Chapter 7 - MPLAB® Mindi™ Analog Simulator - Peak Current Mode Step-Up (Boost) Regulators

This chapter aims to provide a practical introduction to Peak Current Mode Control Step-Up (Boost) Converters. Also, the MPLAB® Mindi™ analog simulator tool will be used in several examples to demonstrate the functionality and the performance of Microchip’s Boost DC-DC devices.

7.1 Prerequisites

Chapter 1 of this workbook, Getting Started with MPLAB® Mindi™.
Review MCP1640, MCP16251 and MCP1642B/D device datasheets.

7.2 Peak Current Mode Control Boost Converter Experiments

The goal of this case study is to understand the impact of input voltage, load current and passive components on the quality of the output voltage, stability, and analyze PFM and PWM mode switching waveforms.

7.3 Case Study: Peak Current Mode Control Boost Converter Start-Up

MPLAB Mindi analog simulator provides start-up examples for several parts from Microchip’s boost portfolio, allowing any of them to be used for testing purposes. In this section, MCP1640 will be used for this test, but the procedure is similar for the other parts.

Open the '(MCP1640) Synchronous boost example, startup' application schematic from Power Management > Switching Regulators > MCP1640.

Delete the waveform generator from the EN pin and tie EN to VBATT.
Set output voltage to 3.3 V with the feedback resistors RTOP and RBOT.

Set output current to 0.025 A by editing the load resistor : {3.3/0.025}.

Run the simulation and view the results.

7.3.1 Additional Exercises

Pulsing ENABLE: this test consists in applying a square signal on the EN pin. The frequency of this signal and the simulation time have to be chosen while considering the converter’s start-up time.

EN Thresholds: this test consists of applying a triangular signal on the EN pin. The frequency of this signal and the simulation time have to be chosen considering the start-up time of the converter. The relevant voltage probes can be altered such that they are plotted on the same graph.

7.4 Case Study: PFM and PWM Switching Modes

The MCP16251/2 devices use an automatic switchover from PWM to PFM mode for light load conditions. During PFM mode, a controlled peak current is used to pump the output up to the threshold limit. While operating in PFM or PWM mode, the P-Channel switch is used as a synchronous rectifier, turning off when the inductor current reaches 0 mA, in order to maximize efficiency. In PFM mode, a comparator is used to terminate switching when the output voltage reaches the upper threshold limit. Once switching has ended, the output voltage will decay or coast down. During this period, which is called Sleep period, 1 µA (typically) is consumed from the input source, which keeps power efficiency high at light load (4 µA are consumed from the output). PFM mode has higher output ripple voltage than PWM mode and variable frequency. The PFM mode frequency is a function of input voltage, output voltage and load current. While in PWM mode, the boost converter periodically pumps the output with a fixed switching frequency of 500 kHz.

Open the '(MCP16251) Synchronous boost example, startup' application schematic from Power Management > Switching Regulators > MCP16251.

Modify the schematic in such a way that the PFM-PWM threshold can be determined. For this setup, the output current has to be linearly increased; this can be done using a PWL Current source.

The advantage of PFM operation is low input current consumption at light loads (high efficiency). The tradeoff is the higher output voltage ripple (~100 mV), as seen in the previous graph compared to the PWM ripple at about 20-30 mV.