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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. Buck converter control loop topology has great impact on the converter transient behavior, stability criteria and switching frequency behavior. 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.
Richtek Buck converters can be divided into 5 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. The 24V rated HV Buck converters are often used for applications that run from 18V. We also provide parts up to 80V input range for industrial supplies or automotive applications with large input voltage fluctuation.
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 RDS(ON) : The MOSFET conduction losses are I2 * RDS(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.
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.
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.
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 RDS(ON) due to the absence of bonding wires.
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.
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.
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.
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.
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.
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.
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.
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: