LTC4310 Просмотр технического описания (PDF) - Linear Technology

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LTC4310

Hot-Swappable I2C Isolators

Linear Technology 

LTC4310-1/LTC4310-2

Applications Information

SDA, SCL Bus Pull-Up Resistor Value Selection

When the SDA (or SCL) bus is rising between 0V and

0.35 • VCC, the LTC4310 controls the bus rise rate to

(0.35 • VCC)/900ns for the LTC4310-1 and to (0.35 • VCC)/

300ns for the LTC4310-2. Users must quantify their

parasitic bus capacitance, CBUS, and choose a bus pull-

up resistor, RBUS, based on their bus pull-up supply

voltage and maximum bus switching frequency to en-

sure that each bus rises faster than the controlled rise

rate. For bus frequencies up to 100kHz, choose the

LTC4310-1 and refer to Figure 2 for the maximum pull-up

resistance to use. For bus frequencies between 100kHz

and 400kHz, choose the LTC4310-2 and refer to Figure 3

for the maximum pull-up resistance to use. Be sure to

include worst-case resistor tolerance when selecting

resistor value.

Rise Time Accelerators

The LTC4310’s rise time accelerator circuitry on the SDA

and SCL lines turns on during rising edges to reduce the

bus rise time. When the bus has risen above 0.45 • VCC, the

LTC4310 turns on a strong, slew-limited pull-up current,

IBOOST, to help even heavily loaded buses meet the rise time

specifications. See the Typical Performance Characteristics

section for the rise time accelerator pull-up current as a

function of temperature and bus capacitance. When either

the bus has risen above (VCC – 1V) or 300ns after the

pull-up current has turned on (whichever comes first), the

LTC4310 deactivates its pull-up current to deter fighting

with the subsequent falling edge. Users must ensure that the

bus pull-up supply voltage VBUS ≥ VCC, so that the accelera-

tors do not overdrive the SDA, SCL bus and source current

into VBUS. The rise time accelerators are deactivated during

start-up, thermal shutdown, shutdown and after disconnec-

tion due to a stuck bus or failure to receive a transmission

within 4.6ms.

18

16

14

VCC = 5V

12

10

VCC = 3.3V

8

6

4

2

0

1

10

100

CBUS(MAX) (pF)

1000

431012 F02

Figure 2. Maximum SDA,SCL Bus Pull-Up Resistor Value as a

Function of Parasitic Bus Capacitance for the LTC4310-1

18

16

14

12

VCC = 5V

10

VCC = 3.3V

8

6

4

2

0

1

10

100

CBUS(MAX) (pF)

1000

431012 F03

Figure 3. Maximum SDA,SCL Bus Pull-Up Resistor Value as

a Function of Parasitic Bus Capacitance for the LTC4310-2

10

431012fa

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