LTC4308
Low Voltage, Level Shifting
Hot Swappable 2-Wire Bus
Buffer with Stuck Bus Recovery
DESCRIPTION
FEATURES
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Optimized for Low Voltage Systems Down to 0.9V
Bidirectional Buffer with Stuck Bus Recovery
–200mV Offset In-Out/+300mV Offset Out-In
30ms Stuck Bus Timeout
Compatible with Non-Compliant VOL I2C Devices
Prevents SDA and SCL Corruption During Live
Board Insertion and Removal from Backplane
±6kV Human Body Model (HBM) ESD Protection
Isolates Input SDA and SCL Lines from Output
Compatible with I2C™, I2C Fast Mode and SMBus
READY Open-Drain Output
1V Precharge on SDAOUT and SCLOUT Lines
Small 8-Lead (3mm × 3mm × 0.75mm) DFN and
8-Lead MSOP Packages
APPLICATIONS
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Live Board Insertion
Servers
Capacitance Buffer/Bus Extender
RAID Systems
ATCA
The LTC®4308 hot swappable, 2-wire bus buffer allows
I/O card insertion into a live backplane without corruption of the data and clock busses. The LTC4308 provides
bidirectional buffering, keeping the backplane and card
capacitances isolated. Negative offset from output to
input allows communication between output bus devices
with high VOL and devices on the low voltage input side,
where bus supplies can be as low as 0.9V. If SDAOUT or
SCLOUT are low for 30ms, the LTC4308 will automatically break the Input-Output connection. At this time the
LTC4308 automatically generates up to 16 clock pulses
on SCLOUT in an attempt to free the bus. A connection
will resume if the stuck bus is cleared.
During insertion, the SDAOUT and SCLOUT lines are precharged to 1V to minimize bus disturbances. When driven
high, the ENABLE input allows the LTC4308 to connect after
a stop bit or bus idle condition. Driving ENABLE low breaks
the connection between SDAIN and SDAOUT, SCLIN and
SCLOUT. READY is an open-drain output which indicates
that the backplane and card sides are connected.
L, LT, LTC and LTM are registered trademarks of Linear Technology Corporation. Hot Swap
is a trademark of Linear Technology Corporation. All other trademarks are the property of their
respective owners. Protected by U.S. Patents including 7032051, 6650174, 6356140.
TYPICAL APPLICATION
The LTC4308 in a 1.2V
Microcontroller Application
1.2V to 5V Level Shifting
SDAOUT
5V
1.2V
2V/DIV
CH1
0.01μF
2.7k
2.7k
VCC
10k
2.7k
2.7k
LTC4308
MICROCONTROLLER
SCLIN
SCLOUT
CARD_SCL
SDAIN
SDAOUT
CARD_SDA
0.5V/DIV
CH2
4308 TA01a
ENABLE
READY
GND
SDAIN
READY
1μs/DIV
4308 TA01b
4308f
1
LTC4308
ABSOLUTE MAXIMUM RATINGS (Notes 1, 7)
VCC to GND ................................................. – 0.3V to 6V
SDAIN, SCLIN, SDAOUT, SCLOUT,
READY, ENABLE .......................................... –0.3V to 6V
Maximum Sink Current (SDAIN, SCLIN, SDAOUT,
SCLOUT, READY) .............................................. 50mA
Operating Temperature Range
LTC4308C ................................................ 0°C to 70°C
LTC4308I..............................................– 40°C to 85°C
Storage Temperature Range
DFN....................................................– 65°C to 125°C
MSOP ................................................– 65°C to 150°C
Lead Temperature (Soldering, 10 sec)
MSOP ............................................................... 300°C
PIN CONFIGURATION
TOP VIEW
ENABLE 1
SCLOUT 2
SCLIN 3
8
9
GND 4
TOP VIEW
VCC
7
SDAOUT
6
SDAIN
5
READY
ENABLE
SCLOUT
SCLIN
GND
8
7
6
5
1
2
3
4
VCC
SDAOUT
SDAIN
READY
MS8 PACKAGE
8-LEAD PLASTIC MSOP
DD PACKAGE
8-LEAD (3mm × 3mm) PLASTIC DFN
TJMAX = 125°C, θJA = 200°C/W
TJMAX = 125°C, θJA = 43°C/W
EXPOSED PAD (PIN 9) CONNECTION TO GROUND IS OPTIONAL
ORDER INFORMATION
LEAD FREE FINISH
TAPE AND REEL
PART MARKING*
PACKAGE DESCRIPTION
TEMPERATURE RANGE
LTC4308CDD#PBF
LTC4308CDD#TRPBF
LBTT
8-Lead (3mm × 3mm) Plastic DFN
0°C to 70°C
LTC4308IDD#PBF
LTC4308IDD#TRPBF
LBTT
8-Lead (3mm × 3mm) Plastic DFN
–40°C to 85°C
LTC4308CMS8#PBF
LTC4308CMS8#TRPBF
LTBTS
8-Lead Plastic MSOP
0°C to 70°C
LTC4308IMS8#PBF
LTC4308IMS8#TRPBF
LTBTS
8-Lead Plastic MSOP
–40°C to 85°C
Consult LTC Marketing for parts specified with wider operating temperature ranges. *Temperature grades are identified by a label on the shipping container.
