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LTC4307-1

High Defi nition Multimedia Interface (HDMI) Level-Shifting 2-Wire Bus Buffer

Linear Technology 

LTC4307-1

OPERATION

Start-Up

When the LTC4307-1 first receives power on its VCC pin

during power-up, it starts in an undervoltage lockout

(UVLO) state, ignoring any activity on the SDA or SCL

pins until VCC rises above 2V (typ). This is to ensure that

the LTC4307-1 does not try to function until it has enough

voltage to do so.

Once the LTC4307-1 comes out of UVLO, it monitors both

2-wire busses for either a stop bit or bus idle condition to

indicate the completion of data transactions. When both

sides are idle or one side has a stop bit condition while the

other is idle, the input-to-output connection circuitry is acti-

vated, joining SDAIN to SDAOUT and SCLIN to SCLOUT.

Connection Circuitry

Once the connection circuitry is activated, the functionality

of the SDAIN and SDAOUT pins is identical. A low forced

on either pin at any time results in both pin voltages being

low. The LTC4307-1 is tolerant of I2C bus DC logic low

voltages up to the 0.3VCC VIL I2C specification.

When the LTC4307-1 senses a rising edge on the bus,

it deactivates its pull-down devices for bus voltages as

low as 0.48V. Care must be taken to ensure that devices

participating in clock stretching or arbitration force logic

low voltages below 0.48V at the LTC4307-1 inputs.

SDAIN and SDAOUT enter a logic high state only when

all devices on both SDAIN and SDAOUT release high.

The same is true for SCLIN and SCLOUT. This important

feature ensures that clock stretching, clock synchroniza-

tion, arbitration and the acknowledge protocol always

work, regardless of how the devices in the system are

tied to the LTC4307-1.

Another key feature of the connection circuitry is that it

provides bidirectional buffering, keeping the capacitances

of the two 2-wire busses isolated from each other. Plac-

ing an LTC4307-1 close to an HDMI port inside an HDMI

transmitter or receiver allows the HDMI device to pass

the capacitance compliance specification. Because of this

isolation, the waveforms on SDAIN and SCLIN look slightly

different than the corresponding waveforms on SDAOUT

and SCLOUT as described here.

Input to Output Offset Voltage

When a logic low voltage, VLOW1, is driven on any of the

LTC4307-1’s data or clock pins, the LTC4307-1 regulates

the voltage on the opposite data or clock pins to a slightly

higher voltage, typically 60mV above VLOW1. This offset is

practically independent of pull-up current (see the Typical

Performance curves).

Propagation Delays

During a rising edge, the rise time on each side is de-

termined by the 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 between

the two sides. Users must account for differences in the

RC time constants between the two 2-wire busses and

ensure that all system timing specifications are met on

both busses.

There is a finite propagation delay through the connection

circuitry for falling waveforms. Figure 2 shows the falling

edge waveforms for VCC = 5.5V, a 10k pull-up resistor on

each side, 150pF parasitic capacitance on the input bus and

50pF on the output pins. An external N-channel MOSFET

device pulls down the voltage on the side with 150pF

capacitance; the LTC4307-1 pulls down the voltage on the

opposite side with a delay of 80ns. 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 Charac-

teristics section shows propagation delay as a function of

temperature and voltage for 10k pull-up resistors and 50pF

equivalent capacitance on both sides of the part. Also, the

tPHL vs COUT curve for VCC = 5.5V shows that increasing the

INPUT SIDE

150pF

1V/DIV

OUTPUT SIDE

50pF

1V/DIV

200ns/DIV

43071 F02

Figure 2. Input-Output Falling Edge Waveforms

43071fa

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