I am using the ADA4700-1 to provide +-40V DC with a 0 to +2.5V input. The output of the ADA4700-1 drives a 47K ohm load (40v=<1ma).
The ADA4700-1 is mounted to a DIP adapter board. I am not able to attach the heat sink pad.
Operating with very little load, I note that the device gets quite hot, and rather rapidly. Not hot enough that it shuts down, but it's still concerning.
Is it expected that the device will get hot, even when a load is not present?
Please allow us to study and analyze your issue. We will keep you posted. Thank you for your patience.
The thermal management of the ADA4700-1 can be found on page 22 of the datasheet.
To calculate the power dissipation and the junction temperature, please use equation (1) and (2).
Notice in equation 2, the power disputation PD depends on 4 terms – Vs, Vout, Isy, Iout. And θJA can be found in table 5 on page 8.
A quick PD calculation without a load, PD = 210mW for Vs =±50 (or 168mW for Vs=±40) assuming ISY=2.1mA. Then, PD × θJA = 9.4°C for Vs =±50 (or 7.5°C Vs =±40), using θJA=45°C/W.
As you can see, even without any load, the high voltage ADA4700-1 has a self-heat temperature of ~10°C for Vs =±50.
That is the reason why thermal management is very important for HV devices. The ADA4700-1 are designed to operate under the specified safe operating area. Refer to the SOA section on page 22 for more information.
The above calculation is assuming that the exposed pad is soldered down to a plane. When the exposed pad is not soldered down, the θJA could easily triple in value, i.e. ~30°C in addition to ambient temperature. This is about 60°C, similar to a hot coffee cup.