Consult LTC Marketing for information on non-standard lead based finish parts.
For more information on lead free part marking, go to: http://www.linear.com/leadfree/
For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/
ELECTRICAL CHARACTERISTICS
The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VCC = 3.3V, unless otherwise noted.
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
Power Supply
VCC
Positive Supply Voltage
ICC
Supply Current
l
VCC = 5.5V, VSCLOUT = VSDAOUT = 0V (Note 6)
l
ISD
Shutdown Supply Current
VCC = 5.5V, ENABLE = 0V
l
VPRE
Precharge Voltage
SDAOUT, SCLOUT Open
l
2.3
0.8
5.5
V
7
11
mA
900
1400
μA
1
1.2
V
4308f
2
LTC4308
ELECTRICAL CHARACTERISTICS
The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VCC = 3.3V, unless otherwise noted.
SYMBOL
PARAMETER
tIDLE
Bus Idle Time
VTHR_EN
ENABLE Threshold Voltage
CONDITIONS
ENABLE Rising Edge
MIN
TYP
MAX
UNITS
l
55
95
175
μs
l
0.45
0.6
0.75
V
VTHR_EN(HYST)
ENABLE Threshold Voltage Hysteresis
(Note 3)
IENABLE
ENABLE Input Current
ENABLE from 0V to VCC
35
tPLH_EN
ENABLE Delay Off-On
(Figure 1)
95
μs
tPHL_EN
ENABLE Delay On-Off
(Note 3), (Figure 1)
10
ns
tPLH_READY
READY Delay Off-On
(Note 3), (Figure 1)
10
ns
l
tPHL_READY
READY Delay On-Off
(Note 3), (Figure 1)
VOL_READY
READY Output Low Voltage
IREADY = 3mA, VCC = 2.3V
l
IOFF_READY
READY Off Leakage Current
VCC = READY = 5.5V
l
0.1
mV
±5
10
0.1
μA
ns
0.4
V
±5
μA
Prop Delay and Rise-Time Accelerators
tPHL
SDA/SCL Propagation Delay High to Low
CLOAD = 50pF, 2.7k to VCC on SDA, SCL,
(Notes 2, 3), (Figure 1)
70
ns
tPLH
SDA/SCL Propagation Delay Low to High
CLOAD = 50pF, 2.7k to VCC on SDA, SCL,
(Notes 2, 3), (Figure 1)
10
ns
tRISE
SDA/SCL Transition Time Low to High
CLOAD = 100pF, 10k to VCC on SDA, SCL,
(Notes 3, 4), (Figure 1)
30
300
ns
tFALL
SDA/SCL Transition Time High to Low
CLOAD = 100pF, 10k to VCC on SDA, SCL,
(Notes 3, 4), (Figure 1)
30
300
ns
IPULLUPAC
Transient Boosted Pull-Up Current
Positive Transition > 0.8V/μs on SDAOUT,
SCLOUT (Note 5)
5
8
mA
Input-Output Connection
VOS
Input to Output Offset Voltage (OUT – IN)
Output to Input Offset Voltage (IN – OUT)
VTHR
l
250
300
380
mV
2.7k to VCC on SDAOUT, SCLOUT,
SDAIN = SCLIN = 0.4V, VCC = 5.5V
l
250
350
450
mV
2.7k to VCC on SDAIN, SCLIN,
SDAOUT = SCLOUT = 0.4V
l
–150
–200
–300
mV
2.7k to VCC on SDAIN, SCLIN,
SDAOUT = SCLOUT = 0.4V, VCC = 5.5V
l
–150
–250
–350
mV
1.4
1.1
1.65
1.35
1.9
1.6
V
V
0.45
0.6
0.75
V
SDAOUT, SCLOUT Logic Input Threshold Voltage VCC ≥ 2.9V
VCC < 2.9V
SDAIN, SCLIN Logic Input Threshold Voltage
VTHR(HYST)
2.7k to VCC on SDAOUT, SCLOUT,
SDAIN = SCLIN = 0.2V
SDAIN, SCLIN Rising Edge, VCC = 2.3V, 5.5V
SDAOUT, SCLOUT Logic Input Threshold Voltage (Note 3)
Hysteresis
50
mV
SDAIN, SCLIN Logic Input Threshold Voltage
Hysteresis
(Note 3)
35
mV
CIN
Digital Input Capacitance SDAIN, SDAOUT,
SCLIN, SCLOUT
(Note 3)
ILEAK
Input Leakage Current
SDA, SCL Pins
l
VOL
Output Low Voltage
SDAOUT, SCLOUT Pins, ISINK = 4mA,
SDAIN = SCLIN = 0V, VCC = 2.7V
l
0
2.7k to VCC on SDAOUT, SCLOUT,
SDAIN = SCLIN = 0V
l
250
SDAOUT, SCLOUT Pins
l
VILMAX
Buffer Input Logic Low Voltage
300
10
pF
±5
μA
400
mV
380
mV
1.2
V
4308f
3
LTC4308
ELECTRICAL CHARACTERISTICS
The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VCC = 3.3V, unless otherwise noted.
