Regulator gets the jitters to cut EMI

A family of low-output power offline switching regulator ICs features a novel "jittering" function that minimises the need for external EMI noise-filtering circuitry.

The new STR-A6251/51M/52 Series from Allegro MicroSystems Europe is a family of low-output power offline switching regulator ICs featuring a novel "jittering" function that minimises the need for external EMI noise-filtering circuitry.

The new devices use a PWM (pulsewidth modulation) topology to provide an output power of up to 20W with a universal input or and 24W with a 230V AC input.

Each device incorporates a variety of functions to realise a simple and reliable design in a switched-mode power supply system, including primary controlled constant-voltage/constant-current operation, automatic standby, soft start, circuit protection and an avalanche-energy-guaranteed MOSFET.

The frequency jittering function enables cost reduction in the EMI noise-filtering circuitry by suppressing EMI noise.

The centre frequency of the PWM operation is set to 50kHz for the STR-A6251/52 and 67kHz for the STR-A6251M.

The frequency jittering provides a variation of +/-2.5% of the centre frequency, and the fluctuation speed is adjusted by an external capacitor for optimum results.

A number of other features reduce the overall costs of designs based on these ICs.

The primary constant output eliminates the need for secondary-side control circuitry which would normally include an operational amplifier, current-sensing resistor and passive components.

In addition, the overload protection circuit incorporates an integral delay timer for automatic re-start, eliminating the need for external components.

In addition to the overload protection, each device includes pulse-by-pulse overcurrent protection, overvoltage protection with latch, and thermal shutdown with latch.

Auto-burst standby operation reduces the standby input power to less than 100mV at zero load and 264V AC, while the soft-start function reduces stress to the integrated power MOSFET during startup.

The proprietary 900V BCD process connected to the internal MOSFET's drain to power the control circuit simplifies the peripheral circuit design and improves the system's efficiency.

Current-mode feedback control and leading-edge blanking eliminate the possibility of malfunctions caused by surge currents in the turn-on period and hence the need for a surge-filtering circuit.

The use of an avalanche energy guaranteed MOSFET simplifies the design of the surge-absorption circuit and eliminates the need for voltage derating.

The STR-A6251/51M/52 Series is targeted at battery-changer, standby and low-output power-supply applications in the consumer, communication, industrial and home appliance markets, and is supplied in an 8-pin dual-inline package.

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