PCB area is always insufficient? Here is a good way to solve it

To meet the increasingly stringent size requirements of compact electronic devices, integrated circuit (IC) designers integrate external components into the device to minimize the number of external components. Among the various circuits required to build all electronic devices, reducing the size of DC-DC converters is equally challenging because they are everywhere (all devices need power), and power supply designers are often faced with the reality that reducing solution size often has a negative impact on performance.

例如，能显著节省PCB面积的一种方法是采用单芯片DC-DC转换器，该转换器将经过精心选择的电源开关器件集成到IC封装之中，从而使所需外部元件减少为少量的无源器件。在许多情况 下，与外部电源开关控制器设计相比，紧凑型设计最终会带来不需要的结果，即在更小的空间中增加了功率损耗，从而产生更高的温升。为了避免产生的热量水平造成困扰，选择合适的单片式DC-DC转换器对于设计紧凑高效的电源系统至关重要。

The LT3942 is one of ADI's most versatile monolithic buck-boost regulator ICs. This boost-buck converter meets the challenge of creating a flexible and compact DC-DC converter solution without sacrificing performance. The LT3942 integrates four 40 V/2 A power switches, two gate driver bootstrap diodes, and all of its control and driver circuitry into a small 4 mm × 5 mm QFN package. With up to 2 MHz operating switching frequency capability, the size of external components can be minimized, saving PCB space while providing high bandwidth operation for a wide range of DC-DC converters.

The LT3942 features the same peak current mode control scheme as the LT8390A/LT8391A family of buck-boost controller ICs and is able to seamlessly transition between 2-switch boost, 4-switch buck-boost (boost-buck), and 2-switch buck operating modes. The converter monitors and compares its input and output voltages to determine the correct operating mode. When the ratio of PV _IN :PV _OUT changes and forces the converter to switch modes, the LT3942 maintains regulation while intelligently switching control between switch pairs.

In addition to regulating the output voltage over a variety of PV _IN :PV _OUT combinations, the LT3942 can also be configured to regulate the input or output current for constant current regulation applications. Current monitoring feedback from the ISMON pin provides a buffered voltage output proportional to the measured current, allowing the connected circuit to monitor the measured current level. This ability to regulate current or voltage makes the LT3942 ideal for use as an LED driver, compact battery charger, converter powered by a tiny solar panel, or general purpose regulator.

Figure 1 shows the complete evaluation circuit for a compact LED driver based on the LT3942. This solution is capable of delivering 1 A of current to a string of four (up to 14 V) white LEDs in series. The input voltage range for maximum power output is 7 V to 36 V, which can drop to 4 V at low operating current, making it ideal for unregulated automotive input supplies. The LT3942 in this solution operates at a switching frequency of 2 MHz, so relatively small inductors and capacitors can be used. As a result, the complete LED driver solution fits into a PCB footprint of 15 mm × 15 mm, with all components located on the same side of the board (including the IC).

Figure 1. This LT3942-based demonstration circuit (DC2404A) shows a high-performance, compact DC-DC regulator solution specifically designed to drive LEDs in this situation.

The solution also features high bandwidth operation, allowing for fast output current regulation. When operating in buck mode, the LT3942 uses an external PWM source to dim the LEDs at 100Hz flicker-free, achieving a dimming ratio of up to 5000:1. If there is no external PWM source, the LT3942 dimming can also be achieved using its internal PWM dimming function. Internal dimming provides up to 128:1 dimming capability without any external PWM signal source, requiring only a resistor to set the dimming frequency, and a DC voltage to control the duty cycle of the output current. Like most of ADI Power's LED drivers, the LT3942 also features analog dimming, which provides up to 20:1 analog dimming by applying a DC voltage on the CTRL pin. Analog and PWM dimming can be combined to achieve higher effective dimming ratios than using either method alone.

To help create a low noise DC-DC converter system, the LT3942 has a selectable spread spectrum frequency modulation (SSFM) feature built into the LT3942. Once enabled, SSFM sweeps the switching frequency between the value set by the RT resistor and up to 25% additional switching frequency. This sweep action disperses the radiation caused by switching over a wide frequency spectrum, rather than concentrating it in a narrow band, thereby reducing the overall EMI peak. When used in conjunction with input and output EMI filters, SSFM helps reduce EMI over a wide frequency range, making it easier to design systems that meet radiation standards.

