Datasheet

LTC4315
10
4315f
If V
CC2
i s t i e d l o w, o u t p u t R TA s a r e d i s a b l e d i n d e p e n d e n t o f
the state of the ACC pin. Using a combination of the ACC pin
and the V
CC2
voltage, the input and output side RTAs can
be controlled independently. The RTAs are also internally
disabled during power up, V
CC2
transitions described in
the Operation section and during a bus stuck low event.
The RTAs when activated pull the bus up to
0.9 • V
CC
and 0.9 • V
CC2
on the input and output sides of
the SDA and SCL pins. Independent supply voltages V
CC
and V
CC2
maximize acceleration range on both inputs and
outputs by allowing the RTA turn-off voltage to be set inde-
pendently on the two sides. In order to prevent bus overdrive
by the RTA, the bus supplies on the input and output sides
of the LTC4315 must be greater than or equal to 0.9 • V
CC
and 0.9 • V
CC2
respectively. An example is shown in Figure 3
where the input bus voltage is greater than V
CC
. During a
rising edge, the input bus rise rate will be accelerated by
the RTA up to a voltage of 2.97V after which the bus rise
rate will reduce to a value that is determined by the bus
current and bus capacitance. The RTA turn-off voltage is
less than the bus supply and the bus is not over driven.
This can also be accomplished by tying V
CC
to the input
bus supply and V
CC2
to the output bus supply as shown
in Figure 4. In this case the input and output busses are
accelerated to 2.97V and 2.25V respectively
APPLICATIONS INFORMATION
PULL-UP RESISTOR VALUE SELECTION
To guarantee that the RTAs are activated during a rising
edge, the bus must rise on its own with a positive slew
rate of at least 0.4V/µs. To achieve this, choose a maximum
R
BUS
using the formula:
R
BUS
≤
V
DD,BUS(MIN)
– V
RTA(TH)
(
)
0.4
V
µs
•C
BUS
(1)
R
BUS
is the bus pull-up resistor, V
DD,BUS(MIN)
is the
minimum bus pull-up supply voltage, V
RTA(TH)
is the
maximum voltage at which the RTA turns on and C
BUS
is
the equivalent bus capacitance. R
BUS
must also be large
enough to guarantee that:
R
BUS
≥
V
DD,BUS(MAX)
– 0.4V
(
)
4mA
(2)
This criterion ensures that the maximum bus current is
less than 4mA.
Figure 3. Level Shift Application Where the SDAIN, SCLIN Bus
Pull-Up Supply Voltages are Higher Than the Supply Voltages of
the LTC4315
Figure 4. Level Shift Application Where the LTC4315 V
CC
and
V
CC2
Pins are Connected to the Bus Pull-Up Supply Voltages
LTC4315
GND
V
CC
V
CC2
4315 F03
READY
SCLOUT
SDAOUT
FAULT
DISCEN
ENABLE
SCL1
SDA1
R2
10k
R5
10k
R6
10k
R4
10k
R3
10k
R1
10k
C1
0.01µF
3.3V
READY
SCL2
SDA2
FAULT
5V
SCLIN
SDAIN
ACC
LTC4315
GND
V
CC
V
CC2
4315 F04
READY
SCLOUT
SDAOUT
FAULT
DISCEN
ENABLE
SCLIN
SDAIN
ACC
SCL1
SDA1
R2
10k
R5
10k
R6
10k
R4
10k
R3
10k
R1
10k
C1
0.01µF
3.3V
READY
SCL2
SDA2
FAULT
2.5V
C2
0.01µF