Datasheet

7–196
Power Consumption
Several factors influence the power consumption of a device: frequency of operation, number of outputs switching, load
capacitance, number of TTL-level inputs, junction temperature, ambient temperature, and thermal resistance of the device. For
BTL and GTL devices, the output power is supplied externally by the output voltage supply (V
TT
). The maximum operating
frequency is limited by the thermal characteristics of the package. TI provides package power-dissipation information in data
sheets under “absolute maximum ratings”. These values are calculated using a junction temperature of 150°C and a board trace
length of 750 mils (no airflow).
2
Traces, power planes, connectors, and cooling fans play an important role in improving heat
dissipation. Figure 5 shows the power consumption of BTL and GTL devices driving the backplane described above. As the
frequency increases, GTL16612 power consumption does not increase as fast as the FB1650. This characteristic is due to the
predominant use of CMOS technology, the lower drive current, and the lower voltage swing of GTL (0.8-V swing for GTL versus
1-V swing for BTL). Lower drive current and lower voltage swing are two of the benefits that GTL provides over BTL drivers.
A power-consumption comparison (see Table 2) illustrates the advantage of GTL over BTL when 160 active inputs and outputs
are switching.
3
Another benefit GTL offers is that the family uses the common 56-pin SSOP and TSSOP packages rather than
the 100-pin thin quad flat package (TQFP) with a heat slug mounted above the die in BTL parts. The pin count on the TQFP
package is almost twice the pin count of the SSOP or TSSOP packages.
50
75
100
125
150
175
200
225
250
20 40 60 80 100
Frequency – MHz
I
T
A
= 25°C, V
CC
= 5 V, V
IH
= 3 V, V
IL
= 0 V
All outputs switching
CC
– Input Current – mA
GTL16612
FB1650
Figure 5. FB1650 and GTL16612 Power Consumption With All Outputs Switching
Table 2. Power Comparison (160 Active Inputs and Outputs)
TECHNOLOGY
POWER
(W)
TERMINATION
(BOTH ENDS)
BTL 11 33 to 2.1 V
GTL 2 50 to 1.2 V