Challenges and Solutions for High-Current DC-DC Converters

In recent years, advances in SoCs, CPUs, GPUs, FPGAs, and other technologies have led to rapid advances in application performance.
As a result, the current supply capacity required of POWER SUPPLIES (DC-DC converters) has increased significantly. This increase in current has brought the following issues to light:

Increased mounting area due to larger components
Increased noise
Heat generation complicates thermal design

In this column, we will introduce specific solutions to these challenges.

1. Multi-phase method

One solution to this problem is the "multi-phase method," which combines the outputs in parallel.
Figures 1 and 2 show examples of single-phase and multi-phase step-down DC-DC converters.

In the multiphase system, the SWITCHES phases of each output are shifted.
Example: 2 phase: 180°, 3 phase: 120°, 4 phase: 90°, 6 phase: 60°, 8 phase: 45°, 12 phase: 30°
The 2-phase and 4-phase SWITCHES waveforms are shown in Figure 3 and Figure 4.

Issues with the single-phase method

When a high-current DC-DC converter is configured as a single-phase converter, the following issues arise:

Larger parts

As current increases, INDUCTORS and FETs also become larger in size and have higher ratings.
This increases the mounting area, and the increased weight also places restrictions on mounters, limits on the height of the housing, and the need for vibration countermeasures.

Increased noise

An increase in input ripple current increases noise, leading to more complex input CAPACITORS and FILTERS, and increased costs for radiated noise countermeasures.

Increased fever

Losses due to the DCR of INDUCTORS and the on-resistance of a FET are proportional to the square of the current, so when the current is large, heat generation increases, making it necessary to take thermal measures such as using a larger heat sink or forced cooling.

Advantages of the multi-phase method

The main benefits of the multi-phase method are as follows:

Reducing output ripple voltage

As shown in the two-phase operating waveforms in Figure 3, the output ripple current, which is the sum of INDUCTORS currents IL1 and IL2, is twice SWITCHES frequency of each output. In this way, with multi-phase operation, the frequency of the output ripple current can be increased to "SWITCHES frequency of each output x the number of phases," which results in a reduction in the output ripple current. As the output ripple current decreases, the output ripple voltage also decreases, making it possible to downsize the output CAPACITORS.
In addition, a reduction in output ripple current allows INDUCTORS value to be lowered, making it easier to select a smaller INDUCTORS.
Furthermore, a lower INDUCTORS value improves the output current slew rate, which also leads to improved load transient response.
In addition, the output ripple current is also affected by the duty ratio (approximately Vout/Vin for a buck circuit). The relationship between the output ripple current and the duty ratio is shown in Figure 5. Depending on the number of phases and the duty ratio, the output ripple current may become zero. Therefore, when the input voltage and output voltage are relatively constant, the output ripple voltage can be minimized by selecting the optimal number of phases.

Input ripple current reduction

As shown in Figure 6, the input current of a step-down DC-DC converter is discontinuous, and in a high-current DC-DC converter, this input ripple current becomes a large noise source (conductive noise).
The input ripple current in a two-phase configuration is shown in Figure 7. By adopting a multiphase system, the frequency of the input ripple current is multiplied by the number of phases, and the current value is also reduced. This reduces noise and simplifies the design of the input FILTERS.
A reduction in ripple current leads to a reduction in the size and quantity of components, as well as a reduction in loss due to the ESR of CAPACITORS, which ultimately extends the life of components.

Figure 6. Input current of step-down DC/DC converter (single-phase operation)

Figure 7. Input current of step-down DC/DC converter (2-phase operation)

Furthermore, like the output ripple current, the input ripple current of a multiphase system is also related to the duty cycle, and this relationship is shown in Figure 8. There is an optimal number of phases depending on the input voltage and output voltage, and this optimal number of phases is the same as for the output ripple current. When the input voltage and output voltage are relatively constant, the input ripple current can also be minimized by selecting the optimal number of phases.

Reduced heat generation and smaller components

By sharing the current among multiple phases, the current per phase is reduced. As a result, it is possible to reduce losses due to INDUCTORS DCR and the FET on-resistance. Furthermore, the required rated current of the components is also reduced, making it possible to use smaller, thinner INDUCTORS and FETs.

