Datasheet

7–202
V
TT
R
C2R
V
REF
Figure 17. Proposed Circuit to Generate V
REF
For the BTL family, four power supplies and two grounds are connected. For live-insertion applications, the power-up sequence
should be: the GND pin should make contact first, followed by BIAS V
CC
. This sequence will precharge the board and the device
capacitance and will establish a voltage between 1.62 V and 2.1 V on the BTL outputs. Next, the V
CC
pin makes contact and,
as V
CC
ramps up, the BIAS V
CC
circuitry starts to turn off. When V
CC
reaches its final value, the BIAS V
CC
circuitry is completely
isolated and does not interfere with the device functionality. BG V
CC
and BG GND pins supply power to the bias generator input
circuitry. BG V
CC
and BG GND must be isolated from the other power supplies to ensure signal integrity at the BTL input. The
2.1-V V
TT
should be regulated from a higher voltage and should supply enough current to switch all 18 outputs (100 mA per
output). V
TT
variation should not exceed ±2% and it is recommended that proper bypass capacitors (0.01 µF or 0.1 µF) be used.
The termination resistor should not exceed ±1% of its resistance value.
Table 3 gives the designer an estimate of the maximum number of loads allowed when designing with GTL and BTL families.
4
Note that crosstalk and poor board layout can degrade the overall quality of the backplane, thereby affecting the number of loads.
Using the formula:
t
r
,t
f
+ 2.2 Z
S
[
(
L C
O
)
)
(
N C
N
)
]
and assuming
t
r
,t
f
+ t
p
+
1
2f
(for worst-case condition), the maximum number of loads on the backplane (N) can be calculated
as follows:
N +
1
4.4 f Z
S
C
N
10
*6
*
L C
O
C
N
Where:
t
r
= Rise time of the device (ns)
t
f
= Fall time of the device (ns)
Z
S
= Output impedance of the source (Ω), 25 Ω for GTL, 16.5 Ω for BTL
C
O
= Characteristic capacitance per unit length of the transmission line (pF/in.) (see Table 3)
L = Length of the backplane (in.)
N = Maximum number of loads on the backplane
C
N
= Capacitance for each load (pF), 5 pF for the device, 5 pF for the connector
t
p
= Pulse width of the signal (ns)
f = Frequency of the signal on the backplane (MHz)
(3)
(4)