AD5751 datasheet(29/32 Pages) AD

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Data Sheet

AD5751

Rev. D | Page 29 of 32

APPLICATIONS INFORMATION

TRANSIENT VOLTAGE PROTECTION

The AD5751 contains ESD protection diodes that prevent damage

from normal handling. The industrial control environment can,

however, subject I/O circuits to much higher transients. To protect

the AD5751 from excessively high voltage transients, external

power diodes and a surge current limiting resistor may be

required, as shown in Figure 55. The constraint on the resistor

value is that during normal operation the output level at IOUT

must remain within its voltage compliance limit of AVDD −

2.75 V and the two protection diodes and resistor must have

appropriate power ratings. Further protection can be added

with transient voltage suppressors if needed.

IOUT

AVDD

AD5751

RLOAD

Figure 55. Output Transient Voltage Protection

THERMAL CONSIDERATIONS

It is important to understand the effects of power dissipation

on the package and how it affects junction temperature. The

internal junction temperature should not exceed 125°C. The

AD5751 is packaged in a 32-lead, 5 mm × 5 mm LFCSP pack-

age. The thermal impedance, θJA, is 42°C/W. It is important that

the devices not be operated under conditions that cause the

junction temperature to exceed its limit. Worst-case conditions

occur when the AD5751 are operated from the maximum AVDD

(55 V) and driving the maximum current (24 mA) directly to

ground. The quiescent current of the AD5751 should also be

taken into account, nominally ~4 mA. The following calculations

estimate maximum power dissipation under these worst-case

conditions, and determine maximum ambient temperature based

on this. These figures assume that proper layout and grounding

techniques are followed to minimize power dissipation, as

outlined in the Layout Guidelines section.

Table 12. Thermal and Supply Considerations

Considerations

32-Lead LFCSP Package

Maximum allowed power dissipation when operating at an ambient

temperature of 85°C

125

−

JMAX

Maximum allowed ambient temperature when operating from a supply of

55 V and driving 24 mA directly to ground (include 4 mA for internal AD5751

current)

TJMAX − (PD × θJA) = 125 − ((55 × 0.028) × 42) = 60.3°C

Maximum allowed supply voltage when operating at an ambient

temperature of 85°C and driving 24 mA directly to ground

(

)

028

125

−

JMAX

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