Voltage-mode Buck converter design example

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.

Fig. 1LM21215A block diagram^[1]

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

Fig. 2LM21215A BoMs^[1]

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

Fig. 3LM21215A model in Simplis

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.

Fig. 4LM21215A time domain waveforms - VSW, VOUT, inductor current

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

Fig. 5LM21215A closed loop transfer function Bode plot

Design procedure

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

Fig. 6Bode plot illustration for the design procedure

To summarize the pole and zero locations:

• The output complexed poles are placed between the two zeros formed by R₁C₃ and R₄C₄.
• The cross over frequency is placed between R₄C₄ and R₃C₃.
• The pole formed by R₃C₃ is placed near the esr zero so that they can cancell with each other.
• The pole formed by R₄C₅ is placed near half of the switching frequency.

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.

Fig. 7Bode plot illustration for the design procedure

Fig. 8 shows the Bode plot of G_c (compensation network), G_vd (duty cycle to output) and closed-loop transfer function.

Fig. 8Bode plot of Gvd, Gc and closed-loop transfer function

References and downloads

[1] LM21215A datasheet

[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