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PMIC (Power Management Integrated Circuit) is an integrated circuit that is used to control and manage functions related to power in a circuit. Voltage regulators are one such component of PMIC which is used to maintain the circuit\'s operating voltage at a specifically standard voltage level. Among voltage regulators, DC-DC switching regulators are high-efficiency power conversion circuits with wide range of application fields. MAX738ACWE is one such DC-DC switching regulator. This application note outlines the purpose and working principle of MAX738ACWE.

Introduction

A switching regulator is a power converter that controls the output voltage by Pulse Width Modulation (PWM). The voltage regulator switch "ON" continuously at a constant duty cycle and the output voltage is governed by the ratio of "on" and "off" period of the switching element. Additionally, the switching regulator works on the principle of converting the input voltage into current, then storing it onto an energy storage component such as an inductor and finally releasing the stored energy to the load as a regulated output voltage. This eliminates the power factor degradation caused by linear regulators and hence provides a high power efficiency.

Application Field

MAX738ACWE is a monolithic step-down switching regulator with a small package. It combines a current-mode PWM controller, a power MOSFET switch, a current sensing circuit and a fold back current protection circuit. With an internal maximum duty cycle of 85%, it supports input voltages up to 16V and output currents up to 200mA. This feature makes it ideal for use in battery-powered portable electronic equipments that require low power to run. This includes portable DVD players, digital cameras, cell phones, PDAs, and various handheld gaming devices. Furthermore, it is suitable for UPS systems that require small, low-cost and efficient energy conversion.

Working Principle

MAX738ACWE has a simple block diagram as shown in figure 1. It consists of an error amplifier, a voltage reference, a PWM comparator and a power MOSFET switch. The power source is connected to the Vin terminal, the output is connected to the Vout terminal and the ground is connected to the ground terminal.

Figure 1: MAX738ACWE Block Diagram

The operation of the MAX738ACWE starts with the voltage reference providing a stable reference voltage which is then compared with the feedback voltage from the output (Vout). If the feedback voltage is lower than the reference voltage, the PWM comparator switches the power MOSFET on and the voltage at the output terminal increases until the feedback voltage becomes equal to the reference voltage. The converse is true when the feedback voltage is greater than the reference voltage. The power MOSFET is then turned off and the output voltage decreases until the feed back voltage becomes equal to the reference voltage. This ensures that the voltage at the output terminal is maintained at a preset value (Vref).

The error amplifier amplifies the difference between the reference voltage and the feedback voltage and the output of the error amplifier is compared with the PWM ramp voltage to turn on and off the power MOSFET. If the feedback voltage is higher than the reference voltage, the output of the error amplifier increases and the PWM comparator will switch off the power MOSFET. If the feedback voltage drops, the output of the error amplifier drops and the PWM comparator will switch on the power MOSFET, increasing the output voltage. The MAX738ACWE also includes a current sensing circuit which prevents excessive current from flowing into the system by monitoring the output current of the switching regulator and setting the peak current limit accordingly.

Conclusion

MAX738ACWE is a monolithic step-down switching regulator with a small package and a high power efficiency. It is suitable for battery-powered devices and UPS systems as it supports input voltages up to 16V and output currents up to 200mA. The operation of the MAX738ACWE is based on a voltage reference, error amplifier, PWM comparator and a power MOSFET switch which controls the output voltage by Pulse Width Modulation.

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