figure 7, we see that Vo(sw)(max) will be at 2.7 V when the PCA9545A supply voltage is 3.5 V or lower, so the PCA9545A supply voltage could be set to 3.3 V. Pull-up resistors can then be used to bring the bus voltages to their appropriate levels (see Figure 14). More Information can be found in Application Note AN262: PCA954X family of I2C/SMBus multiplexers and switches. (1) maximum (2) typical (3) minimum Fig 7. Pass gate voltage versus supply voltage VDD (V) 2.0 5.5 4.5 3.0 4.0 002aaa964 3.0 2.0 4.0 5.0 Vo(sw) (V) 1.0 3.5 5.0 2.5 (1) (2) (3) |
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