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
25
30
35
400
600
UNITS
Bus Stuck Low Timeout
tTIMEOUT
Bus Stuck Low Timer
l
SDAOUT = SCLOUT = 0V
ms
Timing Characteristics
fI2C,MAX
I2C Maximum Operating Frequency
tBUF
Bus Free Time Between Stop and Start Condition (Note 3)
1.3
μs
tHD,STA
Hold Time After (Repeated) Start Condition
(Note 3)
100
ns
tSU,STA
Repeated Start Condition Set-Up Time
(Note 3)
0
ns
tSU,STO
Stop Condition Set-Up Time
(Note 3)
0
ns
tHD,DATI
Data Hold Time Input
(Note 3)
0
ns
tSU,DAT
Data Set-Up Time
(Note 3)
100
ns
(Note 3)
Note 1: Stresses beyond those listed under Absolute Maximum Ratings
may cause permanent damage to the device. Exposure to any Absolute
Maximum Rating condition for extended periods may affect device
reliability and lifetime.
Note 2: See “Propagation Delays” in the Operations section for a
discussion of tPHL and tPLH as a function of pull-up resistance and bus
capacitance.
kHz
Note 3: Determined by design, not tested in production.
Note 4: Measure points are 0.3 • VCC and 0.7 • VCC.
Note 5: IPULLUPAC varies with temperature and VCC voltage as shown in the
Typical Performance Characteristics section.
Note 6: ICC test performed with connection circuitry active.
Note 7: All currents into pins are positive; all voltages are referenced to
GND unless otherwise specified.
TIMING DIAGRAMS
ENABLE, CONNECT, READY Timing
tPHL_READY
tPHL_EN
tPLH_READY
tPLH_EN
ENABLE
CONNECT
READY
4308 TD01
Rising and Falling Propagation Delays and Rise and Fall Times for SDAIN, SDAOUT and SCLIN, SCLOUT
tRISE
tPLH
tPHL
tRISE
tFALL
tFALL
SDAIN/SCLIN
SDAOUT/SCLOUT
4308 TD02
Figure 1. Timing Diagrams
4308f
4
LTC4308
TYPICAL PERFORMANCE CHARACTERISTICS
ICC Enabled Current
vs Temperature
ISD Disabled Current
vs Temperature
8.0
1000
7.5
950
VCC = 5.5V
7.0
6.5
ISD DISABLE CURRENT (μA)
ICC ENABLED CURRENT (mA)
TA = 25°C, VCC = 3.3V, unless otherwise indicated.
VCC = 3.3V
6.0
VCC = 2.3V
5.5
5.0
4.5
900
850
800
750
700
650
4.0
–50
–25
0
50
25
TEMPERATURE (°C)
75
600
–50
100
–25
25
50
0
TEMPERATURE (°C)
4308 G01
4308 G02
Input-Output High to Low
Propagation Delay
vs Temperature
Boost Pull-Up Current
vs Temperature
24
CIN = COUT = 50pF
RPULLUPIN = RPULLUPOUT = 2.7k
BOOST PULL-UP CURRENT (mA)
PROPAGATION DELAY (ns)
140
120
100
VCC = 5.5V
80
VCC = 2.3V
60
40
–50
VCC = 3.3V
–25
25
50
0
TEMPERATURE (°C)
75
100
CIN = 50pF, COUT = 1nF
RPULLUPIN = RPULLUPOUT = 2.7k
20
VCC = 5.5V
16
12
8
VCC = 3.3V
4
VCC = 2.3V
0
–50
–25
0
25
50
TEMPERATURE (°C)
4308 G03
Input-Output High to Low
Propagation Delay vs Output
Capacitance
VCC = 5.5V
95
VCC = 2.3V
80
VCC = 3.3V
50
VPULLUPIN = 1.8V
VPULLUPOUT = VCC
0
–196
304
–198
302
–200
–202
400
600
800
200
OUTPUT CAPACITANCE (pF)
1000
4308 G05
–206
300
298
296
–204
65
100
Output-Input Offset Voltage
vs Pull-Up Resistance
OFFSET VOLTAGE (mV)
110
OFFSET VOLTAGE (mV)
PROPAGATION DELAY (ns)
CIN = 50pF
RPULLUPIN = 2.7k
125 RPULLUPOUT = 2.7k
75
4308 G04
Input-Output Offset Voltage
vs Pull-Up Resistance
140
100
75
294
0
6
4
8
2
INPUT BUS PULL-UP RESISTANCE (k)
10
4308 G06
0
8
2
4
6
10
OUTPUT BUS PULL-UP RESISTANCE (k)
4308 G07
4308f
5
LTC4308
PIN FUNCTIONS
ENABLE (Pin 1): Connection Enable Input. This 0.6V nominal threshold input pin enables or disables the LTC4308.