Examples of products compatible with the multi-phase method

Analog Devices offers a wide range of products that support the multiphase method (see Table 1).

	topology	Input voltage	Number of Outputs	frequency	FET	PKG	AEC-Q100	remarks
MAX20404/5/6	Step-down synchronous rectification	3～36V	1	400kHz/2.1MHz/3MHz (spread spectrum)	〇 (Iout=4A/5A/6A)	17 or 15 pin FC2QFN	〇
MAX20408/10	Step-down synchronous rectification	3～36V	1	400kHz/2.1MHz (spread spectrum)	〇 (Iout=8A/10A)	17 or 15 pin FC2QFN	〇	Pin compatible with MAX20404/5/6
MAX40429	Step-down synchronous rectification	3～5.5V	2	2.1MHz/3.2MHz (spread spectrum)	〇 (Iout=6A/6A)	18-pin FCQFN	〇
MAX25254/5	Step-down synchronous rectification	3～36V	2	400kHz/2MHz (spread spectrum)	〇 (Iout=8A/8A)	23-pin FC2QFN	〇	ASIL-B compliant (MAX25255)
LTC7805	Step-down synchronous rectification	4.5～40V	2	100kHz to 3MHz (spread spectrum)	×	28pin QFN	〇	100% duty possible
LTC7890	Step-down synchronous rectification	4～100V	2	100kHz to 3MHz (spread spectrum)	×	40pin QFN	×	For GaN
LTC7871	Step-down synchronous rectification (bidirectional)	4～100V	6	60kHz to 750kHz (spread spectrum)	×	64pin LQFN	〇	GATE DRIVER required
LTC7872	Step-down synchronous rectification (bidirectional)	4～100V	4	60kHz to 750kHz (spread spectrum)	×	48pin LQFN	〇
LTC8277	Boost asynchronous rectification	4～40V	1 (2 phases)	100kHz to 2MHz (spread spectrum)	×	20pin QFN	〇
LTC7806	Boost Synchronous Rectification	4.5～40V	1 (2 phases)	100kHz to 3MHz (spread spectrum)	×	28pin QFN	〇	Pass-through operation (100% Duty)

Table 1. Products compatible with the multiphase method

The MAX20404/5/6 and MAX20408/10 feature a SYNCOUT TERMINALS that provides an output 180° out of phase with SWITCHES timing.
As shown in Figure 9, two-phase operation is possible by connecting the SYNCOUT TERMINALS of the CONTROLLER IC to the SYNC TERMINALS of the TARGET IC (TERMINALS VEA and OUT TERMINALS are also connected).
In addition, by using a multiphase OSCILLATORS *, multiphase operation other than two phases can also be achieved.
In addition, Factory Automation offers a diverse lineup of multi-phase products, including boost and buck-boost, such as the MAX25254/5, which supports ASIL-B, the LTC7805, which supports 100% duty cycle and can reduce dropout voltage, the LTC7890, a GaN-compatible controller, and the LTC7821/2, which is capable of bidirectional operation.

*Multiphase OSCILLATORS
Multiphase OSCILLATORS with spread spectrum functionality that can reduce noise peaks include the LTC6908 (2-phase), LTC6902 (4-phase), and LTC6909 (8-phase).
For more information on the effects of the spread spectrum function, please also refer to the link below.

Spread Spectrum Frequency Modulation Reduces EMI

2. Silent SWITCHES

As the load current increases, problems such as noise (EMI) and heat generation become apparent.
One technology that can solve these issues is the "Silent SWITCHES." As its name suggests, the Silent SWITCHES is a highly efficient SWITCHES regulator that is extremely quiet (ultra-low EMI performance). The first generation product was released in 2013, and has now evolved into its third generation (2025).

Silent SWITCHES 1

In the circuit configuration of the step-down synchronous rectification DC-DC converter shown in Figure 11, the loop with a large di/dt (hot loop) enclosed in a red frame is the main source of EMI. To reduce EMI, the area of this hot loop must be minimized, but due to various constraints in board design, it is necessary to consider the optimal compromise.
In a silent SWITCHES, the magnetic field is canceled by splitting the hot loop into two smaller loops, as shown in Figure 12, thereby suppressing EMI.
In addition, the use of a copper pillar structure flip-chip package reduces parasitic INDUCTORS.
Furthermore, by shortening the "dead time" during which both the high-side SWITCHES and low-side SWITCHES are off, losses due to the body DIODES of the low-side SWITCHES, which is in the forward direction during the dead time, are reduced, maintaining high efficiency even at large currents.

Silent SWITCHES 2

In the second generation, EMI has been further reduced through advances in packaging technology.
The input CAPACITORS are integrated into the package to minimize hot loops and effectively suppress EMI.
Furthermore, by reducing the number of external components, the mounting area has been made smaller, eliminating dependency on the skill of the board layout.

