Adaptive on-time control powers Semtech’s dc-dc converter platform

Semtech continues to build on its EcoSpeed family with two new devices, the SC174 and SC173 buck regulators. The EcoSpeed dc-dc converter platform is Semtech’s version of adaptive on-time (AOT) architecture.

The most common control topologies for buck converters are voltage mode, current mode, hysteretic mode, constant on-time mode, constant off time mode, and adaptive on-time mode.  Semtech claims its version of AOT control achieves excellent transient response along with high efficiency.

The AOT architecture is a refinement of the relatively straight-forward hysteretic converter topology: The output voltage feedback feeds into a comparator, rather than the more complex error amp of a voltage- or current-mode converter. When the output voltage falls below a certain threshold it gates the device on and when the voltage rises above the threshold it gates the device off. Hence the term hysteretic – it’s the hysteresis in the comparator that determines your control loop. Because there is no error amplifier circuit it has the advantages of simplicity and speed, resulting in a fast transient response to load variations.

The devices maintain their efficiency across a wide load range: Like most converters, at full load its only losses are due to the FET switching losses, but minimizing losses at light to low loads can be more challenging. Energy Star requires efficient power at loads of 10  and 20%. Standby power modes are one answer, but, most electronic products also require fast “wake-up” capability, which demands fast recovery from low-power standby to full-power steady state mode. The SC173 and SC174 have and ultrasonic power-save (UPSAVE) mode. UPSAVE lowers the switching frequency to cater to low-power standby requirements, thereby by reducing the switching losses associated with the power FET gate charge. This results in up to 95% peak standby efficiency. Then, when the load changes from standby to steady state, the EcoSpeed technology’s cycle-by-cycle response quickly exits UPSAVE mode providing fast power-up response time. Additionally, UPSAVE ensures that the device switching frequency stays above the 25kHz threshold, thereby eliminating audible resonance.  

Historically, a down-side of a hysteretic converter is the amount of ripple required in the hysteresis of the comparator: The top of the ripple serves as the upper threshold and the bottom of the threshold is the lower turn-on point. Higher ripple currents call for electrolytic capacitors with their higher ESRs. For designs requiring lower-ESR capacitors, such as ceramics, the devices support a three-component “Virtual ESR” that allows the device to work with any capacitor techno logy.

The SC173 and SC174 offer output currents of 3A and 4A respectively and are packaged in a 10-pad, 3.0 x 3.0 x 1.0 mm, lead-free, halogen-free MLPD package.