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ISL33002IRTZ Datasheet(PDF) 10 Page - Intersil Corporation |
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ISL33002IRTZ Datasheet(HTML) 10 Page - Intersil Corporation |
10 / 19 page 10 FN7560.2 September 30, 2010 Coming out of UVLO but prior to a valid connection state, the SDA and SCL pins are pre-charged to 1V to allow hot insertion. Because the bus at any time can be between 0V and VCC, pre-charging the I/O pins to 1V reduces the maximum differential voltage from the buffer I/O pin and the active bus. The pre-charge circuitry reduces system disturbance when the IC is hot plugged into a live back plane that may have the bus communicating with other devices. Note - For ISL33001 and ISL33003 with EN pins, the pre-charge circuitry is active only after coming out of UVLO and having the device enabled. Connection Circuitry Once a valid connection condition is met, the buffer is active and the input stage of the SDA/SCL pins is controlled by external drivers. The output of the buffer will follow the input of the buffer. The directionality of the IN/OUT pins are not exclusive (bi-directional operation) and functionally behave identical to each other. Being a two channel buffer, the SDA and SCL pins also behave identically. In addition, the SDA and SCL portions of the buffer are independent from each other. The SDA pins can be driven in one direction while the SCL pins can be driven opposite. Refer to Figure 8 for the operation of the bi-directional buffer. When the input stage of the buffer on one side is driven low by an external device, the output of the buffer drives an open-drain transistor to pull the ‘output’ pin low. The ‘output’ pin will continue to be held low by the transistor until the external driver on the ‘input’ releases the bus. To prevent the buffer from entering a latched condition where both internal transistors are actively pulling the I/O pins low, the buffer is designed to be active in only one direction. The buffer logic circuitry senses which input stage is being externally driven low and sets that buffer to be the active one. For example, referring to Figure 8, if SDA_OUT is externally driven low, buffer U2 will be active and buffer U1 is inactive. M1 is turned on to drive SDA_IN low, effectively buffering the signal from SDA_OUT to SDA_IN. The low signal at the input of U1 will not turn M2 on because U1 remains inactive, preventing a latch condition. Buffer Output Low and Offset Voltage By design, when a logic input low voltage is forced on the input of the buffer, the output of the buffer will have an input to output offset voltage. The output voltage of the buffer is determined by Equation 1: Where VOS is the buffer internal offset voltage, RPull-Up is the pull up resistance on the SDA/SCL pin to VCC and RON is the ON resistance of the buffer’s internal NMOS pull-down device. The last term of the equation is the additional voltage drop developed by sink current and the internal resistance of the transistor. The VOS of the buffer can be determined by Figures 17, 18 and is typically 40mV. Reducing the pull-up resistor values increases the sink current and increases the output voltage of the buffer for a given input low voltage (Figures 15, 16, 17, 18). Rise Time Accelerators The ISL33001, ISL33002, ISL33003 buffer rise time accelerators on the SDA/SCL pins improve the transient performance of the system. Heavy load capacitance or weak pull-up resistors on an Open-Drain bus cause the rise time to be excessively long, which leads to data errors or reduced data rate performance. The rise time accelerators are only active on the low to high transitions and provide an active constant current source to slew the voltage on the pin quickly (Figure 19). The rise time accelerators are triggered immediately after the buffer release threshold (approximately 30% of VCC) on both sides of the buffer is crossed. Once triggered, the accelerators are active for a defined pulse width (Figure 20) with the current source turning off as it approaches the supply voltage. Enable Pin (ISL33001 and ISL33003) When driven high, the enable pin puts the buffer into its normal operating state. After power-up, EN high will activate the bus pre-charge circuitry and wait for a valid connection state to enable the buffer and the accelerator circuitry. Driving the EN pin low disables the accelerators, disables the buffer so that signals on one side of the buffer will be isolated from the other side, disables the pre-charge circuit and places the device in a low power shutdown state. READY Logic Pin (ISL33001 Only) The READY pin is a digital output flag for signaling the status of the buffer. The pin is the drain of an Open-Drain NMOS. Connect a resistor from the READY pin to VCC1 to provide the high pull-up. The recommended value is 10kΩ. When the buffer is disabled by having the EN pin low or if the start-up sequencing is not complete, the READY pin will be pulled low by the NMOS. When the buffer has the EN pin high and a valid connection state is made at the SDA/SCL pins, the READY pin will be pulled high by the pull-up resistor. The READY pin is capable of sinking 3mA when pulled low while maintaining a voltage of less than 0.4V. ACC Accelerator Pin (ISL33002 Only) The ACC logic pin controls the rise time accelerator circuitry of the buffer. When ACC is driven high, the accelerators are enabled and will be triggered when crossing the buffer release threshold. When ACC is driven low, the accelerators are disabled. For lightly loaded buses, having the accelerators active may cause ringing or noise on the rising edge transition. Disabling the accelerators will have the buffers continue V OUT V IN V OS V CC RPULL-UP R ON × ⁄ [] ++ = (EQ. 1) ISL33001, ISL33002, ISL33003 |
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