LM25118 - the voltage of VIN

Daisuke,

The LM25118 requires at least 5V to start-up in order to surpass the VCC under voltage lockout, VCC > 3.7V. Looking at figure 3 of the datasheet it can be seen that at VIN=5V this VCC under voltage lockout is surpassed.   This is using the internal VCC regulator.

Operation down to VIN=3V can be achieved if and external VCC is supplied to the part. This can be done by using Vout or an external voltage rail as your external VCC. The time of operation at VIN=5V is only the time it takes for VCC to surpass the VCC undervoltage lockout

 Figure 3

 

-Garrett

Hello Garrett,

Thank you for your reply.

 

If VCCX is connected to an external voltage (4.75V < VCCX < 14V), can LM25118 start in the case of "3V < VIN < 5V"?

 

Best regards,

Daisuke

Daisuke,

The part should start in that configuration.

Garrett

Hello Garrett,

Thank you for your reply.

 

Please teach me the best external voltage of VCCX.

Is it 7V?

 

And please teach me the necessary current of VCCX.

 

Best regards,

Daisuke

Daisuke,

As long as VCCX voltage is within the recommended operating conditions it should be fine. You should also make sure that VCCX is large enough to turn the FETs all the way on.

VCCX current should be the same as VCC in normal operation. Figure 4 shows that VCC current limits around 35mA.

Thanks,
Garrett