LTC4306C View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC4306C' PDF : 20 Pages View PDF
U
OPERATIO
LTC4306
Fall Time Control
Per the I2C Fast Mode (400kHz) Specification, the two-
wire bus digital interface circuitry provides fall time con-
trol when forcing logic lows onto the SDAIN bus. The fall
time always meets the limits:
where tf is the fall time in ns and CB is the equivalent bus
capacitance in pF. Whenever the Upstream-Downstream
Buffer Circuitry is active, its output signal will meet the fall
time requirements, provided that its input signal meets the
fall time requirements.
(20 + 0.1 • CB) < tf < 300ns
APPLICATIO S I FOR ATIO
Design Example
A typical LTC4306 application circuit is shown in Figure 5.
The circuit illustrates the level-shifting, multiplexer/switch
and capacitance buffering features of the LTC4306. In this
application, the LTC4306 VCC voltage and downstream
bus 1 are powered from a 3.3V supply voltage; down-
stream bus 4 is powered from 5V, and the upstream bus
is powered from 2.5V. Channels 2 and 3 are omitted for
simplicity. The following sections describe a methodology
for choosing the external components in Figure 5.
SDA, SCL Pull-Up Resistor Selection
The pull-up resistors on the SDA and SCL pins must be
strong enough to provide a minimum of 100µA pull-up
current, per the SMBus Specification. In most systems,
the required minimum strength of the pull-up resistors is
determined by the minimum slew requirement to guaran-
tee that the LTC4306’s rise time accelerators are activated
during rising edges. At the same time, the pull-up value
should be kept low to maximize the logic low noise margin
and minimize the offset voltage of the Upstream-Down-
stream Buffer circuitry. The LTC4306 is designed to func-
tion for a maximum DC pull-up current of 4mA. If multiple
downstream channels are active at the same time, this
means that the sum total of the pull-up currents from these
channels must be less than 4mA. At supply voltages of
2.7V and 5.5V, pull-up resistor values of 10k work well for
capacitive loads up to 215pF and 420pF, respectively. For
larger bus capacitances, refer to equation (1) below. The
LTC4306 works with capacitive loads up to 2nF.
VBACK = 2.5V
MICROCONTROLLER
VCC
VCC = VBUS1 = 3.3V
C1
R1 R2 R3
10k 10k 10k
0.01µF
6
4
VCC
SCLIN SCL1
16
2 SDA1N SDA1 17
1 ALERT ALERT1 18
R10
1k D1
LTC4306UFD
8
GPIO1
12 ADR2
11 ADR1
10 ADR0
3 GND
SCL4 15
SDA4 14
ALERT4 7
4306 F05
ADDRESS = 1000 100
R4 R5 R6
10k 10k 10k
SFP
MODULE 1
ADDRESS = 1111 000
VBUS4 = 5V
R7 R8 R9
10k 10k 10k
SFP
MODULE 4
ADDRESS = 1111 001
Figure 5. A Level Shifting Circuit
4306f
15
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