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| TCA6408A-Q1 Datasheet, PDF (28/40 Pages) Texas Instruments – Low-Voltage 8-Bit I2C and SMBus I/O Expander With Interrupt Output | |||
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TCA6408A-Q1
SCPS234 â SEPTEMBER 2016
Typical Application (continued)
3.3 V
5V
www.ti.com
VCC
Px
LED
Figure 33. Device Supplied by a Low Voltage
9.2.2 Detailed Design Procedure
The pull-up resistors, RP, for the SCL and SDA lines need to be selected appropriately and take into
consideration the total capacitance of all slaves on the I2C bus. The minimum pull-up resistance is a function of
VCC, VOL,(max), and IOL as shown in Equation 5.
Rp(min)
=
VCC
- VOL(max)
IOL
(5)
The maximum pull-up resistance is a function of the maximum rise time, tr (300 ns for fast-mode operation,
fSCL = 400 kHz) and bus capacitance, Cb as shown in Equation 6.
Rp(max)
=
tr
0.8473 ´ Cb
(6)
The maximum bus capacitance for an I2C bus must not exceed 400 pF for standard-mode or fast-mode
operation. The bus capacitance can be approximated by adding the capacitance of the TCA6408A-Q1, Ci for
SCL or Cio for SDA, the capacitance of wires, connections, traces, and the capacitance of additional slaves on
the bus.
9.2.3 Application Curves
25
Standard-mode
Fast-mode
20
15
10
5
0
0 50 100 150 200 250 300 350 400 450
Cb (pF)
D008
Standard-mode: fSCL= 100 kHz, tr = 1 µs
Fast-mode: fSCL= 400 kHz, tr= 300 nsb
Figure 34. Maximum Pull-Up Resistance (Rp(max)) vs Bus
Capacitance (C)
1.8
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0
0 0.5 1 1.5 2 2.5 3 3.5 4
VCC (V)
VOL = 0.2 à VCC, IOL = 2 mA when VCC ⤠2 V
VOL = 0.4 V, IOL = 3 mA when VCC > 2 V
VCC > 2V
VCC <= 2
4.5 5 5.5
D009
Figure 35. Minimum Pull-Up Resistance (Rp(min)) vs Pull-Up
Reference Voltage (VCCI)
28
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