The LT3942 is not limited to driving LEDs. It is a powerful, compact regulator that is well suited to solving the problem of generating a regulated output from a wide range of unregulated supplies. The 12 V, 1 A regulator design shown in Figure 4 is similar to the 14 W LED driver solution in Figure 2, but with a few minor changes. As with the LED driver application, the regulator maintains output regulation over a wide input voltage range, delivering full output power down to 7 V and maintaining low output power operation down to 4 V.

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

Figure 3. The LT3942’s high bandwidth operation enables high ratio PWM dimming over a wide dynamic brightness range in LED lighting applications. Without the need for an EMI filter, the DC2404A can achieve up to 4000:1 dimming at 120 Hz and up to 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 has an excellent peak efficiency approaching 95%, with efficiencies of 85% or higher across most of its input voltage range. Even when powering its output at one-tenth of its total output power, efficiency of over 80% is maintained, demonstrating that the device can still operate efficiently under light load conditions.

(a). Input Voltage Efficiency Sweep.

(b). Load Current Efficiency Sweep.

Figure 4. The LT3942 configured as a 12 W regulator has excellent line regulation and load efficiency characteristics over a wide input range.

The current sensing and control features of the LT3942 make it suitable not only for LED dimming control, but also work well in other situations where voltage regulation and current control are required. When the sense resistor is configured at the output, the LT3942 can be easily configured as a compact constant current, constant voltage battery charger. For applications with strict input current limits, such as circuits powered by small batteries, capacitor banks, or photovoltaic cells, the monitoring resistor can be moved to the input side of the regulator, providing input current limiting and monitoring for the system. The LT3942 can seamlessly transition from CC mode to CV mode (and vice versa), ensuring that the input and/or output are always regulated.

Animated sequential turn signal lights, commonly seen on new luxury and high-performance vehicles, are rapidly gaining popularity, gradually replacing traditional flashing indicators. Early implementations of sequential turn signals used multiple buck converters or linear regulators to power the LEDs in the turn signal cluster, resulting in complex, relatively inefficient and overly large solutions that greatly limited the application areas of lighting designs. Reducing the number of power ICs required and using a single, efficient device is an obvious way to expand the range of options available to lighting designers.

A single converter solution requires a device that can maintain output regulation for string voltages across a variety of LED combinations, from all LEDs on to a single LED on, and everything in between. As the animated light changes through various configurations of connected LEDs, the input voltage can be above, below, or equal to the output voltage. This type of application requires a boost-buck converter that can intelligently select and seamlessly transition between operating modes while maintaining output regulation. The LT3942’s buck-boost topology and high bandwidth operation enable it to easily handle these changes without glitches.

The sequential turn signal design shown in Figure 5 uses the LT3942 to power eight LEDs at 330mA from the car battery, with the option of powering a string of amber LEDs (for turn signal operation), a string of white LEDs (for daytime running lights), or other decorative lighting (for use in headlight/taillight designs).

Figure 5. In sequential turn signal applications, LEDs are illuminated one at a time, forcing the DC-DC converter to quickly adapt to the new PV _IN :PV _OUT combination. This is not a problem for the LT3942, as it seamlessly transitions from boost, buck-boost, and buck operation during sequential turn mode, ensuring that stable LED current is maintained in each mode.

The microcontroller acts as the interface between the user input turn signal and the lighting system. This gives the lighting designer (or end user, if desired) full control over all the timing and signals required to perform the LED animation sequence, and can control which color LED string is powered at any given time.

In this design, turn signal LEDs are introduced to the string one at a time during sequential lighting mode to generate a turn signal. As LEDs are added to the string by the microcontroller, the LT3942 maintains regulation of the output current to maintain consistent light brightness. After all LEDs are lit, the LT3942 stops switching and pulls the output voltage low, allowing the converter to set up for the next sequential lighting cycle. When the turn signal is not in use, the microcontroller reconnects to the decorative lighting LED string and continues to wait for turn signal user input, combining two lighting functions into a single LED driver solution.

Electronic devices are forcing engineers to continually seek smaller integrated devices to meet the growing space-constrained demands. The LT3942 monolithic buck-boost converter and LED driver integrates numerous space-saving features to address space-constrained electrical design challenges without sacrificing performance. Its monolithic design and 2 MHz operating switching frequency reduce the size of the solution, making it suitable for compact PCB designs. The device is highly flexible and can be used as either a constant current regulator or a constant voltage regulator, making it suitable for a wide variety of applications.

For designs that require low-noise power supplies to meet stringent EMI requirements, the LT3942’s SSFM feature helps reduce conducted and radiated emissions, and its convenient IC package pinout enables a compact switching hot loop. These features, along with a wide input range, simplify the work of designers when faced with compact power supply requirements.