Buck Converter Selection Guide

Buck Converter Definition

Buck converters are switch-mode step-down converters which can provide high efficiency and high flexibility at higher VIN/VOUT ratios and higher load current.

Most Buck converters contain an internal high side MOSFET and synchronous rectifier MOSFET, which are in turn switched on and off via internal duty-cycle control circuit to regulate the average output voltage. The switching waveform is filtered via an external LC filter stage.

Buck converter features

Due to the fact that the MOSFETs are either ON or OFF, Buck converters dissipate very little power, and the duty-cycle control makes large VIN/VOUT ratios possible. The internal MOSFETs R_DS(ON) mainly determines the current handling capabilities of the Buck converter, and the MOSFET voltage ratings determine the maximum input voltage. The switching frequency together with the external LC filter components will determine the ripple voltage on the output. Higher switching frequency buck converters can use smaller filter components, but switch losses will increase. Buck converters with Pulse Skipping Mode (PSM) will reduce their switching frequency at light load, thereby increasing light load efficiency, which can be important in applications with low power standby modes.

The Richtek DC/DC portfolio contains a wide range of Buck converters with different control topologies, including Current Mode (CM), Current Mode-Constant On Time (CMCOT) and Advanced Constant On Time (ACOT®) control topologies. Each topology is suitable for different applications and requirements. For example, Richtek unique ACOT® family has extremely fast transient response compared to CM and CMCOT topologies, which makes it ideal for applications that exhibit very fast load transients, such as DDR, Core SoC, FPGA and ASIC supplies.

Comparison table of Richtek buck topologies

Key features	CM Current Mode	CMCOT Current Mode Constant-On-Time	ACOT® Advanced Constant-On-Time
Topology
Steady-state & Step load
V_IN/I_OUT range	2.5Vin to 5.5Vin / up to 6A for low Vin and battery-powered applications Up to 36Vin / up to 10A Up to 60Vin for Industrial and 36Vin for Automotive applications	2.5Vin to 5.5Vin / up to 3A	Up to 23Vin / up to 12A
Response to Load Steps	moderate	fast	extremely fast
Current Sense	current sense limits min. ON time	low side current sense	Not required
Min. ON Time	Larger, limits the min. achievable duty-cycle	Small min. On time allows small duty-cycles	Small min. On time allows small duty-cycles
Frequency	stable fixed f_SW	constant average f_SW	constant average f_SW
Stable with MLCC	√	√	√
Slope Compensation	√	not required	not required
Synchronized to ext. Clock	√	X	X
Richtek Designer™ simulation tool	Create your account and get your designs up and running in no time. (partial parts have been released)	Coming soon	Create your account and get your designs up and running in no time. (partial parts have been released)
Applications	For applications with steady load conditions. Also for industrial and automotive applications.	For applications with moderate load transients, or applications that require small minimum ON times (i.e. high switching frequency in combination with larger step-down ratios)	For applications with severe fast load transients , such as DDR, Core SoC, FPGA and ASIC supplies

Buck Converter Selection Criteria

In order to select a suitable Buck converter for your application, some key criteria are important to be considered.

Application input voltage

Which upper bound of input voltage fits for your application best?
Richtek Buck converters can be divided into 3 main groups to fulfill different application requirements. Richtek LV Buck converters are suitable for running off single cell Li-Ion batteries as well as supplies from 5V rails. The 18V rated HV Buck converters are often used for applications that run from 12V. We also provide parts up to 36V input range for industrial supplies or automotive applications with large input voltage fluctuation.

Which upper bound of input voltage fits for your application best?

Richtek Buck converters can be divided into 3 main groups to fulfill different application requirements. Richtek LV Buck converters are suitable for running off single cell Li-Ion batteries as well as supplies from 5V rails. The 18V rated HV Buck converters are often used for applications that run from 12V. We also provide parts up to 36V input range for industrial supplies or automotive applications with large input voltage fluctuation.

