OPA551: OPA551

Hello Shlomi,

I reviewed your OPA551.TSC circuit and here are my thoughts regarding your questions:

1. Can I use OPA551?

It is a little difficult to know for sure of the OPA551 will be able to meet the 5 us switching time. Turing the PIN diode on and off in under 5 us comes down to what sort of load the diode represents to the OPA551 output. The voltage that will be dropped across the output series resistor and the forward voltage of the diode will push the OPA551 into the large signal operating region. The Figure 20. OPA551 Large-Signal Step Response shows the output slewing from -12 V to + 12 V in about 2 us, but that was with a 3 kohm/ 100 pF load; the current load is pretty light. The forward biased PIN diode at 100 mA load current and whatever capacitance it exhibits is certainly a much different load. Realistically, the only way to know for sure if it meets the requirement is to set the circuit up and see what it does.

2. Do I need to add feedback resistor to lower the gain? since it is now open loop.

Its usually not a good idea to operate an op amp in an open loop condition. They are designed to operated in their linear input and output ranges. The output will slam from rail to rail in open-loop, and when it hits a rail the open-loop gain collapses and nearly all electrical parameters are much degraded.

I would set the op amp to operate at the lowest closed-loop gain as possible; a gain that allows the output to reach the needed levels of performance for the given input level. Doing so will assure optimal ac performances which are needed for this PIN diode application.

3. I need SMT package, please confirm 100mA current.

Your application is running high current and the power dissipation will be high. I recommend the DDPAK/TO-263 (7) package. It is the larger surface mount package and has a wider Safe Operating Area than the other packages. See datasheet Figures 35 through 37.

4. Is OPA454 better option?

The OPA454 is specified for a +/-50 mA output. It will usually output more current, but that under all conditions may not deliver 100 mA. The OPA551 has about 2x the slew rate of the OPA454 which helps with the speed issue.

Regards, Thomas
Precision Amplifiers Applications Engineering

Hi Thomas,

What you recommended if I decrease to 50mA?

Thanks,

Shlomi

Hi Shlomi,

If the required output current drive is reduced to 50 mA the OPA454 enters as a good possibility for the application. However, I still give a small speed edge to the OPA551 so I would focus on using it. Since the power dissipation would be lower the smaller SOIC package is a good option for either device. It takes up much less board space than the OPA551 DDPAK.

Regards, Thomas

Precision Amplifiers Applications Engineering

Hi Thomas,

Please approve it, I would like to use SOIC package of OPA552

The rational is the pin diode bias at 100mA is with -5V Power supply The +48V is only for reverse

Can you confirm?

Thanks, 

Shlomi

OPA552.TSC

Hi Shlmoi,

The OPA552 does provide wider bandwidth and higher slew rate than the OPA551, but it is not unity gain stable. You would have to operate it with a closed-loop gain of 5 or greater to assure stability. Do note that the faster operation of the OPA552 does sacrifice some of the phase margin that the OPA551 provides in a similar application. The result is more overshoot and a longer settling time than you might achieve with the OPA551, but the only way to really evaluate the difference is to compare the OPA 551 and OPA552 in your particular PIN diode circuit.

The power dissipation in the application would have to be evaluated and then compared to the Safe Operating Area curves in Section 10.4 of the OPA551/OPA552 datasheet. It may very well be that the SOIC has sufficient power dissipation capability to handle your application.

Regards, Thomas
Precision Amplifiers Applications Engineering

Hi Thomas,
Can I get schematic and TINA?
Thanks,
Shlomi

Hi Shlomi,

again, are you really sure that you have to apply a reverse voltage of 48V to the PIN diode??

Kai

Hi Shlomi,

I am a little confused what you need. You provided an OPA552 TINA Spice file (OPA552.TSC) which includes your schematic. Please clarify what you would like from us.

Regards, Thomas

Precision Amplifiers Applications Engineering