2 MHz, Monolithic, Buck-Boost DC/DC Converter and LED Driver

Author:
Kyle Lawrence, Applications Engineer, Analog Devices

Date
06/01/2020

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The LT3942 addresses space-constrained electrical designs by integrating a number of space-saving features without compromising performance.

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Figure 1. This LT3942-based demonstration circuit (DC2404A) showcases a high performance, compact DC/DC regulator solution specifically for, in this case, driving LEDs.

 

To meet increasingly stringent size requirements for compact electronic devices, IC designers bring external components inside of their devices to minimize the external component count. Of the various circuits required to build any electronic device, DC/DC converters are as challenging to shrink in size as they are ubiquitous—everything needs power—as power supply designers often face the reality that shrinking solution size typically negatively affects performance.

One way to save significant PCB real estate is to use a monolithic DC/DC converter that integrates carefully selected power switch devices within the IC package, reducing required external components to a few passive devices. In many cases, the resulting compact design brings the undesirable result of increased power loss in a smaller space for much higher temperature rise when compared with external power switch controller designs. Selecting the right monolithic DC/DC converter is critical in designing a power system that is both compact and efficient in order to avoid generating problematic levels of heat.

The LT3942 step-up/step-down converter meets the challenge of creating a flexible and compact DC/DC converter solution without sacrificing performance. The device integrates four 40 V/2 A power switches, two gate driver bootstrap diodes, and all its control and driver circuitry into a small 4 mm × 5 mm QFN package. With the ability of operating at up to 2 MHz switching frequency, external component sizing is kept to a minimum, conserving PCB space as well as providing high bandwidth operation for any DC/DC converter.

The LT3942 shares the same peak current-mode control scheme used by the LT8390A/LT8391A family of buck-boost controller ICs and exhibits seamless transitions between 2-switch boost (step-up), 4-switch buck-boost (step-up/step-down), and 2-switch buck (step-down) modes of operation. The converter observes and compares its input and output voltages to determine the correct mode of operation. As the ratio of PVIN:PVOUT changes and forces the converter to mode transition, the LT3942 maintains regulation while it intelligently shifts control between switch pairs.

In addition to regulating the output voltage over a variety of PVIN:PVOUT combinations, the LT3942 can also be configured to regulate input or output current for use in constant current regulation applications. Current monitoring feedback from the ISMON pin provides a buffered voltage output proportional to the measured current, allowing connected circuitry to inspect the measured current levels. This ability to regulate current or voltage makes the LT3942 perfect for use as an LED driver, compact battery charger, miniature solar panel-powered converter, or general-purpose voltage regulator.

14 V, 1 A LED Driver

Figure 1 shows a complete evaluation circuit for an LT3942-based compact LED driver. This solution is capable of providing 1 A to a string of four (up to 14 V) white LEDs connected in series. The maximum power delivery input voltage range is 7 V to 36 V, with reduced current operation down to 4 V, which is ideal for unregulated automotive input supplies. The LT3942 in this solution operates at a 2 MHz switching frequency, enabling the use of relatively small inductors and capacitors. The result is that the entire LED driver solution fits a 15 mm × 15 mm PCB footprint, with all components placed on the one side of the board, including the IC.

This solution also boasts high bandwidth operation, allowing for quick adjustments of the output current. While in buck operation, the LT3942 achieves dimming ratios as high as 5000:1 using an external PWM source to dim the LEDs at a flicker-free 100 Hz. If an external PWM source is not available, the LT3942 can also be dimmed using its internal PWM dimming feature. Internal dimming provides up to 128:1 dimming without the need for any external PWM signal source, requiring only a single resistor to set dimming frequency and a dc voltage to control the duty cycle of the output current. Like most Power by Linear™ LED drivers, the LT3942 also features analog dimming by applying a dc voltage at the CTRL pin for up to 20:1 analog dimming. Analog and PWM dimming can be combined to achieve higher effective dimming ratios than either method can alone.

Spread Spectrum Frequency Modulation Reduces EMI Peaks

To help create a low noise DC/DC converter system, the LT3942 includes an optional spread spectrum frequency modulation (SSFM) feature. When enabled, SSFM sweeps the switching frequency between the value set by the RT resistor up to 25% additional switching frequency. This sweeping action distributes the emissions caused by switching over a broad spectrum of frequencies rather than concentrating those emissions into narrow bands, reducing EMI peaks overall. SSFM, when used in combination with input and output EMI filters, can help reduce EMI over a wide range of frequencies, making it easier to design an emissions-compliant system.

12 V, 1 A Voltage Regulator

The LT3942 is not limited to driving LEDs. It is a capable, compact voltage regulator, well suited to solve the problem of producing stable outputs from wide-ranging, unregulated power sources. The 12 V, 1 A voltage regulator design shown in Figure 4 is similar to the 14 W LED driver solution in Figure 2, with a few small modifications. Like the LED driver application, the voltage regulator can maintain output regulation over a wide input voltage range, delivering full output power as low as 7 V and maintaining operation as low as 4 V with reduced output power.

 

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Figure 2. DC2404A utilizes the LT3942 to create a compact 14 W LED driver application capable of providing regulated output current over a wide input range.

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Figure 3. The LT3942’s high bandwidth operation helps achieve high ratio PWM dimming for LED lighting applications with wide dynamic brightness ranges. With EMI filters removed, DC2404A achieves up to 4000:1 dimming at 120 Hz and as high as 5000:1 dimming at 100 Hz.

The efficiency curves in Figure 4 show that even when operating at a 2 MHz switching frequency, the LT3942 12 V regulator boasts an impressive peak efficiency of nearly 95%, with most of its input voltage range exhibiting efficiencies of 85% and above. Over 80% efficiency is maintained even when powering its output at a tenth of its total output power, demonstrating its ability to operate efficiently in light load conditions.

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Figure 4. Configured as a 12 W voltage regulator, the LT3942 exhibits excellent line and load efficiency properties over a wide input range.

The LT3942’s current sensing and control feature is geared toward LED dimming control, but it serves well in other situations requiring both voltage regulation and current control. When the sensing resistor is configured at the output, the LT3942 can easily be configured to act as a compact constant-current, constant-voltage battery charger. For applications that have strict input current limitations, such as circuitry powered from small batteries, capacitor banks, or photovoltaic cells, the sensing resistor can be moved to the input side of the regulator, providing an input current limit to the system, as well as monitoring. The LT3942 seamlessly transitions from CC mode to CV mode (and vice versa), ensuring regulation of input and/or output at all times.

Analog Devices

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