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6.6.1  Power Dissipation

As a general rule a plastic package can dissipate about 1 W, and more expensive ceramic packages can dissipate up to about 2 W. Table 6.3 shows the thermal characteristics of common packages. In a high-speed (high-power) design the ASIC power consumption may dictate your choice of packages. Actel provides a formula for calculating typical dynamic chip power consumption of their FPGAs. The formula for the ACT 2 and ACT 3 FPGAs are complex; therefore we shall use the simpler formula for the ACT 1 FPGAs as an example ¹ :

TABLE 6.3  Thermal characteristics of ASIC packages.
Package 2	Pin count	Max. power P max /W	q JA /°CW –1 (still air) 3 , 4	q JA /°CW –1 (still air) 5
CPGA	84		33	32–38
CPGA	100		35
CPGA	132		30
CPGA	175		25	16
CPGA	207		22
CPGA	257		15
CQFP	84		40
CQFP	172		25
PQFP	100	1.0	55	56–75
PQFP	160	1.75	33	30–33
PQFP	208	2.0	33	27-32
VQFP	80		68
PLCC	44		52	44
PLCC	68		45	28–35
PLCC	84	1.5	44
PPGA	132			33–34

Total chip power = 0.2 (N ¥ F1) + 0.085 (M ¥ F2) + 0.8 ( P ¥ F3) mW

(6.7)

where

F1 = average logic module switching rate in MHz

F2 = average clock pin switching rate in MHz

F3 = average I/O switching rate in MHz

M = number of logic modules connected to the clock pin

N = number of logic modules used on the chip

P = number of I/O pairs used (input + output), with 50 pF load

As an example of a power-dissipation calculation, consider an Actel 1020B-2 with a 20 MHz clock. We shall initially assume 100 percent utilization of the 547 Logic Modules and assume that each switches at an average speed of 5 MHz. We shall also initially assume that we use all of the 69 I/O Modules and that each switches at an average speed of 5 MHz. Using Eq.  6.7 , the Logic Modules dissipate

P LM = (0.2)(547)(5) = 547 mW ,

(6.8)

and the I/O Module dissipation is

P IO = (0.8)(69)(5) = 276 mW .

(6.9)

If we assume the clock buffer drives 20 percent of the Logic Modules, then the additional power dissipation due to the clock buffer is

P CLK = (0.085)(547)(0.2)(5) = 46.495 mW .

(6.10)

The total power dissipation is thus

P D = (547 + 276 + 46.5) = 869.5 mW ,

(6.11)

or about 900 mW (with an accuracy of certainly no better than ± 100 mW).

Suppose we intend to use a very thin quad flatpack ( VQFP ) with no cooling (because we are trying to save area and board height). From Table 6.3 the thermal resistance, q _JA , is approximately 68 °CW ^–1 for an 80-pin VQFP. Thus the maximum junction temperature under industrial worst-case conditions (T _A = 85 °C) will be

T J = (85 + (0.87)(68)) = 144.16 °C ,

(6.12)

(with an accuracy of no better than 10 °C). Actel specifies the maximum junction temperature for its devices as T _Jmax = 150 °C (T _Jmax for Altera is also 150 °C, for Xilinx T _Jmax = 125°C). Our calculated value is much too close to the rated maximum for comfort; therefore we need to go back and check our assumptions for power dissipation. At or near 100 percent module utilization is not unreasonable for an Actel device, but more questionable is that all nodes and I/Os switch at 5 MHz.

Our real mistake is trying to use a VQFP package with a high q _JA for a high-speed design. Suppose we use an 84-pin PLCC package instead. From Table 6.3 the thermal resistance, q _JA , for this alternative package is approximately 44 °CW ^–1 . Now the worst-case junction temperature will be a more reasonable

T J = (85 + (0.87)(44)) = 123.28 °C ,

(6.13)

It is possible to estimate the power dissipation of the Actel architecture because the routing is regular and the interconnect capacitance is well controlled (it has to be since we must minimize the number of series antifuses we use). For most other architectures it is much more difficult to estimate power dissipation. The exception, as we saw in Section 5.4 “Altera MAX,” are the programmable ASICs based on programmable logic arrays with passive pull-ups where a substantial part of the power dissipation is static.

6.6.2 Power-On Reset

Each FPGA has its own power-on reset sequence. For example, a Xilinx FPGA configures all flip-flops (in either the CLBs or IOBs) as either SET or RESET. After chip programming is complete, the global SET/RESET signal forces all flip-flops on the chip to a known state. This is important since it may determine the initial state of a state machine, for example.