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Data Sheet

ADR3625/ADR3630/ADR3650

THEORY OF OPERATION

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Rev. A | 19 of 26

The ADR3625\ADR3630\ADR3650 incorporate a proprietary,

low-noise design and advanced, curvature correction tech-

nology. Combined with micropower design techniques, the

ADR3625\ADR3630\ADR3650 have unmatched performance.

The circuit shown in Figure 60 shows the

ADR3625\ADR3630\ADR3650 simplified schematic.

Figure 60. Simplified Schematic

POWER DISSIPATION

While the ADR3625\ADR3630\ADR3650 are a micropower voltage

reference, these devices are capable of sourcing 50 mA and sink-

ing current up to 10 mA of load current at room temperature across

the rated input voltage range. However, when used in applications

subject to high ambient temperatures, the input voltage and load

current must be monitored carefully to ensure that the devices do

not exceed their maximum allowable power dissipation. To calculate

the maximum allowable power dissipation of these devices, use the

following equation:

PD=TJ−TAθJA

where:

PD is the power dissipation of the devices.

TJ is the junction temperature of the devices.

TA is the ambient temperature of the devices.

The ADR3625\ADR3630\ADR3650 have a thermal resistance of

132.5°C/W for the 8-lead MSOP. A curve that illustrates the allowed

power dissipation vs. temperature for the MSOP is shown in Figure

61. The power dissipation of the ADR3625 as a function of the

input voltage is shown in Figure 62. The top curve shows power

dissipation with a 50 mA load, and the bottom curve shows power

dissipation with no load. When operated within its specified limits

of VIN = 16 V and sourcing is 50 mA, the ADR3625 consumes

about 800 mW at room temperature. The power derating curve in

Figure 61 shows that the ADR3625\ADR3630\ADR3650 can only

safely dissipate 120 mW at 125°C, which is less than its maximum

power output. Care must be taken when designing the circuit so

that the maximum junction temperature is not exceeded. For best

performance, keep the junction temperature less than 150°C.

The ADR3625\ADR3630\ADR3650 includes output current-limit cir-

cuitry, as well as thermal limit circuitry, to protect the reference

from damage in the event of excessive power dissipation. The

ADR3625\ADR3630\ADR3650 is protected from damage by a ther-

mal shutdown circuit. However, changes in performance can occur

as a result of operating at above the maximum rating.

Figure 61. Maximum Allowed Power Dissipation

Figure 62. ADR3625 Typical Power Dissipation

INTERNAL PROTECTION

The ADR3625\ADR3630\ADR3650 have two internal protection

circuits for monitoring the output current and internal junction tem-

perature.

The ADR3625\ADR3630\ADR3650 have a current-limit circuit that

limits the output source current in the event of a short circuit from

the VOUT FORCE pin to ground. The limit is set to 90 mA (typical)

and does not vary much over temperature or input voltage.

An overtemperature shutdown circuit disables the output source

current when the internal junction temperature reaches 178°C.

There is 3°C of hysteresis. Once the junction temperature reduces

to less than the hysteresis, the output drive circuit re-enables. Deg-

radation can occur or reliability can be affected when the junction

temperature of the devices exceeds 150°C.

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