PL138-48OC-R datasheet(12/20 Pages) MICROCHIP

12 / 20 page

PL138-48

DS20005543B-page 12

 2016 Microchip Technology Inc.

FIGURE 5-12:

LVPECL Termination

Schematic #2.

Schematic #2 is an alternative simplified termination.

•VCC = 3.3V

- Ideal value: RT = 48.7Ω

- Commercial value: RT = 50Ω (E24: 51Ω)

•VCC = 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:

Core power dissipation

Output buffer power dissipation

5.3.1

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 × 45 mA = 163 mW.

5.3.2

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

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.

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 + 73 × 0.291 = 106°C.

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 + 60 × 0.291 = 102°C.

TABLE 5-4:

20-PIN TSSOP THERMAL

RESISTANCE

Air Flow Velocity in

Linear Feet/Minute

θJA Value for JEDEC

Standard Multi-Layer PCB

073°C/W

200

67°C/W

500

64°C/W

TABLE 5-5:

16-PIN QFN THERMAL

RESISTANCE

Air Flow Velocity in

Linear Feet/Minute

θJA Value for JEDEC

Standard Multi-Layer PCB

060°C/W

200

53°C/W

500

46°C/W

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