# Voltage-mode Buck converter design example

Ming Sun / December 10, 2022

7 min read • ––– views

## LM21215A

**Ref. 1** shows the datasheet from **TI (Texas Instruments)**. It is a **2.95-V to 5.5-V,15-A, Voltage-Mode Synchronous Buck Converter**. The block diagram is as shown in **Fig. 1**.

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From **LM21215A** datasheet, we can get the following values in a typical application.

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From there, we can create **LM21215A** model in Simplis as shown in **Fig. 3**.

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The time domain waveforms are as shown in **Fig. 4**. The **VOUT** is regulated at **1.2V** and the inductor current is the same as the load current, which is **15A**.

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The closed loop Bode plot from AC simulation is as shown in **Fig. 5**.

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## Design procedure

**Fig. 6** shows the pole and zero locations for the design procedure.

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**To summarize the pole and zero locations:**

- The output complexed poles are placed between the two zeros formed by
**R**and_{1}C_{3}**R**._{4}C_{4} - The cross over frequency is placed between
**R**and_{4}C_{4}**R**._{3}C_{3} - The pole formed by
**R**is placed near the esr zero so that they can cancell with each other._{3}C_{3} - The pole formed by
**R**is placed near half of the switching frequency._{4}C_{5}

The closed-loop voltage-mode Buck converter model is as shown in **Fig. 7**. Given the compensation network components value, every resistor and capacitor from the Type-III compensator should be able to be integrated into the silicon.

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**Fig. 8** shows the Bode plot of **G _{c}** (compensation network),

**G**(duty cycle to output) and closed-loop transfer function.

_{vd}^{}

## References and downloads

[2] Power stage transfer function cheatsheet - Voltage Mode

[3] Type-III compensator design tutorial

[4] LM21215A model in Simplis - pdf

[5] LM21215A model in Simplis - download

[6] Redesigning LM21215A compensation network in Simplis - pdf

[7] Redesigning LM21215A compensation network in Simplis - download