Feature Story
Choosing the optimal Buck topology for your application
The Richtek DC/DC portfolio contains a wide range of buck converters, including conventional Current Mode (CM), Current Mode Constant-On-Time (CMCOT) and Advanced Constant-On-Time (ACOT®) topologies. This feature story will discuss the advantages and drawbacks of each control topology and how to select a most optimal buck topology for your application.
Understanding Richtek Buck Topologies
|
Current Mode
|
Current Mode-COT |
Advanced-COT
|
| CM buck converters control the ON time of the MOSFET. An internal clock controls the switching cycle. ON time is controlled by comparing the sensed inductor peak current with the output of the error amplifier. Control bandwidth is set by the error amplifier and limited to much less than the switching frequency. |
CMCOT buck converters have fixed ON time and regulate the output by controlling the OFF time of the MOSFET. They also contain an error amplifier and current sense, but here the OFF-time is controlled by comparing the sensed inductor current valley with the output of the error amplifier. Converter reaction speed is faster than CM. |
ACOT buck converters have fixed ON time and control the OFF time. But they don’t use current sense or error amplifier: instead, the sensed output voltage is directly compared with the reference.
An internally generated PSR ramp provides the required ripple to make ACOT stable with MLCC capacitors. The fast comparator action and absence of error amplifier makes ACOT control extremely fast. |
.png) |
.png) |
.png) |
Comparison Measurement
The transient behaviour of the three topologies was compared by using three Richtek low voltage (Vin up to 5.5V) Buck converters in identical 5V to 1.2V / 1A applications. The table below shows a comparison measurement on load transient response. Very fast dI/dt rates similar to the rates as seen in MCU core and DDR loads were used.
Fast Step Load Response
|
Topology |
CM |
CMCOT |
ACOT |
| Product |
RT8059 1.5MHz / 1A |
RT8096A 1.5MHz / 1A |
RT5784A 1.5MHz / 2A |
with 550mA
fast step load |
.png) |
.png) |
.png) |
| Vout sag |
65mV or 5% |
49mV or 4% |
24mV or 2% |
| Transient Response |
Moderate |
Fast |
Ultra fast |
Find our products:
RT8059 is a 1A / 5.5V, 1.5MHz CM Buck converter in TSOT-23-5 package.
RT8096A is a 1A / 5.5V, 1.5MHz CMCOT Buck converter with PGOOD in T/SOT23-5/6 packages.
RT5784A
(coming soon) is a 2A / 5.5V,1.5MHz ACOT Buck converter with PGOOD in UDFN2x1.5-8 package.
Also see
RT8095, a 2A / 5.5V, 2.7MHz, 30μA I
Q ACOT Buck converter with PGOOD in WDFN2x2 package.
Key characteristics comparison table of CM, CMCOT and ACOT topologies
| Topology
|
CM
|
CMCOT
|
ACOT
|
| 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 fsw |
constant average fsw |
constant average fsw |
| Stable with MLCC |
√ |
√ |
√ |
| Slope Compensation |
√ |
not required |
not required |
| Synchronized to ext. Clock |
√ |
x |
x |
Which Buck topology is most suitable for your application?
Input voltage, output voltage and nature of the load will in most cases determine which topology is most suitable. Systems with
steady load conditions can use
CM buck converter. To avoid sensitive frequency bands, some CM buck converters can be
synchronized to an external clock. Applications with
severe fast load transients (such as seen in
Core and DDR rails), should be driven by
ACOT buck converters. Note that switching frequency can deviate considerably during dynamic loads.
CMCOT buck converter performance lies in between CM and ACOT buck converters, so it can be used in 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). The dynamic frequency deviation during load transients is less than ACOT.
For more information, please see the application note ’’
Comparing Buck converter topologies’’
New Products
RT9067 is a
14V/200mA, 2μA low Iq LDO with
Enable
function control. The ultra low
2μA Iq and zero disable current makes it ideal for battery powered equipment.
RT8128 is a
0.6V-5V output Buck controller in SOP-8 package. The Mach Response™ provides 100ns instant-on response to load transient and is MLCC stable. Ideal for CPU core, chipset, DRAM or other low voltage high current supplies.
Did you know…?
Richtek and iST unveiled the successful joint-development on the 2nd generation MEMS G-sensor analysis technology, which successfully helped clarify the failure factors in design stage and won ISQED (international quality electronic Design Workshop) recognition.