For normal operation, pull or connect ENABLE high. Driving
ENABLE below the 0.45V threshold isolates SDAIN from
SDAOUT, SCLIN from SCLOUT, asserts READY low, and
prohibits automatic clock and stop bit generation during a
fault condition. A rising edge on ENABLE after a fault has
occurred forces a connection between SDAIN, SDAOUT
and SCLIN, SCLOUT. Connect to VCC if unused.
SCLOUT (Pin 2): Serial Clock Output. Connect this pin to a
SCL bus segment where bus stuck low recovery is desired.
A pull-up resistor should be connected between this pin
and a bus pull-up supply greater than or equal to VCC.
SCLIN (Pin 3): Serial Clock Input. Connect this pin to a SCL
bus segment where isolation from bus stuck low issues is
desired. A pull-up resistor should be connected between
this pin and a bus pull-up supply greater than 0.9V.
GND (Pin 4): Device Ground. Connect this pin to a ground
plane for best results.
READY (Pin 5): Connection Ready Status Output. This
open-drain N-channel MOSFET pin pulls low when
ENABLE is low, when the startup and connection sequence
described in the Operation section has not been completed,
or when the LTC4308 disconnects the input and output
pins due to a bus stuck low condition. READY goes high
when ENABLE is high and connection is made between the
input and output pins. Connect a pull-up resistor, typically
10k, from this pin to the bus pull-up supply. This pin can
be left open if unused.
SDAIN (Pin 6): Serial Data Input. Connect this pin to a SDA
bus segment where isolation from bus stuck low issues is
desired. A pull-up resistor should be connected between
this pin and a bus pull-up supply greater than 0.9V.
SDAOUT (Pin 7): Serial Data Output. Connect this pin to a
SDA bus segment where bus stuck low recovery is desired.
A pull-up resistor should be connected between this pin
and a bus pull-up supply greater than or equal to VCC.
VCC (Pin 8): Supply Voltage Input. Place a bypass capacitor
of at least 0.01μF close to VCC for best results.
Exposed Pad (Pin 9, DFN Package Only): Exposed Pad
may be left open or connected to device ground.
4308f
6
LTC4308
BLOCK DIAGRAM
Low Voltage Level Shifting 2-Wire Bus Buffer with Stuck Bus Recovery
8mA
CONNECT
6
VCC 8
IBOOSTSDA
SDAIN
SDAOUT
7
SLEW RATE
DETECTOR
100k
CONNECT
PRECHARGE
PC_CONNECT
CONNECT
3
100k
8mA
IBOOSTSCL
SCLIN
SCLOUT
2
SLEW RATE
DETECTOR
+
CONNECT
–
1.65V/1.6V
1.35V/1.3V
30ms
TIMER
+
+
0.6V
–
IBOOSTSCL
IBOOSTSDA
+
–
1.65V/1.6V
1.35V/1.3V
LOGIC
0.6V
–
READY
PC_CONNECT
1
ENABLE
CONNECT
5
+
0.6V
–
UVLO
95μs
DELAY
CONNECT
GND
4
4308 BD
4308f
7
LTC4308
OPERATION
Start-Up
When the LTC4308 first receives power on its VCC pin,
either during power-up or live insertion, it starts in an
under voltage lockout (UVLO) state, ignoring any activity
on the SDA or SCL pins until VCC rises above 2V (typical).
This ensures the LTC4308 does not try to function until
enough supply voltage is present.
During this time, the 1V precharge circuitry is actively
forcing 1V through 100k nominal resistors to the SDAOUT
and SCLOUT pins. Because SDAOUT and SCLOUT pins
may be plugged into a live backplane, where the voltage
on the backplane SDA and SCL busses can be anywhere
between 0V and VCC, precharging SCLOUT and SDAOUT
to 1V minimizes the worst-case voltage differential these
pins will see at the moment of contact, therefore minimizing
the amount of disturbance caused by the I/O card.
Once the LTC4308 exits from UVLO, it monitors both the
input and output pins for either a stop bit or a bus idle
condition to indicate the completion of data transactions.
When both sides are idle or one side has a stop bit while the
other is idle, the connection circuitry is activated, joining
the SDA and SCL pins on the input bus with those on the
output bus. Because SDAIN and SCLIN are monitored for
a stop bit or bus idle as a condition for connection, they
may also be used for Hot-Swapping, but note that these
pins are not precharged.
Connection Circuitry
Once the connection circuitry is activated, the functionality
of the input and output bus of the respective SDA or SCL
pins is identical. A low forced on either output or input pin
at any time results in both pin voltages forced low. The
LTC4308 SCLOUT and SDAOUT busses are tolerant of I2C
bus DC logic low voltages up to the VIL specification of
0.3 • VCC, while the SCLIN and SDAIN busses are tolerant
of bus logic low voltages up to 0.6V. A high occurs when
all devices on the input and output pins release high.
