PL138-28 Datasheet, PDF (9/11 Pages) Micrel Semiconductor – 2.5V-3.3V Low-Skew 1-2 Differential PECL Fanout Buffer

PL138-28
2.5V-3.3V Low-Skew 1-2 Differential PECL Fanout Buffer
POWER CONSIDERATIONS
Driving LVPECL outputs requires an amount of power that can warm up the chip significantly.
The general requirement for the chip is that the junction temperature should not exceed +110°C.
The power consumption can be divided into two parts:
1) Core power dissipation
2) Output buffers power dissipation
CORE POWER DISSIPATION
The chip core power is equal to VCC×IEE. With a worst case VCC and IEE the power dissipation in the core is
3.63V×45mA=163mW.
OUTPUT BUFFER POWER DISSIPATION
The output buffers are not exposed to the full VCC-VEE voltage. On the differential output, one line is at logic 1
with a small voltage across the buffer and a large output current. The other line is at logic 0 with a larger voltage
across the buffer and a smaller output current. The power dissipation per output buffer is 32mW. Only buffers that
are loaded will have power dissipation. With both buffers loaded the worst case output buffer power dissipation
will be 64mW.
Total Chip Power Dissipation, worst case, is 163mW + 64mW = 227mW.
JUNCTION TEMPERATURE
How much the chip is warmed up from the power dissipation depends upon the thermal resistance from the chip to
the environment, also known as “junction to ambient”. The thermal resistance depends upon the type of package,
how the package is assembled to the PCB and if there is additional air flow for improved cooling. For the LQFP
package with use of the Thermal Relief pad, the thermal resistance is as follows:
JEDEC Standard Multi Layer PCB
SOIC 8-pin Package
TSSOP 8-pin Package
Air Flow Velocity in Linear Feet per Minute
0
200
500
θJA = 97°C/W
θJA = 85°C/W
θJA = 77°C/W
θJA = 120°C/W
θJA = 105°C/W
θJA = 95°C/W
SOIC 8-pin:
The temperature of the chip (junction) will be higher than the environment (ambient) with an amount equal to θJA ×
Power. For an ambient temperature of +85°C, all outputs loaded and no air flow, the junction temperature TJ =
85°C+97×0.227 = 107°C.
TSSOP 8-pin:
For an ambient temperature of +85°C, all outputs loaded and 200LFM air flow, the junction temperature TJ =
85°C+105×0.227 = 109°C. It is recommended to use at least 200LFM air flow to prevent the junction temperature
from increasing above +110°C.
For use up to +80°C or lower no air flow is needed: TJ = 80°C+120×0.227 = 107°C.
Micrel Inc. • 2180 Fortune Drive • San Jose, CA 95131 • USA • tel [phone] • fax [phone] • www.micrel.com Rev 11/26/13 Page 9