AD8178 datasheet(33/40 Pages) AD

33 / 40 page

AD8178

Rev. 0 | Page 33 of 40

QNPN

QPNP

VNEG

VPOS

VOUTPUT

IOUTPUT

IO, QUIESCENT

Figure 51. Simplified Output Stage

Example

With an ambient temperature of 85°C, all nine RGB output

channels driving 1 Vrms into 100 Ω loads, and power supplies at

±2.5 V, follow these steps:

Calculate the power dissipation of the AD8178 using data

sheet quiescent currents, neglecting the VDD current because

it is insignificant.

PD,QUIESCENT = (VPOS × IVPOS) + (VNEG × IVNEG)

(4)

PD,QUIESCENT = (2.5 V × 460 mA) + (2.5 V × 460 mA) = 2.3 W

Calculate power dissipation from loads. For a differential

output and ground-referenced load, the output power is

symmetrical in each output phase.

PD,OUTPUT = (VPOS − VOUTPUT,RMS) × IOUTPUT,RMS

(5)

PD,OUTPUT = (2.5 V − 1 V) × (1 V/100 Ω) = 15 mW

There are 15 output pairs, or 30 output currents.

nPD,OUTPUT = 30 × 15 mW = 0.45 W

Subtract quiescent output stage current for the number of

loads (30 in this example). The output stage is either standing

or driving a load, but the current needs to be counted only

once (valid for output voltages > 0.5 V).

PDQ,OUTPUT = (VPOS − VNEG) × IOUTPUT,QUIESCENT

(6)

PDQ,OUTPUT = (2.5 V − (−2.5 V)) × 1.65 mA = 8.25 mW

There are 15 output pairs, or 30 output currents.

nPD,OUTPUT = 30 × 8.25 mW = 0.25 W

Verify that the power dissipation does not exceed the

maximum allowed value.

PD,ON-CHIP = PD,QUIESCENT + nPD,OUTPUT − nPDQ,OUTPUT

(7)

PD,ON-CHIP = 2.3 W + 0.45 W − 0.25 W = 2.5 W

From Figure 50 or Equation 1, this power dissipation is below

the maximum allowed dissipation for all ambient temperatures

up to and including 85°C.

In a general case, the power delivered by the digital supply and

dissipated into the digital output devices has to be taken into

account following a similar derivation. However, because the

loads driven by the H and V outputs are high and because the

voltage at these outputs typically sits close to either rail, the

correction to the on-chip power estimate is small. Furthermore,

the H and V outputs are active only briefly during sync

generation and returned to digital ground thereafter.

Short-Circuit Output Conditions

Although there is short-circuit current protection on the AD8178

outputs, the output current can reach values of 80 mA into

a grounded output. Any sustained operation with too many

shorted outputs can exceed the maximum die temperature

and can result in device failure (see the Absolute Maximum

Ratings section).

Crosstalk

Many systems (such as KVM switches) that handle numerous

analog signal channels have strict requirements for keeping the

various signals from influencing any of the other signals in the

system. Crosstalk is the term used to describe the coupling of

the signals of other nearby channels to a given channel.

When there are many signals in close proximity in a system,

as is undoubtedly the case in a system that uses the AD8178,

the crosstalk issues can be quite complex. A good understanding

of the nature of crosstalk and some definition of terms is required

to specify a system that uses one or more crosspoint devices.

Types of Crosstalk

Crosstalk can be propagated by means of any of three methods.

These fall into the categories of electric field, magnetic field, and the

sharing of common impedances. This section explains these effects.

Every conductor can be both a radiator of electric fields and

a receiver of electric fields. The electric field crosstalk mechanism

occurs when the electric field created by the transmitter propagates

across a stray capacitance (for example, free space) and couples

with the receiver and induces a voltage. This voltage is an unwanted

crosstalk signal in any channel that receives it.

Currents flowing in conductors create magnetic fields that circulate

around the currents. These magnetic fields then generate voltages

in any other conductors whose paths they link. The undesired

induced voltages in these other channels are crosstalk signals.

The channels that crosstalk can be said to have a mutual inductance

that couples signals from one channel to another.

The power supplies, grounds, and other signal return paths

of a multichannel system are generally shared by the various

channels. When a current from one channel flows in one of these

paths, a voltage that is developed across the impedance becomes

an input crosstalk signal for other channels that share the common

impedance.

All these sources of crosstalk are vector quantities, so the magni-

tudes cannot simply be added together to obtain the total crosstalk.

In fact, there are conditions where driving additional circuits in

parallel in a given configuration can actually reduce the crosstalk.

The fact that the AD8178 is a fully differential design means that

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