AN246 Ver la hoja de datos (PDF) - Philips Electronics

Número de pieza

componentes Descripción

fabricante

AN246

Transmission lines and terminations with Philips Advanced Logic families

Philips Electronics 

Philips Semiconductors

Transmission lines and terminations

with Philips Logic families

Application Note

AN246

Typical characteristic impedances on PC boards can be from 50 Ω

to 100 Ω. The impedance can be determined by

Eq. 2

Ǹ ZO +

LO

CO

where L0 and C0 are the characteristic inductance and capacitance

per unit length of the trace.

The line propagation delay can be determined by

Eq. 3

TO + ǸLOCO

Distributed capacitive loads lower the effective impedance of a

transmission line and increase the line propagation delay. Consider

a bus structure with equally spaced loads of the same value as in

Figure 2. The capacitors represent the input capacitance of each

receiver.

ABT244

L = 10 in.

ZO = 65 Ω

LO = 9.2 nH/in.

CO = 2.2 pF/in.

TO = 142 ps/in.

C’s = 5 pF

SH00115

Figure 2. Equally spaced capacitive loads

If the driver’s rise or fall time is longer than the electrical length of

the spacing between the loads, the effects of individual capacitors

distribute evenly across the waveform edge. This adds capacitance

to the line’s characteristic capacitance. The board interconnect at

the receiver pin has capacitance also: via, connector, etc., and the

values are added to the receiver’s capacitance to form a lumped

value. Suppose the interconnect capacitance is 5 pF, then the

lumped distributed capacitance is 10 pF per every 2 inches or 5 pF

per inch. The new line impedance, ZO’, can be calculated and will

be

Eq. 4

Ǹ ZOȀ +

ZO

1

)

CLU

CO

Ǹ +

65 W

1

)

5 pFń in.

2.2 pFń in.

+ 36 W

where CLU = load capacitance per unit length, pF/in.

Likewise, the new line propagation delay will be

Eq. 5

Ǹ TOȀ + TO

1

)

CLU

CO

Ǹ + 142 psńin.

1

)

5 pFń in.

2.2 pFń in.

+ 257 psńin.

Since the effective line impedance can be reduced with more

loading, a driver with sufficient source and sink capability should be

chosen to drive that particular impedance. This is discussed in the

next section.

INCIDENT WAVE SWITCHING AND DRIVER I-V

CHARACTERISTICS

When launching a pulse down the line, the driver needs sufficient

current to change the voltage on the line. For TTL level input

receivers, the guaranteed VIH and VIL levels are 2.0 V and 0.8 V.

This means that the leading edge incident wave launched down the

line should meet those levels to switch all receivers on the line and

switch them only once. The drive current required is

Eq. 6

IAV

at

VOH

+ VIH

min * VOL

ZOȀ

typ

Eq. 7

IAV

at

VOL

+ VOH

typ * VIL

ZOȀ

max

As an example of incident wave switching capability, refer back to

the bus structure in Figure 2. The effective line impedance is 34 Ω.

Using Equations 6 and 7, the drive current required to switch the line

is determined as follows:

IAV

at

VOH

+ VIH

min * VOL

ZOȀ

typ

+

2

V*

36

0.2

W

V

+ 50 mA

and

IAV

at

VOL

+ VOH

typ * VIL

ZOȀ

max

+

3.4

V * 0.8

36 W

V

+ 72 mA

ABT products are rated for +32 mA source current at 2 V and –64

mA sink current at 0.55 V. By referring to I-V curves you can

determine if the dynamic drive current is enough to switch the line

on the incident wave. From the following curves in Figures 3 and 4,

note that the –76 mA at 2 V and +167 mA at 0.8 V satisfies the

requirements in the above formulas. To compare the drive strength

of other product families, Figures 5 through 9 show IOL and IOH

currents for a typical ‘244 driver for the ABT16, ALVC, ALVT, LVC,

LVT, and LVT16 families.

1998 Feb 05

3

Share Link: