LM74502H: Abs max voltage on SRC

Shotaro Sakai

Intellectual 2560 points

Part Number: LM74502H

Hello team,

My customer is considering the below circuit and have a question on abs max voltage on SRC.

The datasheet says SRC must not exceed VS voltage.

But when VIN turn off immediately and FET is switched off, it may be under condition of SRC voltage > VS voltage. Because SRC pin has Cout and it has some voltage level. What do you think about this concern?

Best regards,

Shotaro

5 months ago

+1 Shiven Dhir 5 months ago

TI__Genius 12775 points

Hi Shotaro,

The circuit shared is the load switch configuration only and when the VIN drops immediately, reverse current will flow through the body diode of the FET and COUT will be discharged.

Even when using back-to-back FETs, this device does not support reverse current blocking and hence allows free flow of reverse current. Because of which COUT will not hold any value when VIN drops as it will be discharged.

In all the cases, SRC is within the range specified.

Regards,

Shiven Dhir

0 Shotaro Sakai 5 months ago in reply to Shiven Dhir

TI__Intellectual 2560 points

Hello Shiven,

We understood what you want to say.

But during flow of reverse current, body diode has some forward voltage. And it means V(VS) < V(SRC) condition occurs. What do you think about this?

Best regards,

Shotaro Sakai

+1 Shiven Dhir 5 months ago in reply to Shotaro Sakai

TI__Genius 12775 points

Hi Shotaro,

That will not harm the device.

Regards,

Shiven Dhir

0 Shotaro Sakai 5 months ago in reply to Shiven Dhir

TI__Intellectual 2560 points

Hi Shiven,

Sorry, again. You means the time under V(VS) < V(SRC) condition is very short. So this is no issue, right?

Best regards,

Shotaro

0 Shiven Dhir 5 months ago in reply to Shotaro Sakai

TI__Genius 12775 points

Hi Shotaro,

The diode drop will be around 0.3V - 0.7V only and SRC is rated for (VS+0.3). Since the overshoot is very small, it will not harm the device.

Regards,

Shiven Dhir

0 Shotaro Sakai 5 months ago in reply to Shiven Dhir

TI__Intellectual 2560 points

Hi Shiven,

Can you tell me if the below understanding is correct?

V(VS)≦0: The condition is equal to FET turns OFF

V(VS)>0: The condition is equal to FET turns ON

Best regards,

Shotaro

0 Shiven Dhir 5 months ago in reply to Shotaro Sakai

TI__Genius 12775 points

Hi Shotaro,

To turn on the external FET, there are some conditions which needs to be satisfied.

V(OV) < 1.25V

V(VS)>3.9V is one of the conditions to turn on the FET.

Regards,

Shiven Dhir

0 Shotaro Sakai 5 months ago in reply to Shiven Dhir

TI__Intellectual 2560 points

Hi Shiven,

I understood.

When FET turns OFF, V(VS) may be 0<V(VS)<3.9V. So abs max of SRC must be V(VS). But SRC will be V(VS)+0.3V...

It is no makes sense for my customer... What do you think?

Best regards,

Shotaro

0 Shiven Dhir 5 months ago in reply to Shotaro Sakai

TI__Genius 12775 points

Hi Shotaro,

I will check with my team and internally and confirm accordingly.

Regards,

Shiven Dhir

0 Shiven Dhir 5 months ago in reply to Shiven Dhir

TI__Genius 12775 points

Hi Shotaro,

The understanding is correct here. V(SRC) will be V(VS) + 0.3 when the output capacitor is discharging.

But the table was formed on the basis of ON state condition. Hence when the controller is ON and gate is high, SRC cannot be higher than VS. During discharge when V(VS)<V(SRC), nothing will happen to the device and can be used confidently.

Regards,

Shiven Dhir

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LM74502H: Abs max voltage on SRC