ics8745b-21 Integrated Device Technology, ics8745b-21 Datasheet - Page 15

• Manufacturer Part Number: ics8745b-21
• Description: 1 1 Differential-to-lvds Zero Delay Clock Generator
• Manufacturer: Integrated Device Technology

Power Considerations
This section provides information on power dissipation and junction temperature for the ICS8745B-21.
Equations and example calculations are also provided.
1.
The total power dissipation for the ICS8745B-21 is the sum of the core power plus the analog power plus the power dissipated in the
load(s).
The following is the power dissipation for V
NOTE: Please refer to Section 3 for details on calculating power dissipated in the load.
Total Power_
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 devices is 125°C.
In order to calculate junction temperature, the appropriate junction-to-ambient thermal resistance θ
air flow of 200 linear feet per minute and a multi-layer board, the appropriate value is 39.7°C/W per Table 7below.
Therefore, Tj for an ambient temperature of 70°C with all outputs switching is:
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. Thermal Resitance
IDT™ / ICS™ LVDS ZERO DELAY CLOCK GENERATOR
Linear Feet per Minute
Single-Layer PCB, JEDEC Standard Test Boards
Multi-Layer PCB, JEDEC Standard Test Boards
NOTE: Most modern PCB designs use multi-layered boards. The data in the second row pertains to most designs.
ICS8745B-21
1:1 DIFFERENTIAL-TO-LVDS ZERO DELAY CLOCK GENERATOR
Power Dissipation.
•
•
70°C + 0.696W * 39.7°C/W = 97.6°C. This is well below the limit of 125°C.
Power (core)
Power (outputs)
The equation for Tj is as follows: Tj = θ
Tj = Junction Temperature
θ
Pd_total = Total Device Power Dissipation (example calculation is in section 1 above)
T
JA
A
= Ambient Temperature
= Junction-to-Ambient Thermal Resistance
MAX
= 492mW + 204mW = 696mW
MAX
MAX
= V
= V
DD_MAX
DDO_MAX
θ
JA
for 20 Lead TSSOP, Forced Convection
* (I
DD_MAX
* I
DD
DDO_MAX
= 3.3V + 5% = 3.465V, which gives worst case results.
JA
+ I
* Pd_total + T
DDA_MAX
= 3.465V * 59mA = 204mW
θ
) = 3.465V * (125mA + 17mA) = 492mW
JA
83.2°C/W
46.2°C/W
vs. Air Flow
A
0
15
65.7°C/W
39.7°C/W
200
ICS8745BM-21REV. C OCTOBER 27, 2008
JA
must be used. Assuming a moderate
57.5°C/W
36.8°C/W
500