Datasheet LTC4303 (Analog Devices) - 8

LTC4303
U U W U APPLICATIO S I FOR ATIO Resistor Pull-Up Selection
In most applications the LTC4303 will be used with a staggered connector where V The system pull-up resistors must be strong enough CC and GND will be long pins. SDA and SCL are medium length pins to ensure that to provide a positive slew rate of 0.8V/µs on the SDA the V and SCL pins, in order to activate the rise time accelera- CC and GND pins make contact fi rst. This will allow the precharge circuitry to be activated on SDA and SCL tors during rising edges. Choose maximum resistor value before they make contact. ENABLE is a short pin that is RPULL-UP(MAX) using the formula: pulled down when not connected. This is to ensure that where VBUSMIN is the minimum operating pull-up sup- the connection between the backplane and the cards data ply voltage, and CBUS the total capacitance on respec- and clock busses is not enabled until the transients as- tive bus line. sociated with live insertion have settled. V – 0.8V • 1250 ns [ /V ( BUSMIN ( ) ] ) R k Figure 3 shows the LTC4303 in a CompactPCITM confi gu- PULLUP MAX ( )[ Ω] = C p [ F BUS ] ration. Connect VCC and ENABLE to the output of one of the CompactPCI power supply Hot Swap circuits. Use a For example, assume VBUS = VCC = 3.3V, and assuming pull-up resistor to ENABLE for a card side enable/disable. ±10% supply tolerance, VBUSMIN = 2.97V. With CBUS = VCC is monitored by a fi ltered UVLO circuit. With the VCC 100pF, RPULL-UP, MAX = 27.1k. Therefore a smaller pull-up voltage powering up after all the other pins have established resistor than 27.1k must be used, so 10k works fi ne. connection, the UVLO circuit ensures that the backplane
Live Insertion and Capacitance Buffering Application
and the card data and clock busses are not connected until the transients associated with live insertion have settled. Figures 3 through 6 illustrate applications of the LTC4303 Owing to their small capacitance, the SDAIN and SCLIN that take advantage of both its Hot SwapTM controlling and pins cause minimal disturbance on the backplane busses capacitance buffering features. In all of these applications, when they make contact with the connector. note that if the I/O cards were plugged directly into the backplane without the LTC4303 buffer, all of the backplane Figure 4 shows the LTC4303 in a PCI application where all and card capacitances would add directly together, making of the pins have the same length. In this case, a RC fi lter rise- and fall-time requirements diffi cult to meet. Placing a circuit on the I/O card with a product of 10ms provides LTC4303 on the edge of each card, however, isolates the a fi lter to prevent the LTC4303 from becoming activated card capacitance from the backplane. For a given I/O card, until the transients associated with live insertion have the LTC4303 drives the capacitance on the card side and settled. Connect the capacitor between ENABLE and GND, the backplane must drive only the digital input capacitance and the resistor from VCC to ENABLE. of the LTC4303, which is less than 10pF. Hot Swap is a trademark of Linear Technology Corporation. 4303fb 8