Datasheet

LTC4151
10
4151ff
For more information www.linear.com/LTC4151
Using Opto-Isolators with LTC4151-1 and LTC4151-2
The LTC4151-1/LTC4151-2 split the SDA line into SDAI
(input) and SDAO (LTC4151-1 inverted output) or SDAO
(LTC4151-2 output) for convenience of opto-coupling
with a host controller that sits at a different ground level.
When using opto-isolators with the LTC4151-1, connect
the SDAI to the output of the incoming opto-coupler and
connect the SDAO to the anode of the outgoing opto-
coupler (see Figure 9). With the outgoing opto-coupler
clamping SDAO and internal 6V (5.5V minimum) clamps
on SDAI and SCL, the pull-up resistors on these three pins
can be directly connected to V
IN
. In this way (with SDAO
rather than conventional SDAO), the need for a separate
low voltage supply for pull-ups is eliminated.
Figure 11 shows the LTC4151-2 with high speed opto-
couplers for faster bus speeds. The LTC4151-2 has a non-
inverter SDAO output. Powered from V
IN
, the high voltage
LT3010-5 low dropout regulator provides the supply for the
opto-couplers as well as the bus lines pull-up. If the SDAI
and SDAO on the master controller are not tied together,
the ACK bit of the SDAO must be
returned back to SDAI.
Start and Stop Conditions
When the I
2
C bus is idle, both SCL and SDA must remain
in the high state. A bus master signals the beginning of a
transmission with a Start condition by transitioning SDA
from high to low while SCL stays high. When the master
has finished communicating with the slave, it issues a Stop
condition by transitioning SDA from low to high while SCL
stays high. The bus is then free for another transmission.
Stuck-Bus Reset
The LTC4151 I
2
C interface features a stuck-bus reset
timer. The low conditions of the SCL and the SDA/SDAI
pins are OR’ed to start the timer. The timer is reset when
both SCL and SDA/SDAI are pulled high. If the SCL pin or
the SDA/SDAI pin is held low for over 33ms, the stuck-bus
timer will expire and the internal I
2
C state machine will be
reset to allow normal communication after the stuck-bus
condition is cleared. The stuck-bus timer can be disabled
by clearing control register bit G2.
applicaTions inForMaTion
Figure 9. Opto-Isolation of the I
2
C Interface Between LTC4151-1 and a
Microcontroller (1.5kHz Data Rate of I
2
C is Limited by Slew Rate of Opto-Isolators)
4151 F09
3.3V
R
S
0.02Ω
µ-CONTROLLER
GND
LTC4151-1
SCL
SCL
V
IN
48V
V
IN
V
DD
V
ADIN
SDAI
ADIN
ADR1
R5
0.51k
R6
10k
R7
10k
R4
0.51k
R1
20k
R2
20k
R3
5.1k
18
27
36
45
81
72
63
54
SDA
ADR0
GND
SENSE
+
SENSE
–
SDA0
MOCD207M
MOCD207M