LTM2887 datasheet(23/38 Pages) LINER

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LTM2887

2887fb

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The isolated side bidirectional serial data pin, SDA2,

simplified schematic is shown in Figure 11. An internal

1.8mA current source provides a pull-up for SDA2. Do not

connect any other pull-up device to SDA2. This current

source is sufficient to satisfy the system requirements for

bus capacitances greater than 200pF in FAST mode and

greater than 400pF in STANDARD mode.

APPLICATIONS INFORMATION

CBUS (pF)

100

2887 F12

1000

V = 3V

V = 3.3V

V = 3.6V

V = 4.5V TO 5.5V

CBUS (pF)

100

2887 F13

1000

V = 3V

V = 3.3V

V = 3.6V

V = 4.5V TO 5.5V

Figure 12. Maximum Standard Speed Pull-Up Resistance on SDA

Figure 13. Maximum Fast Speed Pull-Up Resistance on SDA

Additional proprietary circuitry monitors the slew rate on

the SDA and SDA2 signals to manage directional control

across the isolation barrier. Slew rates on both pins must

be greater than 1V/µs for proper operation.

The logic side bidirectional serial data pin, SDA, requires a

pull-up resistor or current source connected to VL. Follow

the requirements in Figures 12 and 13 for the appropri-

ate pull-up resistor on SDA that satisfies the desired rise

time specifications and VOL maximum limits for FAST and

STANDARD modes. The resistance curves represent the

maximum resistance boundary; any value may be used

to the left of the appropriate curve.

The isolated side clock pin, SCL2, has a weak push-pull

output driver; do not connect an external pull-up device.

SCL2 is compatible with I2C devices without clock stretch-

ing. On lightly loaded connections, a 100pF capacitor

form SCL2 to GND2 or RC lowpass filter (R = 500Ω,

C = 100pF) can be used to increase the rise and fall times

and minimize noise.

Some consideration must be given to signal coupling

between SCL2 and SDA2. Separate these signals on a

printed circuit board or route with ground between. If

these signals are wired off board, twist SCL2 with VCC2

and/or GND2 and SDA2 with GND2 and/or VCC2, do not

twist SCL2 and SDA2 together. If coupling between SCL2

and SDA2 is unavoidable, place the aforementioned RC

filter at the SCL2 pin to reduce noise injection onto SDA2.

Figure 11. Isolated SDA2 Pin Schematic

2887 F11

SDA2

1.8mA

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