MPC941 datasheet(7/13 Pages) IDT

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REVISION 10 3/18/15

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MPC941 DATA SHEET

Power Consumption of the MPC941 and Thermal

Management

The MPC941 AC specification is guaranteed for the entire

operating frequency range up to 250 MHz. The MPC941

power consumption and the associated long-term reliability

may decrease the maximum frequency limit, depending on

operating conditions such as clock frequency, supply voltage,

output loading, ambient temperture, vertical convection and

thermal conductivity of package and board. This section

describes the impact of these parameters on the junction

temperature and gives a guideline to estimate the MPC941

die junction temperature and the associated device reliability.

For a complete analysis of power consumption as a function

of operating conditions and associated long term device

reliability, please refer to the Freescale application note

AN1545. According the AN1545, the long-term device

reliability is a function of the die junction temperature:

Increased power consumption will increase the die

junction temperature and impact the device reliability

(MTBF). According to the system-defined tolerable MTBF,

the die junction temperature of the MPC941 needs to be

controlled, and the thermal impedance of the board/package

should be optimized. The power dissipated in the MPC941 is

represented in equation 1.

Where ICCQ is the static current consumption of the

MPC941, CPD is the power dissipation capacitance per

output.

C

L represents the external capacitive output

load, and N is the number of active outputs (N is always 27 in

case of the MPC941). The MPC941 supports driving

transmission lines to maintain high signal integrity and tight

timing parameters. Any transmission line will hide the lumped

capacitive load at the end of the board trace, therefore,

C

is zero for controlled transmission line systems and can be

eliminated from equation 1. Using parallel termination output

termination results in equation 2 for power dissipation.

In equation 2, P stands for the number of outputs with a

parallel or thevenin termination. VOL, IOL, VOH and IOH are a

function of the output termination technique, and DCQ is the

clock signal duty cyle. If transmission lines are used,

C

L is

zero in equation 2 and can be eliminated. In general, the use

of controlled transmission line techniques eliminates the

impact of the lumped capacitive loads at the end lines and

greatly reduces the power dissipation of the device.

Equation 3 describes the die junction temperature TJ as a

function of the power consumption.

Where Rthja is the thermal impedance of the package

(junction to ambient), and TA is the ambient temperature,

according to Table 7, the junction temperature can be used to

estimate the long-term device reliability. Further, combining

equation 1 and equation 2 results in a maximum operating

frequency for the MPC941 in a series terminated

transmission line system.

TJ,MAX should be selected according to the MTBF system

requirements, and Table 7, Rthja can be derived from Table 8.

The Rthja represent data based on 1S2P boards. Using 2S2P

boards will result in a lower thermal impedance than indicated

below.

If the calculated maximum frequency is below 250 MHz, it

becomes the upper clock speed limit for the given application

conditions. The following eight derating charts describe the

safe frequency operation range for the MPC941. The charts

were calculated for a maximum tolerable die junction

temperature of 110

C (120C), corresponding to a estimated

MTBF of 9.1 years (4 years), a supply voltage of either 3.3 V

or 2.5 V, and series terminated transmission line or capacitive

loading. Depending on a given set of these operating

conditions and the available device convection, a decision on

the maximum operating frequency can be made.

Table 7. Die Junction Temperature and MTBF

Junction Temperature (C)

MTBF (Years)

100

20.4

110

9.1

120

4.2

130

2.0

Table 8. Thermal Package Impedance of the 48ld LQFP

Convection, LFPM

Rthja (1P2S board), K/W

Still air

100 lfpm

200 lfpm

300 lfpm

400 lfpm

500 lfpm

PTOT = [ ICCQ + VCC · fCLOCK · ( N · CPD +  CL ) ] · VCC

PTOT = VCC · [ ICCQ + VCC · fCLOCK · ( N · CPD +  CL ) ] +  [ DCQ · IOH · (VCC – VOH) + (1 – DCQ) · IOL · VOL ]

TJ = TA + PTOT · Rthja

fCLOCK,MAX =

CPD · N · V

[

– (ICCQ · VCC)

]

Rthja

Tj,MAX – TA

Equation 1

Equation 2

Equation 3

Equation 4

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