LTC2946
23
2946fa
For more information www.linear.com/LTC2946
APPLICATIONS INFORMATION
Figure 18. Opto-Isolation of a 1.5kHz I
2
C Interface Between LTC2946-1 and Microcontroller (SCL Omitted for Clarity)
Figure 19. Opto-Isolation of a I
2
C Interface with Low Power, High Speed Opto-Couplers (SCL Omitted for Clarity)
regulator applications mentioned. Figure 18 shows an
alternate connection
for use with low speed opto-couplers
and the LTC2946-1. This circuit uses a limited-current
pull-up on the internally clamped SDAI pin and clamps
the SDAO pin with the input diode of the outgoing opto-
isolator, removing the need to use INTV
CC
for biasing in
the absence of an auxiliary low voltage supply. For proper
clamping:
S(MAX)
SDA,SCL(MIN)
I
SDA,SCL(MAX)
≤ R4 ≤
S(MAX)
SDA,SCL(MAX)
I
SDA,SCL(MIN)
V
S(MAX)
− 5.9V
≤ R4 ≤
V
S(MAX)
− 6.9V
(3)
As an example, a supply that operates from 36V to 72V
would require the value of R4 to be between 13k and 58k.
The LTC2946-1 must be used in this application to ensure
that the SDAO signal polarity is correct.
The LTC2946-1 can also be used with high speed opto-
couplers with push-pull outputs and inverted logic as
shown in Figure 19. The incoming opto-isolator draws
power from the INTV
CC
, and the data output is connected
directly to the SDAI pin with no pull-up required. Ensure
the current drawn does not exceed the 10mA maximum
capability of the INTV
CC
pin. The SDAO pin is connected
to the cathode of the outgoing opto-coupler with a current
limiting resistor connected back to INTV
CC
. An additional
discrete N-channel MOSFET is required at the output of
the outgoing opto-coupler to provide the open-drain pull-
down that the I
2
C bus requires. Finally, the input of the
incoming opto-isolator is connected back to the output
as in the low speed case.
LTC2946-1
SDAI
SDAO
GND
3.3V
GND
SDA
µP
1/2 MOCD207M
1/2 MOCD207M
V
IN
48V
R4
20k
R5
5.6k
R6
0.47k
R7
2k
V
DD
2946 F18
V
IN
48V
1/2 ACPL-064L*
1/2 ACPL-064L*
*CMOS OUTPUT
ISO_SDA
C
1
1µF
C2
1µF
R5
2k
V
DD
INTV
CC
LTC2946-1
SDAO
SDAI
GND
V
CC
V
CC
GND
GND
BS170
Q1
R6
2k
R7
2k
2946 F19
3.3V
GND
SDA
µP
V
DD
