LM27761: +3.5v input to (-3.5v) output ?

Hello Ohad,

For a start, What exactly would be the load of your system ?

If you are looking for a simple inverter which would convert +3.5V input to -3.5V output, you should have a look at the TPS60400 family, maximum 60mA output current.

Thank you for reaching out to us!

Best Regards,

Dorian

Hi Dorian, 

The load for that inverter is 150mA @ -3.5v. 

What regard the LM27761 ? Can it make this ? In what output current ? 

Thanks,

Ohad

Hi Ohad,

As you can observe on Figure 12, you should expect a Dropout of about 50mV minmum at the output of the LDO.

To this can be added the resistance of  the parasitic resistance Rdson of the MOSFET switches.

If you want to obtain 150mA @ -3.5V, you would need to have a greater input voltage considering the losses previously mentioned.

 If ripple is less of a concern you could also take a look at the LM2776, which would have a lower drop from input to output.

Thank you!

Best Regards,

Dorian

Hi, 

So from what you say, when using the LM27762/1 which is known to be "Charge pump" in order to get outputs of +3.5v or -3.5v I need to take into account LDO Drop voltage and and the Rdson of the switching MOSFET and to add these two voltage loose valuse to the target I want which is absolute of 3.5v and therefore to put a bigger absolute voltage than 3.5v in the input. 

How come is that ?? if LM27762/1 is a charge pump which shall multiply a smaller voltage to a bigger just like a boost converter but in a different technique. 

Why aren't there graphs which display Output voltage Vs Input voltage when the charge pump acts as a multiplier and Vin is samaller than the Vout ? 

Can you explain ? 

Ohad

Hello Ohad,

I don't know what exactly is your point here. You need to know that LM27762/1 is only a Voltage Inverter, it Inverts and Regulates the Input Supply Voltage as mentioned in the datasheet. I understand some topologies can act as multipliers, the LM2776 family don't.

If you have a look at the 8.2.2.2 (Charge-Pump Voltage Inverter) section of the datasheet, there you can see that the input voltage is only inverted and not boosted-down before going to the LDO .

Does it answer your question?

Thank you!

Best Regards,

Dorian

Hi, 

I did not expect the LM27762 as a charge pump to boost input voltage down, I expected it to boost voltage up since it is a charge pump. 

So I understand from you, the LM27762 device does not act as an upconverter but only as a voltage inverter and LDO ? 

Ohad

Hi Ohad,

LM27762 regulates two outputs. One positive and one negative. The positive rail is regulated via a (positive) LDO and the negative rail uses a charge pump to invert the input voltage and then regulates the inverted voltage with a (negative) LDO.

Thank you,

Dorian