LM2776: Cascade connection

Hello Omer,

Thank you for reaching out,

Omer YILDIRIM said:

Can I use two or more LM2776s by connecting them to the cascade?

Yes, you can use, please take care of the maximum output current drawn by the device. 

Please share the schematic of the cascaded LM2776 for review.

Thank you & regards,

Moheddin.

Hi Moheddin,

The schematic of the circuit will be as above. I want to get -16.2V and 60mA output.

Regards.

Hi Moheddin,

Thanks for supporting.

I will take it from here.

Hi Omer,

Let me support you further.

Some key points to remember while cascading Charge Pumps:

- We cannot achieve 100% efficiency in any converter. There is always a power loss during energy transfer. Which means the final Vout will be less than what you estimated depending on the Rout, parasitics, Load etc.

- The output impedance increases as you cascade more no.of devices.

- The parasitics from the layout needs to be accounted for, inorder to estimate the final Vout you can achieve.

- Theoretically, above cascade connection looks feasible. However, I need to double check. Please indicate the input/output capacitor connections in the schematic for all the three stages.

- What is the application and what is the load for LM2776?

Regards,

Febin

Hi Febin,

I am designing a circuit that can output up to 50mA between -5V and +5V using OPA2197. When I examine the datasheet of OPA2197, in case of 50mA output current, the output voltage rails drop to 3V below the supply voltage. Therefore, I need a supply voltage of at least +-9V. Also, I will put the PRG21AR420MB1RA element to limit the current to the output of OPA2197 in case of short circuit. Since there will be some voltage on it, the supply voltage will need to be above +-9V. The system has 5.4V to supply many components. I can easily increase +5.4V to +16.2V with boost-up converter circuits. However, I could not find a complete solution in negative voltage. I have the LM2776. But its maximum supply voltage is 5.5V. Cascade connection was possible in ICs such as ICL7660. But their output current is also very low. That's why I'm trying to find a solution to this problem using the LM2776.

Sorry for being late in replying.

Regards.

Hi Omer,

Febin said:

Theoretically, above cascade connection looks feasible. However, I need to double check. Please indicate the input/output capacitor connections in the schematic for all the three stages.

The schematic you shared is feasible. However, you might not be able to get -16.2V due to the losses.

Care should be taken for the input/output capacitor connections.

For eg: GND of Input & Output cap of second stage should be the GND pin of the second stage LM2776.

            GND of Input & Output cap of third stage should be the GND pin of the third stage LM2776.

Febin said:

- What is the application and what is the load for LM2776?

Unfortunately, I could not understand your explanation. Could you please share a system level block diagram?

Regards,

Febin

Hi Febin,

The system will be roughly as follows.

Hi Febin,

As you said, a voltage of around 10V may be enough. First I'll try with two LM2776s. Thanks for your support.

Regards.

Hi Febin,

Thank you for your support. I will try as you said. You can close the thread.

Regards.