FXMA2104 Datasheet, PDF (7/15 Pages) Fairchild Semiconductor – Dual-Supply, 4-Bit Voltage Translator / Buffer / Repeater

Buffer / Repeater Performance
The FXMA2104 dynamic drivers have enough current-
sourcing capability to drive a 400pF capacitive bus. This
is beneficial when an I²C buffer / repeater is required.
The I²C specification stipulates a maximum bus
capacitance of 400pF. If an I²C segment exceeds
400pF, an I²C buffer / repeater is required to split the
segment into two segments, each of which is less than
400pF. Figure 4 is a scope shot of an FXMA2104
driving a lumped load of 600pF. Notice the (30% - 70%)
rise time is only 112ns (total R_PU = 2.2K). This is well
below the maximum edge rate of 300ns. Not only does
the FXMA2104 drive 400Pf; it also provides excellent
headroom below the I²C specification maximum edge
rate of 300ns.
V_OL vs. I_OL
The I²C specification mandates a maximum V_IL (I_OL of
3mA) of V_CC •
is a master on
0.3
the
and a
A port
maximum
of an I²C
V_OL of 0.4V. If
translator with
there
a V_CC
of 1.65V and a slave on the I²C translator B port with a
V_CC of 3.3V, the maximum V_IL of the master is (1.65V x
0.3) 495mV. The slave could legally transmit a valid
logic LOW of 0.4V to the master.
If the I²C translator’s channel resistance is too high, the
voltage drop across the translator could present a V_IL to
the master greater than 495mV. To complicate matters,
the I²C specification states that 6mA of I_OL is
recommended for bus capacitances approaching
400pF. More I_OL increases the voltage drop across the
I²C translator. The I²C application benefits when I²C
translators exhibit low V_OL performance. Figure 5
depicts typical FXMA2104 V_OL performance vs. a
competitor, given a 0.4V V_IL.
0.65
0.6
0.55
0.5
0.45
0.4
0
V_OL: FXMA2104 vs. Device B, V_IL = 0.4V
Device B
VIL = 0.4V
FXMA2104
VIL = 0.4V
2
4
6
8
10
I_OL (mA):
Figure 5. V_OL vs. I_OL
© 2011 Fairchild Semiconductor Corporation
FXMA2104 • Rev. 1.0.1
7
www.fairchildsemi.com