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LTC4309CGN-PBF

Level Shifting Low Offset Hot Swappable 2-Wire Bus Buffer with Stuck Bus Recovery

Linear Technology 

LTC4309

APPLICATIONS INFORMATION

to the pull-up impedance, multiple LTC4309 buffers can

be used in a single system. This allows the user to divide

the line and device capacitances into more sections with

buffering and meet rise and fall times.

The LTC4309 disconnects when both bus I/O’s are above

0.48V and rising. In systems with large ground bounce,

if many devices are cascaded, the 0.48V threshold can be

exceeded, and the transients associated with the ground

bounce can appear to be a rising edge. Under this condition,

the LTC4309 with inputs above 0.48V may disconnect.

Level Shifting Applications

Systems requiring different supply voltages for the

backplane side and the card side can use the LTC4309

for bidirectional level shifting, as shown in Figure 6. The

LTC4309 can level shift between bus pull-up supplies as

low as 1.7V, with the accelerators disabled, to as high as

5.5V. Level shifting allows newer designs that require low

voltage supplies, such as EEPROMs and microcontrollers,

the capability to interface with legacy backplanes which

may be operating at higher supply voltages.

Systems with Supply Voltage Droop

In large 2-wire systems, the supply voltages seen by devices

at various points in the system can differ by a few hundred

millivolts or more. For proper operation, make sure that

the VCC2(LTC4309) is ≥ 1.8V, and VCC(LTC4309) ≥ 2.3V.

Additional Pull-Up Supply Options

In typical applications, a pull-up resistor connected from

the LTC4309’s bus output pins to VCC and bus input pins

to VCC2 or VCC, if VCC2 is grounded, is sufficient. However,

for unique applications, additional flexibility is available for

bus pull-up supplies other than VCC or VCC2. One example

is shown in Figure 8. The expanded bus pull-up range is

dependent on the user configuration of the rise time ac-

celerators and the supply voltage, VCC.

If the rise time accelerators are enabled, the bus pull-up

supply can be greater than or equal to VCC for the output

busses and accordingly, the input pull-up supply can

be greater than or equal to VCC2 for the input busses.

This ensures the LTC4309’s rise time accelerators do

not source current through the pull-up resistors into the

pull-up supply. If the rise time accelerator circuitries are

disabled, the bus pull-up supply can be as low as 2V for

VCC ≥ 2.9V and for VCC < 2.9V, the bus pull-up supply can

be as low as 1.7V. The bound on the lower supply limit

exists to ensure the bus signal range exceeds the logic

input threshold voltage, VTHR.

Resistor Pull-Up Value Selection

To guarantee the rise time accelerators are activated during

a rising edge, the bus must rise on its own with a positive

slew rate of at least 0.8V/μs. To achieve this, choose a

maximum resistor value RPULLUP using the formula:

RPULLUP

(VBUS(MIN)

– 0.8V)•

CBUS

1250

ns

V

Where RPULLUP is the pull-up resistor value in kilo ohms,

VBUS(MIN) is the minimum bus pull-up supply voltage and

CBUS is the equivalent bus capacitance in pico-Farads

(pF).

To estimate the value of CBUS, use a general rule of 20pF

of capacitance per device on the bus (10pF for the device

and 10pF for interconnect).

In addition, RPULLUP must be strong enough to overcome

the precharge voltage and provide logic highs on SDAOUT

and SCLOUT for the start-up and connection circuitry to

connect the backplane to the card. Regardless of the bus

capacitance, always choose

RPULLUP

VBUS(MAX) –

100μA

VTHR

4309fa

11

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