Application current consumption

How to calculate the power loss and maximum application peak current?
When considering the Buck converter current rating, there are two factors to consider: The application average current consumption and the application peak current. The application average current will determine the average heat in switching MOSFETs which is related to conduction losses and switching losses. Conduction losses are related to the internal MOSFET R_DS(ON) : The MOSFET conduction losses are I² * R_DS(ON) ; Switching losses are mostly related to the current, the input voltage and the switching frequency. In most standard applications, the switching losses are roughly 30% of the total losses, but in applications with higher input voltage or high frequency, the switching losses can increase considerably. The application total power losses can be derived from the datasheet efficiency curve: . The device maximum rated current and over-current protection level must be considered when checking application peak load current. The difference between load current and inductor peak or valley current is ½ the inductor ripple current, so be sure to include this when checking the application maximum load current in relation to OCP current levels.

How to calculate the power loss and maximum application peak current?

When considering the Buck converter current rating, there are two factors to consider: The application average current consumption and the application peak current.

The application average current will determine the average heat in switching MOSFETs which is related to conduction losses and switching losses. Conduction losses are related to the internal MOSFET R_DS(ON) : The MOSFET conduction losses are I² * R_DS(ON) ; Switching losses are mostly related to the current, the input voltage and the switching frequency. In most standard applications, the switching losses are roughly 30% of the total losses, but in applications with higher input voltage or high frequency, the switching losses can increase considerably. The application total power losses can be derived from the datasheet efficiency curve: .
The device maximum rated current and over-current protection level must be considered when checking application peak load current. The difference between load current and inductor peak or valley current is ½ the inductor ripple current, so be sure to include this when checking the application maximum load current in relation to OCP current levels.

Light load efficiency

When to select Force-PWM mode or PSM mode?
For supply rails that need to be active in low power standby modes, it is desirable to make the Buck converter efficiency at light load as high as possible. Force-PWM Buck converters keep the switching frequency fixed over the entire load range while Pulse Skip Mode (PSM) will reduce switching frequency at light load, thereby improving light load efficiency since the majority of losses at light load are caused by switching loss. Read more for PSM operation principle and its advantages and drawbacks.

When to select Force-PWM mode or PSM mode?

For supply rails that need to be active in low power standby modes, it is desirable to make the Buck converter efficiency at light load as high as possible. Force-PWM Buck converters keep the switching frequency fixed over the entire load range while Pulse Skip Mode (PSM) will reduce switching frequency at light load, thereby improving light load efficiency since the majority of losses at light load are caused by switching loss.

Switching frequency

How to select suitable switching frequency?
Higher switching frequency makes it possible to use smaller inductor and capacitors, and improves the step load behaviour of the converter. However, it also increases switching losses and extends the EMI radiation frequency range. Higher switching frequency can also limit the maximum step-down ratio that can be achieved : The minimum duty-cycle is limited by the converter minimum ON time and the frequency: , so In general, higher VIN applications should use lower switching frequency devices.

How to select suitable switching frequency?

Higher switching frequency makes it possible to use smaller inductor and capacitors, and improves the step load behaviour of the converter. However, it also increases switching losses and extends the EMI radiation frequency range. Higher switching frequency can also limit the maximum step-down ratio that can be achieved : The minimum duty-cycle is limited by the converter minimum ON time and the frequency:

, so

In general, higher VIN applications should use lower switching frequency devices.

Low BOM cost

How to reduce BOM cost?
Choosing the right Buck topology together with the most optimal IC package can bring you cost savings on both passive components and IC cost. ACOT® topology offers superior load transient response, making it possible to reduce the size of your output capacitors and still meet the load transient voltage undershoot requirement. Flip-Chip in TSOT-23-6 package offers lowest package cost, while maintaining good thermal performance and low R_DS(ON) due to the absence of bonding wires.

How to reduce BOM cost?

Choosing the right Buck topology together with the most optimal IC package can bring you cost savings on both passive components and IC cost. ACOT® topology offers superior load transient response, making it possible to reduce the size of your output capacitors and still meet the load transient voltage undershoot requirement. Flip-Chip in TSOT-23-6 package offers lowest package cost, while maintaining good thermal performance and low R_DS(ON) due to the absence of bonding wires.

