40MX and 42MX FPGA Families
1-10 Revision 11
Power Dissipation
The general power consumption of MX devices is made up of static and dynamic power and can be
expressed with the following equation:
General Power Equation
P = [ICCstandby + ICCactive] * VCCI + IOL* VOL* N + IOH * (VCCI – VOH) * M
where:
ICCstandby is the current flowing when no inputs or outputs are changing.
ICCactive is the current flowing due to CMOS switching.
IOL, IOH are TTL sink/source currents.
VOL, VOH are TTL level output voltages.
N equals the number of outputs driving TTL loads to VOL.
M equals the number of outputs driving TTL loads to VOH.
Accurate values for N and M are difficult to determine because they depend on the family type, on design
details, and on the system I/O. The power can be divided into two components: static and active.
Static Power Component
The static power due to standby current is typically a small component of the overall power consumption.
Standby power is calcula ted for commercial, worst-case conditions. The static power dissipation by TTL
loads depends on the numbe r of outputs driving, and o n the DC load current. For in stance, a 32-bit bus
sinking 4mA at 0.33V will generate 42mW with all outputs driving LOW, and 140mW with all outputs
driving HIGH. The actual dissipation will average somewhere in between, as I/Os switch states with time.
Active Power Component
Power dissipation in CMOS devices is usually dominated by the dynamic power dissipation. Dynamic
power consumption is frequency-dependent and is a function of the logic and the external I/O. Active
power dissipation results from charging internal chip capacitances of the interconnect, unprogrammed
antifuses, module inputs, and module outputs, plus external capacitances due to PC board traces and
load device inputs. An additi onal component of the active power dissipation is the totem pole current in
the CMOS transistor pairs. The net effect can be associated with an equivalent capacitance that can be
combined with frequency and voltage to represent active power dissipation.
The power dissipated by a CMOS circuit can be expressed by the equation:
Power (µW) = C
EQ
* VCCA2 * F(1)
where:
C
EQ
= Equivalent capacitance expressed in picofarads (pF)
VCCA = Power supply in volts (V)
F = Switching frequency in megahertz (MHz)
Equivalent Capacitance
Equivalent capacitance is calculated by measuring ICCactive at a specified frequency and voltage for
each circuit component of interest. Measurements have been made over a range of frequencies at a
fixed value of VCC. Equivalent capacitance is frequency-independent, so the results can be used over a
wide range of operating conditions. Equivalent capacitance values are shown below.
