PL138-48 Datasheet, PDF (12/20 Pages) Microchip Technology – 2.5V to 3.3V, Low-Skew, 1:4 Differential PECL Fanout Buffer

PL138-48
FIGURE 5-12:
Schematic #2.
LVPECL Termination
Schematic #2 is an alternative simplified termination.
• V_CC = 3.3V
- Ideal value: RT = 48.7Ω
- Commercial value: RT = 50Ω (E24: 51Ω)
• V_CC = 2.5V
- Ideal value: RT = 18.7Ω
- Commercial value: RT = 18Ω
5.3 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 buffer power dissipation
5.3.1 CORE POWER DISSIPATION
The chip core power is equal to V_CC × I_EE. With a worst
case V_CC and I_EE, the power dissipation in the core is
3.63V × 45 mA = 163 mW.
5.3.2
OUTPUT BUFFER POWER
DISSIPATION
The output buffers are not exposed to the full V_CC –
V_EE 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
32 mW. Only buffers that are loaded will have power
dissipation. With all 4 buffers loaded the worst case
output buffer power dissipation will be 128 mW.
Total chip power dissipation, worst case, is 163 mW +
128 mW = 291 mW.
5.3.3 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.
TABLE 5-4:
20-PIN TSSOP THERMAL
RESISTANCE
Air Flow Velocity in
θ_JA Value for JEDEC
Linear Feet/Minute Standard Multi-Layer PCB
0
73°C/W
200
67°C/W
500
64°C/W
The temperature of the chip (junction) will be higher
than the environment (ambient) with an amount equal
to θ_JA × Power. For an ambient temperature of +85°C,
all outputs loaded and no air flow, the junction
temperature T_J = 85°C + 73 × 0.291 = 106°C.
TABLE 5-5:
16-PIN QFN THERMAL
RESISTANCE
Air Flow Velocity in
θ_JA Value for JEDEC
Linear Feet/Minute Standard Multi-Layer PCB
0
60°C/W
200
53°C/W
500
46°C/W
The temperature of the chip (junction) will be higher
than the environment (ambient) with an amount equal
to θ_JA × Power. For an ambient temperature of +85°C,
all outputs loaded and no air flow, the junction
temperature T_J = 85°C + 60 × 0.291 = 102°C.
DS20005543B-page 12
 2016 Microchip Technology Inc.