LM76003: LM76003RNPR Schematic Review

Hi Kapil,

Kapil K said:

1. The signal 24V_SHUTDOWN is coming from TPS2378 PD Controller via CDB pin using MOSFET logic implementation. I have kept the 80.6K resistor connected to this pin as well. Do I need this resistor or not?

I think you have the connection to the wrong pin. RT is a pin that sets the switching frequency of the buck converter. You would want to connect the signal from the PD controller to the EN pin of the LM76003 device. Are you designing the LM76003 to switch around 500kHz? If so, the 80.6kohm resistor is the correct value.

Looking at the above table, you inductor looks correct, the feedback resistors are correct, but the amount of output capacitance is lacking. I suggest adding more output capacitors. Remember, the capacitance of a ceramic capacitor has capacitance derating when a voltage is applied to it. For example, a 10uF/50V/X7R/1210 capacitor from Murata has a capacitance value of 59.4% of its rated capacitance. See below:

This means at 12V output, the capacitor has a net capacitance of around ~6uF. Therefore to have 33uF of capacitance shown in the table, you will need 6 X 10uF capacitors for your design.  

Kapil K said:

For Pgood Pin, I have used the voltage divider by using output voltage to proivde the voltage to pgood pin to track the o/p.

Looks goods.

Ben

Hi Ben,

Thanks for the review comments. I will follow the same and implement this.

• As you mentioned about the mapping of the EN pin and 24V_SHUTDOWN signal. I will connect the 24V_SHUTDOWN signal to the enable pin, but will the enable pin be connected to Pvin (Vinput)? Or it will only connect to the 24V shutdown signal.

• There is one more option given for enabling the pin: Precision enable. I went to the datasheet and tried to understand this. It seems that we need to select the voltage divider and the enable voltage value. But in that case, how can I select the resistor values and voltage on the enable pin? And how can I map this 24V shutdown signal?

• How will this enable pin be used for UVLO feature, resistor selection, and 24V_shutdown signal mapping?

Can you clarify the above queries and explain the things with one example

Regards

Kapil

Hi Kapil,

Kapil K said:

• As you mentioned about the mapping of the EN pin and 24V_SHUTDOWN signal. I will connect the 24V_SHUTDOWN signal to the enable pin, but will the enable pin be connected to Pvin (Vinput)? Or it will only connect to the 24V shutdown signal.

• There is one more option given for enabling the pin: Precision enable. I went to the datasheet and tried to understand this. It seems that we need to select the voltage divider and the enable voltage value. But in that case, how can I select the resistor values and voltage on the enable pin? And how can I map this 24V shutdown signal?

• How will this enable pin be used for UVLO feature, resistor selection, and 24V_shutdown signal mapping?

Above is an example of how to calculate the values of the EN resistors. In this example, I assume a VIN of 30V that will enable the LM76002 device. When VIN is rising, the LM76002 will turn on at VIN=30V.

The picture below shows when VIN is falling, the LM76002 will turn off at 26.3V. Therefore, the EN circuit provides a VIN_UVLO to turn on and off the LM76002 device.

 To use the 24V_SHUTDOWN signal, see below. Depending on the logic of the 24V_SHUTDOWN, you may need to invert the signal.

Ben

Hi Ben, 

Thanks for the clarification for Enable pin. 

But as you mentioned about the output capacitor calculation, which is 33uF (equivalent value at output voltage) This value will be for full load condition. I have done the simulation for 24V @2A requirement. 

Vin Range: 44V to 50V

Vout: 24V @ 2A 

So, the O/P cap calculation comes around 10 uF with ECR value 857.38 nOhm for 22.056 mV Peak-to-peak output ripple voltage. But I am good for higher value of ripple requirement. I need to meet above calculated value.

Correct me if I am missing anything.  

Hi Kapil,

You might want to use the quickstart calculator (found in the LM76005 product folder) to check to see if the loop is stable at the cross over frequency with the smaller amount of output capacitance.

Ben

Hi Ben,

Thanks for your valuable inputs. I have implemented the inputs in schematic and sharing the schematic for final review. 

I have one query for the RT Pin Resistor value. As per the datasheet, 

PLACE A RESISTOR FROM THIS PIN TO GROUND TO SET THE SWITCHING frequency.  IF FLOATING, THE DEFAULT SWITCHING FREQUENCY IS 500 KHZ. DO NOT SHORT TO GROUND.

So Do we require 80.6K resistor? 

It would be highly appreciable if you could review the design.

Hi Kapil,

Kapil K said:

I have one query for the RT Pin Resistor value. As per the datasheet, 

PLACE A RESISTOR FROM THIS PIN TO GROUND TO SET THE SWITCHING frequency.  IF FLOATING, THE DEFAULT SWITCHING FREQUENCY IS 500 KHZ. DO NOT SHORT TO GROUND.

So Do we require 80.6K resistor? 

No, if your switching frequency is set at 500kHz, there is no need for the 80.6kohm resistor.

Kapil K said:

It would be highly appreciable if you could review the design.

The schematic looks good to me.

Ben

Hi Ben,

Thank you!!

Regards

Kapil

Hi Kapil,

If there are no further questions, please close the thread by clicking on "resolved". Thanks and good luck to you!

Ben