Integrated
Circuit
Systems, Inc.
ICS8431-21
350MH
Z
, L
OW
J
ITTER
, C
RYSTAL
O
SCILLATOR
-
TO
-3.3V LVPECL F
REQUENCY
S
YNTHESIZER
P
OWER
C
ONSIDERATIONS
This section provides information on power dissipation and junction temperature for the ICS8431-21.
Equations and example calculations are also provided.
1. Power Dissipation.
The total power dissipation for the ICS8431-21 is the sum of the core power plus the power dissipated in the load(s).
The following is the power dissipation for V
CC
= 3.3V + 5% = 3.465V, which gives worst case results.
NOTE:
Please refer to Section 3 for details on calculating power dissipated in the load.
•
•
Power (core)
MAX
= V
CC_MAX
* I
EE_MAX
= 3.465V * 155mA =
537.1mW
Power (outputs)
MAX
=
30mW/Loaded Output pair
If all outputs are loaded, the total power is 1 * 30mW =
30mW
Total Power
_MAX
(3.465V, with all outputs switching) = 537.1mW + 30mW =
567.1mW
2. Junction Temperature.
Junction temperature, Tj, is the temperature at the junction of the bond wire and bond pad and directly affects the reliability of the
device. The maximum recommended junction temperature for HiPerClockS
TM
devices is 125°C.
The equation for Tj is as follows: Tj =
θ
JA
* Pd_total + T
A
Tj = Junction Temperature
θ
JA
= Junction-to-Ambient Thermal Resistance
Pd_total = Total Device Power Dissipation (example calculation is in section 1 above)
T
A
= Ambient Temperature
In order to calculate junction temperature, the appropriate junction-to-ambient thermal resistance
θ
JA
must be used. Assuming a
moderate air flow of 200 linear feet per minute and a multi-layer board, the appropriate value is 39.7°C/W per Table 7 below.
Therefore, Tj for an ambient temperature of 85°C with all outputs switching is:
85°C + 0.567W * 39.7°C/W = 107.5°C. This is below the limit of 125°C.
This calculation is only an example. Tj will obviously vary depending on the number of loaded outputs, supply voltage, air flow,
and the type of board (single layer or multi-layer).
Table 7. T
HERMAL
R
ESISTANCE
θ
JA
FOR
28-
PIN
SOIC, F
ORCED
C
ONVECTION
θ
JA
by Velocity (Linear Feet per Minute)
0
Single-Layer PCB, JEDEC Standard Test Boards
Multi-Layer PCB, JEDEC Standard Test Boards
76.2°C/W
46.2°C/W
200
60.8°C/W
39.7°C/W
500
53.2°C/W
36.8°C/W
NOTE:
Most modern PCB designs use multi-layered boards. The data in the second row pertains to most designs.
8431AM-21
www.icst.com/products/hiperclocks.html
12
REV. A APRIL 27, 2005
ICS [ INTEGRATED CIRCUIT SYSTEMS ]
