4

APPLICATION NOTES CONT'D

HEAT SINKING

Radiation performance curves for TID testing have been generated

for all radiation testing performed by MS Kennedy. These curves

show performance trends throughout the TID test process and can

be located in the MSK5977RH radiation test report. The complete

radiation test report is available in the RAD HARD PRODUCTS section

on the MSK website.

APPLICATION NOTES CONT'D

TOTAL DOSE RADIATION TEST

PERFORMANCE

IMPROVING INITIAL ACCURACY AND

REDUCING TEMPERATURE DRIFT

ADDING SHUTDOWN

The MSK5979RH can be easily shutdown by either reducing

Rset to 0Ω or connecting a transistor from the set pin to ground.

By connecting two transistors, as shown in Figure 6, a low current

voltage source is all that is required to take the set pin to ground

as well as pull the output voltage to ground. Q2 pulls the output

voltage to ground when no load is present and only needs to sink

10mA. Use a low leakage switching diode between Vout and Set

to avoid overstress during shutdown transitions.

FIGURE 6

The initial output accuracy of the MSK5979RH due to set pin current

tolerance and set point resistor accuracy can be reduced to 0.2% us-

ing the MSK109RH radiation hardened precision reference. Minimal

drift of the MSK109RH from temperature extremes and irradiation

ensure very tight regulation. The circuit can be configured to use

the 2.5V reference to directly set the output at 2.5V or with a slight

variation it can provide any output within the operating range of the

MSK5979RH down to 0V output. Select Rs to maintain between

1mA and 10mA of current through the reference; see Figure 5 below.

Rs may be tied to VIN or another power source. The optional trim

resistor can be used to further trim out initial output and system

error. Reference the MSK109RH data sheet for application circuits

that provide stable output voltages across the full operating range

of the MSK5979RH including down to 0V output and the operating

characteristics of the MSK109RH.

FIGURE 5

ADDITIONAL APPLICATION INFORMATION

For additional applications information, please reference Linear Tech-

nology Corporation's® LT3080 and RH3080 data sheets.

To determine if a heat sink is required for your application and if so,

what type, refer to the thermal model and governing equation below.

Governing Equation: Tj = Pd x (Rθjc + Rθcs + Rθsa) + Ta

WHERE

Tj = Junction Temperature

Pd = Total Power Dissipation

Rθjc = Junction to Case Thermal Resistance

Rθcs = Case to Heat Sink Thermal Resistance

Rθsa = Heat Sink to Ambient Thermal Resistance

Tc = Case Temperature

Ta = Ambient Temperature

Ts = Heat Sink Temperature

EXAMPLE:

This example demonstrates the thermal calculations for the TO-257

package with the regulator operating at one-half of its maximum

rated output current.

Conditions for MSK5979RH:

1.) Assume 45° heat spreading model.

2.) Find regulator power dissipation:

Pd = (VIN - VOUT)(Iout)

Pd = (3-1)(0.45)

= 0.9W

3.) For conservative design, set Tj = +125°C Max.

4.) For this example, worst case Ta = +90°C.

5.) Rθjc = 5.0°C/W from the Electrical Specification Table.

6.) Rθcs= 0.15°C/W for most thermal greases.

7.) Rearrange governing equation to solve for Rθsa:

Rθsa= ((Tj - Ta)/Pd) - (Rθjc) - (Rθcs)

= (125°C - 90°C)/0.9W - 5.0°C/W - 0.15°C/W

= 33.7°C/W

In this case the result is 33.7°C/W. Therefore, a heat sink with a ther-

mal resistance of no more than 33.7°C/W must be used in this appli-

cation to maintain regulator circuit junction temperature under 125°C.

For enhanced radiation tolerance the die has a glassivation thick-

ness of 4KA and is not in accordance with MIL-PRF-38534.

DIE GLASSIVATION

8548-133 Rev. B 8/15