IC package consideration

Which IC package is most suitable for your application?
Richtek Buck converters are available in many types of packages: from tiny CSP 1.3x2.1mm to cost effective TSOT-23-6 to larger TSSOP-14 thermally enhanced package. SOP-8 (exposed pad) and DFN2x2 and DFN3x3 packages are often used in Buck converters: Their pin count ranges from 6 ~ 12 pins for extra functionality, and they offer good thermal performance due to exposed thermal pad. They are cost effective, making them a popular choice for many applications. It is possible to use these parts in single sided layout, but for better thermal and electrical performance multi-layer PCB layouts are recommended. TSSOP-14 or WDFN-14L 4x3 have larger thermal pads, which allow them to dissipate more power. Read more to find design considerations for other packages.

Which IC package is most suitable for your application?

Richtek Buck converters are available in many types of packages: from tiny CSP 1.3x2.1mm to cost effective TSOT-23-6 to larger TSSOP-14 thermally enhanced package.

SOP-8 (exposed pad) and DFN2x2 and DFN3x3 packages are often used in Buck converters: Their pin count ranges from 6 ~ 12 pins for extra functionality, and they offer good thermal performance due to exposed thermal pad. They are cost effective, making them a popular choice for many applications. It is possible to use these parts in single sided layout, but for better thermal and electrical performance multi-layer PCB layouts are recommended. TSSOP-14 or WDFN-14L 4x3 have larger thermal pads, which allow them to dissipate more power. Read more to find design considerations for other packages.

Other considerations


External soft-start All Richtek Buck converters have a soft-start function. After enabling the converter, the duty-cycle is gradually increased to allow a smooth rising output voltage, which avoids inrush current due to sudden charging of the output capacitors. Converters with internal soft-start have a fixed soft-start time. If the application uses very large output capacitance or requires a specific soft-start time, it is better to select a converter with externally programmable soft-start; the soft-start time can be set by an external capacitor. External compensation Current mode converters need error amplifier compensation to ensure stable operation. The type-II compensation components determine the converter bandwidth and the phase boost frequency. Converters with external compensation have more flexibility in setting the desired bandwidth and phase margin with different types of output capacitors over a wider range of input and output voltage conditions. Programmable frequency Some converters have a programmable frequency function: The switching frequency can be set by means of an external resistor. This gives more flexibility in choosing the best switching frequency for the application; higher frequency to reduce ripple or component size or get better transient behavior, or lower frequency to improve efficiency or reduce higher harmonics. External sync input Some current mode converters have an external sync input that allows the internal clock to be synchronized to an external clock signal. This makes it possible to set the switching frequency at a very precise value (for avoiding noise at sensitive frequency bands), and also make it possible to run several converters at the same frequency. Low-Dropout mode or 100% duty-cycle mode Many current mode Buck converters from the LV series have Low Dropout mode function: When the input voltage drops, these Buck converters gradually increase the duty-cycle and will continuously switch-on the high side MOSFET when the input voltage drops below the regulated output voltage. This function is especially suitable in battery powered applications, and can extend application operation time when the battery is almost depleted. Power Good function The Power Good function will monitor the Buck converter output signal and provide a means of telling the system when the output voltage is within a certain operating range. Power Good can be used for system initialization, fault detection or start-up sequence. Over Current Protection All Richtek Buck converters have Over Current Protection (OCP). When the inductor current exceeds the OCP level, the converter duty-cycle is limited. Further load increase will result in output voltage drop. However, there are different ways how the system behaves in overload condition: Latch mode OCP: When during overload the output voltage drops below the Under Voltage Protection (UVP) point, the system shuts down and latches. The converter needs to be re-enabled or cycle the input voltage for restart. This protection ensures zero power after overload, but does not have auto restart. Hiccup mode OCP: When during overload the output voltage drops below the UVP point, the system shuts down and initiates a restart with soft-start. Continuous overload will show continuous shut-down/restart cycle or hiccup mode. The advantage of hiccup mode is low average overload current, and guarantees auto restart after the overload is removed. Non UVP: During overload the output voltage drops, but there is no UVP action. The system continues to run at OCP current level during overload. The output voltage recovers immediately after the overload is removed. But the continuous OCP current level can lead to increased temperature in longer term overload conditions.