When the LTC4308 senses a rising edge on either of the
output busses, with a slew rate greater than 0.8V/μs, the
internal pull-down device for the respective bus is deactivated at bus voltages as low as 0.48V. This methodology
maximizes the effectiveness of the rise time accelerator
circuitry and maintains compatibility with other devices
in the LTC4300 bus buffer family. Care must be taken to
ensure devices participating in clock stretching or arbitration is capable of forcing logic low voltages below 0.48V
at the LTC4308’s SCLOUT and SDAOUT pins.
These important features ensure the I2C specification
protocols such as clock stretching, clock synchronization, arbitration, and acknowledge function seamlessly
in all cases as specified, regardless of how the devices in
the system are connected to the LTC4308.
Another key feature provided by the connection circuitry
is input and output bus capacitance isolation through
bidirectional buffering. Because of this isolation, the
waveforms on the input busses look slightly different than
the corresponding output bus waveforms, as described
in the next two sections.
Offset Voltages
When a logic low is driven on SDAIN or SCLIN, the LTC4308
regulates SDAOUT or SCLOUT, respectively, to a higher
voltage, typically 300mV above the driven low voltage.
When a logic low is driven on SCLOUT or SDAOUT, the
LTC4308 regulates SCLIN or SDAIN, respectively, to a voltage that is typically 200mV below the driven low voltage.
These offsets are nearly independent of pull-up current
(see Typical Performance Characteristics).
4308f
8
LTC4308
OPERATION
COUT = 50pF
VPULLUP(OUT) = VCC = 3.3V
1V/DIV
1V/DIV
COUT = 50pF
VPULLUP(OUT) = VCC = 3.3V
CIN = 150pF
VPULLUP(IN) = 1.8V
CIN = 150pF
VPULLUP(IN) = 1.8V
200ns/DIV
200ns/DIV
4308 F02
Figure 2. Input-Output Rising Edge Waveforms
Propagation Delays
During a rising edge, the rise time on each side is influenced by rise time acceleration, bus pull-up resistor, and
the equivalent capacitance on the line. If the pull-up resistors are the same, a difference in rise time occurs which is
directly proportional to the difference in capacitance and
the presence of rise time acceleration between the two
sides. This effect is displayed in Figure 2 for VCC = 3.3V
and a 2.7k pull-up resistor on the input (VPULLUP(IN) =
1.8V, CIN = 150pF) and output (VPULLUP(OUT) = 3.3V, COUT
= 50pF). Since the output pin has rise time acceleration
and less capacitance than the input, it rises faster and
the effective propagation delay is negative.
There is a finite propagation delay through the connection circuitry for falling waveforms. Figure 3 shows the
falling edge waveforms for the same pull-up resistors and
equivalent capacitance conditions as used in Figure 2.
An external N-channel MOSFET device pulls down the
voltage on the side with 150pF capacitance; the LTC4308
pulls down the voltage on the opposite side with a delay
of 70ns. This delay is always positive and is a function of
supply voltage, temperature and the pull-up resistors and
equivalent bus capacitances on both sides of the bus.
The Typical Performance Characteristics section shows
propagation delay as a function of temperature and voltage
for 2.7k pull-up resistors and 50pF equivalent capacitance
on both sides of the part. Also, the Propagation Delay as
a function of Output Capacitance curve shows that larger
4308 F03
Figure 3. Input-Output Falling Edge Waveforms
output capacitances translate to longer delays. Users must
quantify the difference in propagation times for a rising
edge versus a falling edge in their systems and adjust
setup and hold times accordingly.
Bus Stuck Low Timeout
SDAOUT and SCLOUT are each connected to an internal
timer. When SDAOUT or SCLOUT is low, its respective
timer is started. Each timer is only reset when its pin goes
high. If the bus stuck low does not go high within 30ms
(typical), the connection circuitry is disabled, breaking
the connection between the respective input and output
pins. In addition, after at least 40μs, up to 16 clock pulses
at 8.5kHz (typical) are generated on the SCLOUT pin by
the LTC4308 in an attempt to free the stuck low bus. The
clock pulses are halted if the bus recovers to a logic high
condition before the completion of the full 16 pulses. A
stop bit is always generated on the SCLOUT and SDAOUT
pins to reset all devices on the bus.
If the stuck low SDAOUT or SCLOUT does not recover to
a logic high condition after the automatic clocking and
stop bit generation, the LTC4308 remains disconnected.
Should the bus free, the LTC4308 will reconnect the input
and output busses if a stop bit or bus idle condition is
detected, as specified in the Start Up section. Alternatively,
a rising edge on ENABLE forces the connection circuitry to
reconnect the input and output busses and reset the 30ms
timer if the bus remains in a stuck bus low condition.
4308f
9
LTC4308
OPERATION
When powering up into a bus stuck low condition, the
connection circuitry connecting the SDA and SCL pins are
not activated. 30ms after UVLO, automatic clocking and
stop bit generation takes place as described above.
