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## How to select an inductor for a given 5V POL application?

Employee
Employee

How to select an inductor for a 5V application?

Is there a app note or User manual that could help provide L and C selection for 5V application?

1 Solution

## Re: How to select an inductor for a given 5V POL application?

Moderator
Moderator

Hello @sbak2023 ,

Thank you for contributing to the Infineon community.

Inductor Selection:
The inductor value can be determined using the following relation:

Where:

PVin(max) = Maximum input voltage;

∆iLmax= Maximum peak-to-peak inductor ripple current;

OCPmax = maximum specification of the OCP limit

Isat = Inductor saturation current.

Vo = Output voltage

Fsw = Operating frequency

Dmin = Minimum Duty cycle

The inductor is selected based on output power, operating frequency, and efficiency requirements.

Operating frequency (Fsw) and peak-to-peak inductor ripple current (∆iL) play a crucial part in the sizing of the inductor.

For applications requiring a high-duty cycle, it is important to make sure a proper switching frequency is selected so that the resulting off-time is longer than the maximum spec of the minimum off-time which can be calculated as shown below.

Where,

Toff(min) is the minimum off time

Vin is the input voltage

fsw is the desired switching frequency

k is the variation of the switching frequency. As a rule of thumb, select k = 1.25 to ensure the design margin.

The resulting maximum duty cycle is therefore determined by the selected on-time and minimum off-time.

∆iL can be 20% of the output current as a starting point of the design. There is no hard specification as long as it meets all other specifications.

It is recommended not to cross 50% under any condition as this may result in a higher saturation current and Output capacitance value required increases.

The saturation current must be higher than the maximum specification of the OCP limit plus the peak-to-peak inductor ripple current. For some core materials, inductor saturation current may decrease with an increase in temperature. It is important to check the inductor saturation current at the maximum operating temperature.

Capacitor Selection:
The output capacitor selection is mainly determined by the output voltage ripple and transient response requirements.

Case 1: Based on the output ripple

To satisfy the Vo ripple requirement, Co should satisfy the following criterion.

Where ∆Vor is the desired peak-to-peak output ripple voltage.

Case 2: Based on transient response requirements

To meet the transient response requirements, the output capacitors should also meet the following criterion.

Where ∆VOL is the allowable Vo deviation during the load transient.

∆Io(max) is the maximum step load current.

We have to pick the highest value of Co out of the above cases

Note:

• The impact of ESL, ESR, control loop response, transient load slew rate, and PWM latency is not considered in the calculation shown above. Extra capacitance is usually needed to meet the transient requirements. As a rule of thumb, we can triple the Co that is calculated above as a starting point, and then optimize the design based on the practical measurement.
• It is suggested to use Multi-Layer Ceramic Capacitor (MLCC) for their low ESR, ESL, and small size.

The datasheet of each iPOL will also provide details on Inductor and capacitor selection.

Kindly choose the active and preferred part from the list of POLs that are available in our portfolio based on your requirement.

Regards,

Nishanth

## Re: How to select an inductor for a given 5V POL application?

Moderator
Moderator

Hello @sbak2023 ,

Thank you for contributing to the Infineon community.

Inductor Selection:
The inductor value can be determined using the following relation:

Where:

PVin(max) = Maximum input voltage;

∆iLmax= Maximum peak-to-peak inductor ripple current;

OCPmax = maximum specification of the OCP limit

Isat = Inductor saturation current.

Vo = Output voltage

Fsw = Operating frequency

Dmin = Minimum Duty cycle

The inductor is selected based on output power, operating frequency, and efficiency requirements.

Operating frequency (Fsw) and peak-to-peak inductor ripple current (∆iL) play a crucial part in the sizing of the inductor.

For applications requiring a high-duty cycle, it is important to make sure a proper switching frequency is selected so that the resulting off-time is longer than the maximum spec of the minimum off-time which can be calculated as shown below.

Where,

Toff(min) is the minimum off time

Vin is the input voltage

fsw is the desired switching frequency

k is the variation of the switching frequency. As a rule of thumb, select k = 1.25 to ensure the design margin.

The resulting maximum duty cycle is therefore determined by the selected on-time and minimum off-time.

∆iL can be 20% of the output current as a starting point of the design. There is no hard specification as long as it meets all other specifications.

It is recommended not to cross 50% under any condition as this may result in a higher saturation current and Output capacitance value required increases.

The saturation current must be higher than the maximum specification of the OCP limit plus the peak-to-peak inductor ripple current. For some core materials, inductor saturation current may decrease with an increase in temperature. It is important to check the inductor saturation current at the maximum operating temperature.

Capacitor Selection:
The output capacitor selection is mainly determined by the output voltage ripple and transient response requirements.

Case 1: Based on the output ripple

To satisfy the Vo ripple requirement, Co should satisfy the following criterion.

Where ∆Vor is the desired peak-to-peak output ripple voltage.

Case 2: Based on transient response requirements

To meet the transient response requirements, the output capacitors should also meet the following criterion.

Where ∆VOL is the allowable Vo deviation during the load transient.

∆Io(max) is the maximum step load current.

We have to pick the highest value of Co out of the above cases

Note:

• The impact of ESL, ESR, control loop response, transient load slew rate, and PWM latency is not considered in the calculation shown above. Extra capacitance is usually needed to meet the transient requirements. As a rule of thumb, we can triple the Co that is calculated above as a starting point, and then optimize the design based on the practical measurement.
• It is suggested to use Multi-Layer Ceramic Capacitor (MLCC) for their low ESR, ESL, and small size.

The datasheet of each iPOL will also provide details on Inductor and capacitor selection.

Kindly choose the active and preferred part from the list of POLs that are available in our portfolio based on your requirement.

Regards,

Nishanth