External soft-start

All Richtek Buck converters have a soft-start function. After enabling the converter, the duty-cycle is gradually increased to allow a smooth rising output voltage, which avoids inrush current due to sudden charging of the output capacitors. Converters with internal soft-start have a fixed soft-start time. If the application uses very large output capacitance or requires a specific soft-start time, it is better to select a converter with externally programmable soft-start; the soft-start time can be set by an external capacitor.

External compensation

Current mode converters need error amplifier compensation to ensure stable operation. The type-II compensation components determine the converter bandwidth and the phase boost frequency. Converters with external compensation have more flexibility in setting the desired bandwidth and phase margin with different types of output capacitors over a wider range of input and output voltage conditions.

Programmable frequency

Some converters have a programmable frequency function: The switching frequency can be set by means of an external resistor.

This gives more flexibility in choosing the best switching frequency for the application; higher frequency to reduce ripple or component size or get better transient behavior, or lower frequency to improve efficiency or reduce higher harmonics.

External sync input

Some current mode converters have an external sync input that allows the internal clock to be synchronized to an external clock signal. This makes it possible to set the switching frequency at a very precise value (for avoiding noise at sensitive frequency bands), and also make it possible to run several converters at the same frequency.

Low-Dropout mode or 100% duty-cycle mode

Many current mode Buck converters from the LV series have Low Dropout mode function: When the input voltage drops, these Buck converters gradually increase the duty-cycle and will continuously switch-on the high side MOSFET when the input voltage drops below the regulated output voltage. This function is especially suitable in battery powered applications, and can extend application operation time when the battery is almost depleted.

Power Good function

The Power Good function will monitor the Buck converter output signal and provide a means of telling the system when the output voltage is within a certain operating range. Power Good can be used for system initialization, fault detection or start-up sequence.

Over Current Protection

All Richtek Buck converters have Over Current Protection (OCP). When the inductor current exceeds the OCP level, the converter duty-cycle is limited. Further load increase will result in output voltage drop. However, there are different ways how the system behaves in overload condition:

Latch mode OCP: When during overload the output voltage drops below the Under Voltage Protection (UVP) point, the system shuts down and latches. The converter needs to be re-enabled or cycle the input voltage for restart. This protection ensures zero power after overload, but does not have auto restart.
Hiccup mode OCP: When during overload the output voltage drops below the UVP point, the system shuts down and initiates a restart with soft-start. Continuous overload will show continuous shut-down/restart cycle or hiccup mode. The advantage of hiccup mode is low average overload current, and guarantees auto restart after the overload is removed.
Non UVP: During overload the output voltage drops, but there is no UVP action. The system continues to run at OCP current level during overload. The output voltage recovers immediately after the overload is removed. But the continuous OCP current level can lead to increased temperature in longer term overload conditions.

Richtek Buck Product Family

Current Mode Buck converters
CMCOT Buck converters
ACOT product family
The proprietary ACOT control scheme not only features ultra-fast transient response but also improves upon legacy constant on-time architectures, achieving constant average frequency over line, load and output ranges to minimize interference and noise problems. The ACOT Buck converters are stable with and optimized for ceramic output capacitors without external components or external ripple injection scheme. The ACOT family includes latch-off, hiccup and constant current protection modes. Richtek has introduced the whole range of ACOT Buck converters up to 12A output current capacity and the input range is from 4.5V to 23V. The ACOT Buck converters are ideal for Set Top Boxes, industrial and commercial low power systems, computer peripherals and LCD Monitors and TVs. See the list of ACOT products.
Wide Input Voltage Range DC-DC power solutions for industrial, Automotive and LED lighting applications
Richtek offers comprehensive power conversion solutions for input voltages ranging up to 60V, suitable for a wide range of applications in the industrial, automotive, and professional lighting field. The key features for Buck converter parts in wide input range are: Robust control architectures for coping with noisy environments, PGOOD and adjustable Soft-start for power sequencing, thermally enhanced packages for operation in higher ambient temperatures, and fully stable with all ceramic input and output capacitors for extended operation life time. We also offer products qualified over -40°C to 85°C industrial temperature range as well as automotive AEC-Q100 standard qualified parts. See the list of selected products.

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