TCA9800 Datasheet, PDF (14/36 Pages) Texas Instruments – Level-Translating I2C Bus Buffer/Repeater

TCA9800
SCPS264 – MARCH 2017
www.ti.com
9.4.1.1.2 R_ILC
The R_ILC parameter describes the maximum allowed pull-down resistance. This parameter comes from the
combination of I_ILC and I_CS, and states the maximum resistance that can satisfy the I_ILC parameter. Note that
series resistors on the bus are going to affect this, as seen with other types of buffers (voltage delta across the
series resistor. This increases the effective V_OL of the external device pulling the bus low).
The calculated resistance of the internal pull-down FET of an external device can be calculated from the V_OL and
I_OL measurements of the external device in question using Equation 1. Take care to consider any series resistors
placed in the path from the TCA980x to any external device. Note that R_PD is the calculated resistance of the
internal pull-down FET, and not a resistor to ground. This is for determining if the external device's output
characteristics meet the TCA980x R_ILC requirement (150 Ω).
R_PD V_OL / I_OL
(1)
9.4.1.2 Input and Output Leakage Current (I_EXT-I/I_EXT-O)
The Input external current (I_EXT-I) and output external current (I_EXT-O) parameters describe the amount of parasitic
current either injected into the device or pulled from the device (such as leakage from ESD cells) without
affecting device operation as shown in Table 4.
PARAMETER
I_EXT-I Input leakage current
I_EXT-O Output leakage current
Table 4. B-Side Input and Output Leakage Current
SHORT DESCRIPTION
Current that is external, but pulled up to supply, leaking current into the TCA980x
B-side. An example is a leaky ESD cell from VCC, or an external pull-up resistor.
Current that is pulled from the TCA980x B-side. ESD cells are the most common
form of output leakage. Care must be taken not to violate this spec, otherwise the
leakage current can create a false low.
DETAILED
INFORMATION
See the I_EXT-I
section
See the I_EXT-O
section
9.4.1.2.1 I_EXT-I
I_EXT-I is a current source that is external to the TCA980x B-side, but leaks current into the device. This type of
input leakage may not exceed the I_EXT-I maximum spec, or else the minimum I_ILC value does not apply.
TCA980x
V_CCB
I²C Slave
I_CS
A-Side
B-Side
Figure 13. I_EXT-I Example
As shown in Figure 13, I_EXT-I is a source of additional current into the device, shown as a leaky ESD cell. The
user must keep I_EXT-I as close to 0 as possible, since the TCA980x has a current source as a pull-up internally,
and uses this current source to help detect which side is driving a low. As I_EXT-I increases, it increases the
minimum I_ILC value, requiring that an external device sink more current from the TCA980x in order to transmit a
low. There must be no external pull-up resistor on the B-side to contribute to I_EXT-I.
14
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