‘Zero'-standby charger needs no help

A second chip maker has got under 5mW standby for a phone charger, this time without push-button intervention.

In October, Power Integrations introduced the first sub-5mW standby off-line power a 3W controller, called LinkZero-AX, which required a push-button or signal from the secondary side to start it.

Now iWatt of California has introduced the iW1700 which claims to slip under 5mW standby without external intervention.

"iWatt's adaptive digital PWM/PFM technology sends the controller into sleep mode when the load is disconnected, cutting no-load power consumption to less than 4mW," said iWatt, "or effectively zero according to IEC 62301 which rounds power usage of 5mW or less to zero."

Uto 5W output is available from 120/230V, although the firm has yet to say if the full output current and full input range are available from a 5mW-capable design simultaneously.

The chip uses a cheap external NPN power switch and the firm said an 80% efficient 5V 1A universal input supply will need 28 external components and fit into a 25x25x25mm cube.

Applications are expected in chargers for phones, audio players and digital cameras.

As with most low-standby controllers, iW1700 features primary-side control to eliminate feed-back opto-couplers.

It also has quasi-resonant switching for low EMI, cycle-by-cycle waveform analysis, and a up to 72kHz switching frequency.

"A proprietary digital algorithm switches between PWM and PFM modes multiple times as the end device charges," said the firm. "This approach eliminates audible noise, improves efficiency, and reduces switching losses by operating at a pre-determined minimum frequency at no-load. When the load is disconnected, the controller enters sleep mode and turns off non-essential circuits."

An active start-up function disconnects the lossy start-up resistor after the IC powers up, using an NPN bipolar or a depletion-mode n-mosfet, "and allows a one-second or less start-up time," said iWatt.

Part of the adaptive digital cycle-by-cycle waveform analysis handles voltage dips during load transients.

"iW1700 maintains better than +3% output voltage and current regulation over the entire operating line, load, and temperature range, regardless of manufacturing variability or component tolerances," said iWatt.

Built-in frequency jitter and quasi-resonant (valley-mode) switching helps to meet EN55022 class-B-conducted EMI with 10dB margin, and the chip comes in a six-pin SOT-23.