This chapter presents an introduction to the simulation of Microchip’s Peak Current Mode Step-Down (Buck) Converters. The following exercises will walk you through a number of exercises with multiple representative parts to observe characteristics, start-up from a rising input voltage, start-up from an enable, and start-up from output load steps. You will also explore Pulse Frequency Modulation (PFM) versus Pulse Width Modulation (PWM) operation and the boundary between them.
4.1 Prerequisites
- Chapter 1 - Getting Started with the MPLAB® Mindi™ Analog Simulator.
- A review of the MCP16301, MCP16331, and MCP16311/2 Datasheets.
4.2 Peak Current Mode Buck Converter
The goal of the following case studies is to understand the impact of the input voltage, the load current, and the passive components to the quality of the output voltage and stability, and to analyze PFM and PWM mode switching waveforms.
4.3 Case Study: Peak Current Mode Buck Converter Start Up
a
Open the 'Buck example, startup' application schematic from Power Management > Switching > MCP16301.
b
Place two voltage probes on VIN and EN, as seen on the next figure:
c
Rename curve’s label as desired. In order to perform this task, double-click on probe’s symbol and use the same name for all graphs.
d
Edit the parameters of each waveform generator, according to a specific test, if necessary.
Double-click on a component’s symbol to pop-up a window similar to what is presented below:
e
Run the simulation and stack the curves when viewing the results.
f
Add cursors and move them around to measure the various timing parameters.
4.3.1 Additional Exercises
a
Perform the tests presented above both for MCP16311 and MCP16331 devices and compare the results.
b
Change RTOP, L1, and converter’s load (RLOAD) in order to get another output voltage (according to the datasheet). Run the simulation and compare the results.
4.4 Case Study: PFM versus PWM Switching Modes
a
Open the 'Synchronous Buck example, AC transient load step' application schematic from Power Management > Switching > MCP16311.
b
Ensure that the transient analysis type is selected.
c
Select and remove the 'Bode Plot Probe' and ‘AC1’ voltage source.
d
Connect RTOP with VOUT to close the feedback loop:
e
Edit the parameters of the ‘ILOAD’ waveform generator. Chose ‘Sawtooth’ as the shape of the signal, because the PFM/PWM transition happens when the load current is increased to a certain point. The waveform’s period should not exceed simulation time, but should be comparable with it for accurate measurements. The amplitude of the signal should sweep from 0 to 200 mA, which is a value high enough to cover the occurrence of PFM/PWM transition (as presented in the datasheet):
f
Change the parameters of ‘RLOAD’. In the following example, {5/0.01} means that at 5 V across the resistor, the current through it is 0.01 A (or 10 mA). This doesn’t mean that it forces converter’s output voltage to be 5 V. If the output voltage changes to 10 V (for example) and the value of the resistor remains unchanged, the current flowing through it will be double (10 mA at 5 V means 20 mA at 10 V, while keeping {5/0.01}). Instead of {5/0.01}, type the actual value of the resistance inside the ‘Result’ box which corresponds to 10 mA at 5 V, which is 500 Ω:
g
Increase the simulation time to 10 ms to correspond with the ILOAD ramp time.
g
Run the simulation and stack all curves.
i
Zoom into the area of interest and add cursors to find the load current (IOUT) where the converter transitions between PFM and PWM. This is indicated by a change in the output voltage ripple, as seen in the figure below:
4.4.1 Additional Exercises
a
Change the value of the VIN and repeat this test several times to determine how this affects the load current threshold corresponding to PFM/PWM boundary.
b
Change the values of RTOP, L1, and RLOAD to modify the output voltage (according to the datasheet) and compare the results.
4.5 Case Study: Load transient response
4.5.1 Analyze Load Transient Response
a
Open the 'Buck example, AC transient load step' application schematic from Power Management > Switching Regulators > MCP16331.
b
Run the simulation and stack the curves in the resulting waveform.
c
Add cursors to measure the undershoot and the overshoot, as depicted in the following image:
d
Switch to the AC analysis graphs to observe the frequency response of the converter, as illustrated below.
4.5.2 Additional Exercises
a
Increase the load current and measure the output voltage undershoot/overshoot, Gain Crossover Frequency, and Phase Margin.
b
Change RTOP and L1. In order to get a different output voltage, re-run the simulation and compare the results.
c
Change the values of the output capacitors (as shown in the next screenshot), run the simulation again and compare the new results.
d
Perform the tests presented above for different switching regulators (MCP16301, MCP16311) and compare the results.
4.6 References
c
MPLAB® Mindi™ Available Models:
- MCP16301/H
- MCP16311
- MCP16331