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.
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.