The CS51221 and CS51227, two new voltage-mode pulse-width modulated (PWM) control ICs from Cherry Semiconductor Corporation, are designed for feed forward operation in sub 150W DC/DC power converters. The CS51221 is designed for applications that require a full set of control and protection features, while the CS51227 is a basic device targeted at more cost sensitive applications. Both controllers have all the features necessary for basic voltage-mode operation. On-chip capabilities substantially reduce external component count, board area requirements, and overall power converter cost.

The full-featured, 16-lead CS51221 provides programmable pulse-by-pulse overcurrent protection, overvoltage protection with programmable hysteresis, undervoltage lock out, accurate programmable duty cycle limit, leading edge current sense blanking, and bi-directional synchronization. It offers a soft start feature for controlled start up of the output voltage and to protect against voltage overshoots. The synchronization output (SYNC) can be used to synchronize the device to other power supplies. Its bi-directional capability allows each supply to be either the master or slave unit.

The 8-lead CS51227 offers fewer options, but still contains many of the control capabilities required for supply operation: undervoltage lockout, leading-edge current sense blanking, and 50ns shutdown propagation delay. Leading edge current sense blanking prevents voltage spikes from causing false current limit triggering. The CS51221 and CS51227 are fixed frequency, 1 MHz devices that drive an external power MOSFET switch with up to 1A of sink/source gate drive current.

I had forgotten, until I had both Cirrus Logic's Crystal parts and these Cherry parts on the desktop at the same time, that the two companies are using the "CS" designator. It will probably cause little confusion in the current market but I can imagine that the replacement marketplace will have some fun in a few years time.

These two products fall into one of the largest demand areas in the marketplace and both have sufficient, basic, protection that they are worth looking at for any new application in the sub-150-W dc-dc conversion arena. The CS51221 is much fuller of features, some of which will look very attractive to different parts of the market. Speedily-changing load environment designers will love the pulse-by-pulse overcurrent protection, for example, while the soft-start will be of use in designs where there are fixed, heavier load conditions. The synchronization ability could be both a feature or a nuisance; in situations where the supplies are operating correctly it is a feature, when a supply goes faulty circuit planning has to take in to account all the possible ramifications, which could be quite tricky. There are also situations where you might want to make one supply always be the Master in a synchronizing environment.