READY Digital Output
This pin provides a digital flag which is low when either
ENABLE is low, the start-up sequence described earlier
in this section has not been completed, or the LTC4308
has disconnected the input and output busses due to a
bus stuck low condition. READY goes high when ENABLE
is high and start-up is complete. The pin is driven by an
open-drain pull-down device capable of sinking 3mA while
holding 0.4V on the pin. Connect a resistor to the bus
pull-up supply to provide the pull-up.
ENABLE
When the ENABLE pin is driven below 0.45V with respect
to the LTC4308’s ground, the input pin is disconnected
from the output pin and the READY pin is pulled low.
When the pin is driven above 0.75V, the part waits for
data transactions on both the input and output pins to be
complete (as described in the Start-Up section) before
connecting the two pins. At this time the internal pull-down
on READY releases.
A rising edge on ENABLE after a bus stuck low condition
has occurred forces a connection between SDAIN, SDAOUT,
and SCLIN, SCLOUT even if the bus stuck low condition
has not been cleared. At this time the 30ms timer is reset,
but not disabled.
Rise Time Accelerators
Once connection has been established, rise time accelerator circuits on SDAOUT and SCLOUT are enabled. During
positive bus transitions of at least 0.8V/μs, the rise time
accelerators provide strong, slew-limited pull-up currents
to force the bus voltage to rise at a rate of 100V/μs.
The rise time accelerators significantly improve reliability and performance in I2C systems in several ways.
First, due to the accelerator’s significantly lower pull-up
impedance, as compared to the bus pull-up resistance,
the system is less susceptible to noise on rising edges,
providing smooth, controlled transitions for both small
and large systems. Second, the accelerators allow users
to choose larger bus pull-up resistors, reducing power
consumption and improving logic low noise margins or
to design with bus capacitances beyond those specified
in the I2C specifications.
For these reasons, it is strongly recommended that users
choose bus pull-up resistors that guarantee the output
busses will rise on their own at a rate of at least 0.8 V/μs to
ensure activation of the accelerators. See the Applications
Information section for selecting pull-up resistor sizes.
It is important to connect SDAOUT and SCLOUT pins to
a bus whose pull-up supply is equal to or greater than
the LTC4308’s supply to ensure the accelerators do not
source current through the pull up resistors into the pullup supply.
The rise time accelerators are internally disabled until the
sequence of events described in the start-up section has
been completed, as well as during automatic clocking and
stop bit generation for a bus stuck low recovery event.
4308f
10
LTC4308
APPLICATIONS INFORMATION
Resistor Pull-Up Value Selection
To guarantee the SDAOUT and SCLOUT rise time accelerators are activated during a rising edge, the bus must rise
on its own with a positive slew rate of at least 0.8V/μs. To
achieve this, choose a maximum resistor value RPULLUP
using the formula:
(VBUS(MIN) − 0.8V)• 1250ns / V
RPULLUP ≤
CBUS
Where RPULLUP is the pull-up resistor value in kΩ, VBUS(MIN)
is the minimum bus pull-up supply voltage and CBUS is
the equivalent bus capacitance in pF.
To estimate the value of CBUS, use a general rule of 20pF
of capacitance per device on the bus (10pF for the device
and 10pF for interconnect).
In addition, RPULLUP must be strong enough to overcome
the precharge voltage and provide logic highs on SDAOUT
and SCLOUT for the start-up and connection circuitry to
connect the backplane to the card. To meet this requirement, always choose
RPULLUP ≤ 75k
VBUS(MIN) − VTHR(MAX)
VTHR(MAX) − 1V
where VTHR(MAX) is the maximum specified Logic Input
Threshold Voltage, VTHR.
Further, on SDAIN and SCLIN and for heavily loaded
systems on SDAOUT and SCLOUT, where the selected
RPULLUP value causes the bus to rise at a rate slower than
0.8V/μs, users must also guarantee
RPULLUP ≤
VBUS(MIN) − VTHR(MAX)
100μA
Live Insertion and Capacitance Buffering Application
Figure 4 and 5 illustrate applications of the LTC4308 that
take advantage of the LTC4308’s Hot Swap™, capacitance
buffering and output pin precharge features. If the I/O
cards were plugged directly into the backplane without the
LTC4308 buffer, all of the backplane and card capacitances
would add directly together, making rise time and fall time
requirements difficult to meet. Placing an LTC4308 on the
edge of each card isolates the card capacitance from the
backplane. For a given I/O card, the LTC4308 drives the
capacitance of everything on the card and the backplane
must drive only the capacitance of the LTC4308, which
is less than 10pF.
Figure 4 shows the LTC4308 used in the typical staggered
connector application, where VCC and GND are the longest
“early power” pins. The “early power” pins ensure the
LTC4308 is initially powered and forcing the 1V precharge
voltage on the medium length SDA and SCL output pins
before they contact with the backplane busses. Coupled
with ENABLE as the shortest pin, passively pulled to ground
by a resistor, the staggered approach provides additional
time for transients associated with live insertion to settle
before the LTC4308 can be enabled.
