Datasheet

SCEA019 - January 2001
7–106 Texas Instruments GTLP Frequently Asked Questions
P
STAT
= 0.5 × 0.5 × V
CC
(I
CCL
+ I
CCH
) + 0.5 × V
CC
(I
CCL
) = V
CC
(0.75 × I
CCL
+ 0.25 ×
I
CCH
)
I
CCL
, I
CCH
, and I
CCZ
are identical for CMOS devices, but different for BiCMOS/bipolar
devices. Normally, these are combined in CMOS-device data sheets, but, because both the
GTL and GTLP families include both CMOS and BiCMOS devices, all values are included to
preclude questions about whether all values are the same or only the maximum value was
included.
Dynamic Power
While switching the outputs, most of the dynamic power is generated in the output structure of
devices that have totem-pole configurations. Because GTL and GTLP devices have
open-drain/collector outputs and no active pullup transistors, dynamic power caused by
current spikes due to internal charging and discharging processes within the GTLP circuit can
be approximated as a very small value that, in comparison to the static and output power, is
negligible.
The Thin Very Small-Outline Package (TVSOP) application report, literature number
SCBA009C, March 1997, at www.ti.com/sc/docs/psheets/abstracts/app/scba009.htm,
provides more information on how to calculate dynamic power consumption for devices with
totem-pole outputs.
Output Power
Output power per bit is:
P
OUT
= I
OL
× V
OL
× (1 – output duty cycle)
Where
I
OL
= (V
TT
– V
OL
) / (R
TT
/ 2)
Example:
For an output duty cycle of 60% (output stays high 60% of the cycle),
assuming V
TT
= 1.5 V, V
OL
= 0.55 V, and R
TT
= 38 Ω, then:
I
OL
= (V
TT
– V
OL
) / (R
TT
/ 2) = (1.5 V – 0.55 V) / (38 Ω / 2) = 50 mA
P
OUT
= I
OL
× V
OL
× (1 – output duty cycle) = 50 mA × 0.55 V × (1 – 0.6) = 11 mW per bit
For an 18-bit device running all 18 bits at 60% output duty cycle,
P
OUT
= 18 × 11 mW = 198 mW
Total Power Consumption
Total power consumption for an 18-bit device operating at 3.3-V V
CC
is calculated using the
following equations. Nine outputs are switching at a 50% duty cycle with the other nine
outputs at a fixed static low: