STDVE001A datasheet(14/49 Pages) STMICROELECTRONICS

14 / 49 page

Functional description

STDVE001A

14/49

Figure 7.

STDVE001A gain vs. frequency

The equalizer of STDVE001A is fully adaptive and automatic in function. The default setting

of EQ = 00 is recommended for optimal operation. The equalizer performance is optimized

for all frequencies over the cable lengths from 1m to 25 m at EQ = 00. If cable lengths

greater than 25 m are desired in application, then EQ = 11 setting is recommended. The

other two EQ settings of 01 and 10 are provided simply for fine-tuning purposes and can be

used for very short external cables or PCB traces only if deemed necessary.

Input termination

The STDVE001A integrates precise 50

Ω ± 5% termination resistors, pulled up to VCC, on all

its differential input channels. External terminations are not required. This gives better

performance and also minimizes the PCB board space. These on-chip termination resistors

should match the differential characteristic impedance of the transmission line. Since the

output driver consists of current steering devices, an output voltage is not generated without

a termination resistor. Output voltage levels are dependent on the value of the total

termination resistance. The STDVE001A produces TMDS output levels for point-to-point

links that are doubly terminated (100

Ω at each end). With the typical 10 mA output current,

the STDVE001A produces an output voltage of 3.3 – 0.5 V = 2.8 V when driving a

termination line terminated at each end. The input terminations are selectable thus saving

power for the unselected ports.

Output buffers

Each differential output of the STDVE001A drives external 50

Ω load (pull-up resistor) and

conforms to the TMDS voltage standard. The output drivers consist of 10 mA differential

current-steering devices.

The driver outputs are short-circuit current limited and are high-impedance to ground when

OE_N = H or the device is not powered. The current steering architecture requires a

resistive load to terminate the signal to complete the transmission loop from VCC to GND

through the termination resistor. Because the device switches the direction of the current

flow and not voltage levels, the output voltage swing is determined by VCC minus the voltage

drop across the termination resistor. The output current drivers are controlled by the OE_N

pin and are turned off when OE_N is a high. A stable 10 mA current is derived by accurate

internal current mirrors of a stable reference current which is generated by band-gap voltage

across the REXT. The differential output driver provides a typical 10 mA current sink

capability, which provides a typical 500 mV voltage drop across a 50

Ω termination resistor.

Gain v/s Freq (STDVE001A)

225

325

410

825

1650

Freq (MHz)

Gain (EQ=00)

Gain (EQ=01)

Gain (EQ=10)

Gain (EQ=11)

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