Silent SWITCHES 3

The third generation achieves noise reduction in the LF band (10Hz to 100kHz) and fast transient response.
Even in noise-sensitive applications, the Silent SWITCHES 3 can be used alone to configure POWER SUPPLIES without using an LDO.
The Silent SWITCHES 3 applies the current reference circuit of the linear regulator LT3045 and configures the error amplifier as a unity-gain amplifier (amplification factor of 1), as shown in Figure 13, thereby achieving significant noise reduction without amplifying the noise generated in the reference.

Silent SWITCHES Products

Analog Devices offers a wide range of products incorporating Silent SWITCHES technology, some of which are listed in Table 2.

	topology	Silent Switcher	Input voltage	Number of Outputs	frequency	FET	PKG	AEC-Q100
LT83205	Step-down synchronous rectification	3	2.8～18V	1	300kHz～6MHz	〇 (Iout=5A)	15-pin LFCSP	×
LT8627SP	Step-down synchronous rectification	3	2.8～18V	1	300kHz～4MHz	〇 (Iout=16A)	24pin LQFN	×
LT8625SP	Step-down synchronous rectification	3	2.7～18V	1	300kHz～4MHz	〇 (Iout=8A)	24pin LQFN	×
LT7200S	Step-down synchronous rectification	2	2.9～18V	4	400kHz to 3MHz (spread spectrum)	〇 (Iout=5A/5A)	48pin LQFN	〇
LT8648S	Step-down synchronous rectification	2	3～42V	1	200kHz to 2.2MHz (spread spectrum)	〇 (Iout=15A)	36pin LQFN	〇
LT7150S	Step-down synchronous rectification	2	3.1～20V	1	400kHz～3MHz	〇 (Iout=20A)	42-pin BGA	〇
LTC3311S	Step-down synchronous rectification	2	2.25～5V	1	500kHz～5MHz	〇 (Iout=12.5A)	18pin LQFN	〇
LT8652S	Step-down synchronous rectification	2	3～18V	2	300kHz～3MHz	〇 (Iout=8.5A/8.5A)	36pin LQFN	〇
LTC3313	Step-down synchronous rectification	1	2.25～5V	1	500kHz～5MHz	〇 (Iout=15A)	18pin LQFN	〇
LT8640A	Step-down synchronous rectification	1	3.4～42V	1	200kHz～3MHz	〇 (Iout=5A)	18pin LQFN	〇

Table 2. Silent SWITCHES Products

3. SWITCHES Capacitor (Charge Pump)

In high current environments, heat generation becomes a major issue.
As a solution, we will introduce a highly efficient SWITCHES capacitor (charge pump).
SWITCHES capacitors have the great advantage of not requiring INDUCTORS, but have traditionally been limited to low-power applications (such as dual POWER SUPPLIES for amplifiers and sensor POWER SUPPLIES).
Analog Devices offers highly efficient SWITCHES capacitor products capable of handling high currents.

LTC7820: A very efficient fixed-magnification SWITCHES-capacitor DC/DC controller (external FETs) with an efficiency peak exceeding 99%, as shown in Figure 14.
LTC7825: A SWITCHES capacitor DC/DC converter with integrated FETs that can handle up to 12A.
LTC7821/LTC7822: These hybrid step-down synchronous controllers combine SWITCHES capacitor and DC/DC controllers to provide constant voltage output. Figure 15 shows the efficiency of the LTC7822.

	topology	Input voltage	Number of Outputs	frequency	FET	PKG	remarks
LTC7820	Buck/Boost/Inverter SWITCHES Capacitor	6～72V	1	100kHz～1MHz	×	28pin QFN	Step-down ratio = 2:1, step-up ratio = 1:2 Inversion ratio = 1:1
LTC7821	blood pressure reduction Hybrid Synchronous Rectification	10～72V	1	200kHz～1.5MHz	×	32pin QFN	Output voltage can be set
LTC7822	blood pressure reduction Hybrid Synchronous Rectification	19～90V	2	150kHz～1.5MHz	×	48pin QFN	Output voltage can be set
LTC7825	blood pressure reduction SWITCHES Capacitor	4.5～38V	1	100kHz～1.4MHz	〇 (Iout=12A)	48pin LQFN	Input/output voltage ratio 2:1

Table 3 SWITCHES Capacitor Products

Summary

In this column, we have introduced the following three technical solutions to the major issues facing high-current DC/DC converters (larger components, noise, and heat generation).

1. Multi-phase method
2. Silent SWITCHES
3. SWITCHES Capacitor (Charge Pump)

Each technology is a promising option for achieving the optimum POWER SUPPLIES design depending on the application and required specifications.
We hope you will refer to the contents of this column when considering your design and use it to help you build a higher performance and more reliable POWER SUPPLIES system.