Figure 5 shows the LTC4308 in an application where all
of the pins have the same length. In this application, a
resistor is used to hold the ENABLE pin low during live
insertion, until the backplane control circuitry can enable
the device.
Level Shifting Applications
Systems requiring different supply voltages for the
backplane side and the card side can use the LTC4308
for bidirectional level shifting, as shown in Figures 4, 5,
and 7. The LTC4308 can level shift between bus pull-up
supplies as low as 0.9V to as high as 5.5V. Level shifting
allows newer designs that require lower voltage supplies,
such as EEPROMs and microcontrollers, the capability to
interface with legacy backplanes which may be operating
at higher supply voltages.
The LTC4308’s negative offset voltage from output to
input allow level shifting applications with high SDAOUT
and SCLOUT VOL to effectively translate to the low voltage
SDAIN and SCLIN busses. Figure 7 shows an application
where 200Ω resistors, used to provide additional ESD
protection for the Temperature Sensor’s internal low
impedance pull-down device, generate high VOL on the
SDAOUT and SCLOUT busses.
4308f
11
LTC4308
APPLICATIONS INFORMATION
Systems with Supply Voltage Droop
LTC4308 and LTC4301L Feature Comparison
In large 2-wire systems, the VCC voltages seen by devices
at various points in the system can differ by a few hundred
millivolts or more. This situation is modeled by a series
resistor in the VCC line, as shown in Figure 6. For proper
operation, make sure that the VCC(LTC4308) is ≥ 2.3V.
Although both, the LTC4308 and LTC4301L are functionally similar Hot Swappable Bus Buffers designed for
Low Voltage Level Translation in 2-wire bus systems, the
LTC4308 provides greater features. These features include
automatic bus stuck low detection and recovery; rise time
accelerators on the output busses, and –200mV In-Out and
300mV Out-In offset voltages that are nearly independent
of pull-up resistors. These and other differences are listed
in Table 1 and must be accounted for if using the LTC4308
in LTC4301L applications.
Table 1: Differences Between LTC4301l and LTC4308
SPECIFICATION
LTC4301L
LTC4308
COMMENTS
VCC(MIN)
2.7V
2.3V
Lower supply voltage allows greater compatibility with low voltage systems.
VOS(TYP)
100mV
–200mV/300mV Negative output-to-input offset voltage provide better noise margin on low voltage bus.
IPULLUPAC(TYP)
N/A
8mA
Output bus rise time accelerators aid heavily loaded busses to meet rise time specifications.
tTIMEOUT
N/A
30ms
Stuck Bus Recovery automatically isolates the input bus from the output bus and attempts to recover
the output bus.
READY
–
–
READY functions identically. In addition, the LTC4308 will pull READY low to indicate when
disconnection has occurred.
CS/ENABLE
Active Low
Active High
When replacing an LTC4301L with an LTC4308, invert the CS signal.
4308f
12
LTC4308
APPLICATIONS INFORMATION
BACKPLANE
CONNECTOR
MIXED VOLTAGE BACKPLANE
CARD
CONNECTORS
LOW VOLTAGE PERIPHERAL I/O CARD 1
3.3V
1V
1V
R1
10k
R2
10k
C1
0.01μF
R3
10k
C2
0.01μF
VCC
SDAOUT
SCLOUT
LTC4308
READY
ENABLE
GND
SDA
SCL
READY
ENA1
R6
10k
R4
2.7k
R5
2.7k
SDAIN
SCLIN
CARD1_SDA
CARD1_SCL
•
•
•
LOW VOLTAGE PERIPHERAL I/O CARD N
1V
C3
0.01μF
C4
0.01μF
R7
2.7k
R8
2.7k
VCC
ENAn
R9
10k
SDAOUT
SCLOUT
LTC4308
READY
ENABLE
GND
SDAIN
SCLIN
CARDn_SDA
CARDn_SCL
4308 F03
Figure 4. The LTC4308 in an Application with Staggered Connectors.
BACKPLANE
CONNECTOR
MIXED VOLTAGE BACKPLANE
CARD
CONNECTORS
LOW VOLTAGE PERIPHERAL I/O CARD 1
3.3V
1V
1V
R1
10k
R2
10k
C1
0.01μF
R3
10k
SDA
SCL
READY
ENA1
R6
10k
VCC
SDAOUT
SCLOUT
LTC4308
READY
ENABLE
GND
C2
0.01μF
R4
2.7k
R5
2.7k
SDAIN
SCLIN
CARD1_SDA
CARD1_SCL
•
•
•
LOW VOLTAGE PERIPHERAL I/O CARD N
1V
C3
0.01μF
ENAn
R9
10k
VCC
SDAOUT
SCLOUT
LTC4308
READY
ENABLE
GND
C4
0.01μF
SDAIN
SCLIN
R7
2.7k
R8
2.7k
CARDn_SDA
CARDn_SCL
4308 F04
Figure 5. The LTC4308 in an Application Where All the Pins Have the Same Length.
4308f
13
LTC4308
TYPICAL APPLICATIONS
VCC(LTC4308)
V(BUS)
RDROOP
VCC
C1
0.01μF
R1
10k
R2
10k
R3
10k
SDA1
SCL1
R4
10k
VCC
LTC4308
ENABLE
SDAIN SDAOUT
SCLIN SCLOUT
R5
10k
SDA2
SCL2
READY
READY
GND
4308 TA02
Figure 6. System with Voltage Droop
1.2V
5V
0.01μF
0.01μF
1.8k
1.8k
VCC
LTC4308
10k
10k
ENABLE
200Ω*
TEMPERATURE
SENSOR
SCLOUT
SCLIN
SDAOUT
SDAIN
SCL
200Ω*
SDA
5V
10k
READY
GND
*200Ω ARE ADDITIONAL ESD
PROTECTION RESISTORS
READY
4308 TA03
Figure 7. High VOL Application
4308f
14
LTC4308
PACKAGE DESCRIPTION
DD Package
8-Lead Plastic DFN (3mm × 3mm)
(Reference LTC DWG # 05-08-1698)
R = 0.115
TYP
5
0.38 ± 0.10
8
0.675 ±0.05
3.5 ±0.05
1.65 ±0.05
2.15 ±0.05 (2 SIDES)
3.00 ±0.10
(4 SIDES)
PIN 1
TOP MARK
(NOTE 6)
1.65 ± 0.10
(2 SIDES)
PACKAGE
OUTLINE
0.25 ± 0.05
0.50
BSC
2.38 ±0.05
(2 SIDES)
4
0.25 ± 0.05
0.75 ±0.05
0.200 REF
0.00 – 0.05
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
1
(DD8) DFN 1203
0.50 BSC
2.38 ±0.10
(2 SIDES)
BOTTOM VIEW—EXPOSED PAD
NOTE:
1. DRAWING TO BE MADE A JEDEC PACKAGE OUTLINE M0-229 VARIATION OF (WEED-1)
2. DRAWING NOT TO SCALE
3. ALL DIMENSIONS ARE IN MILLIMETERS
4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE
MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE
5. EXPOSED PAD SHALL BE SOLDER PLATED
6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION
ON TOP AND BOTTOM OF PACKAGE
MS8 Package
8-Lead Plastic MSOP
(Reference LTC DWG # 05-08-1660)
0.889 ± 0.127
(.035 ± .005)
5.23
(.206)
MIN
3.20 – 3.45
(.126 – .136)
3.00 ± 0.102
(.118 ± .004)
(NOTE 3)
0.65
(.0256)
BSC
0.42 ± 0.038
(.0165 ± .0015)
TYP
8
7 6 5
0.52
(.0205)
REF
RECOMMENDED SOLDER PAD LAYOUT
0.254
(.010)
3.00 ± 0.102
(.118 ± .004)
(NOTE 4)
4.90 ± 0.152
(.193 ± .006)
DETAIL “A”
0° – 6° TYP
GAUGE PLANE
1
0.53 ± 0.152
(.021 ± .006)
DETAIL “A”
2 3
4
1.10
(.043)
MAX
0.86
(.034)
REF
0.18
(.007)
SEATING
PLANE
0.22 – 0.38
(.009 – .015)
TYP
0.65
(.0256)
BSC
0.127 ± 0.076
(.005 ± .003)
MSOP (MS8) 0204
NOTE:
1. DIMENSIONS IN MILLIMETER/(INCH)
2. DRAWING NOT TO SCALE
3. DIMENSION DOES NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS.
MOLD FLASH, PROTRUSIONS OR GATE BURRS SHALL NOT EXCEED 0.152mm (.006") PER SIDE
4. DIMENSION DOES NOT INCLUDE INTERLEAD FLASH OR PROTRUSIONS.
INTERLEAD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.152mm (.006") PER SIDE
5. LEAD COPLANARITY (BOTTOM OF LEADS AFTER FORMING) SHALL BE 0.102mm (.004") MAX
4308f
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
15
LTC4308
TYPICAL APPLICATION
The LTC4308 in a Level Shifting Application.
2.5V
1.8V
0.01μF
0.01μF
2.7k
2.7k
2.7k
VCC
2.7k
LTC4308
MICROCONTROLLER
SCLIN
SCLOUT
SDAIN
SDAOUT
SCL
SDA
3.3V
ENABLE
10k
READY
GND
READY
4308 TA04
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Hot Swappable 2-Wire Bus Buffer with Low Voltage
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Allows Bus Pull-Up Voltages as Low as 1V on SDAIN and SCLIN
LTC4302-1/LTC4302-2 Addressable 2-Wire Bus Buffer
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Low Offset Hot Swappable 2-Wire Bus Buffer with
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60mV Buffer Offset, 30ms Stuck Bus Disconnect and Recovery, Rise
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ThinSOT is a trademark of Linear Technology Corporation
4308f
16
Linear Technology Corporation
LT 0408 • PRINTED IN USA
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