Tiny clock optimized for switching regulators

Milpitas, Calif. — Linear Technology Corp. claims its spread spectrum silicon oscillator is a tiny, robust, low-power device that is optimized for switching regulators.

Using a single resistor, the LTC6908 is programmable to any frequency from 50 kHz to 10 MHz. The LTC6908 comes in two configurations, each with dual outputs. The LT6908-1's outputs are 180 degrees out of phase and the LTC6908-2's outputs are 90 degrees out of phase.

Enabling the pseudo-random spread-spectrum provides a simple, effective way to reduce electromagnetic interference (EMI). In the event that the switcher bandwidth is limited, the LTC6908 modulation rate can be adjusted to one of three settings. “Implementing spread-spectrum for switchers is now trivial,” says Doug LaPorte, design section lead for Linear Technology. “The user sets the frequency with one resistor and selects a modulation rate.”

One resistor sets the frequency. Additional features include optional ±10 percent frequency spreading, ±40 ppm/°C temperature stability, fast start up time <1.5 milliseconds, outputs muted until stable, 2.7 volt to 5.5 V operation and frequency error 1.5 percent maximum.

See related graphic

Fully specified over the temperature range of -40°C to 125°C, these parts are available in a 6-lead ThinSOT package and a 2 x 3 mm DFN. The LTC6908 is in full production with prices starting at $1.65 each in 1,000-piece quantities. Click here for the LTC6908 data sheet.

Linear Technology Corp. , 1-800-454-6327,

The LTC6908 is a complete solid-state clock signal generator, which does not require external oscillation circuitry such as a resistor and capacitor combination or a crystal resonator. A single resistor is used to select the operating frequency.

“Although frequency accuracy does not match that of a crystal oscillator, the LTC6908's accuracy of 1.5 percent is ideal for switching regulators,” said Greg Zimmer, LTC's product marketing engineer.

The major advantages of Linear Technology's silicon oscillators are the tiny size, and the robust, low-power operation over a large frequency range. “Unlike crystal oscillators, these oscillators start up very quickly (typically less than a few hundred microseconds versus tens of milliseconds),” Zimmer said. In a handheld scanner, for instance, the scanner can be powered up to scan bar codes very quickly and then just as quickly powered down, enabling the battery to last longer, he said.

Another benefit of using silicon-oscillators is that a single part can be used for many different frequencies and the frequency can even be changed on-the-fly. Silicon oscillators are also less susceptible to shock and vibration when compared with crystal oscillators, according to Zimmer.

To address EMI issues, the LTC6908, which was made for use in power supplies, has an optional feature called spread-spectrum frequency modulation (SSFM) to vary the output frequency. “The purpose of SSFM is to lower the peak amount of radiated energy at the switching frequency,” Zimmer said.

See related SSFM enabled/disabled graphic

Linear's first silicon oscillator aimed at switching regulators, the LTC6902, came out about five years ago. The LTC6908 has 6 pins and is less than half the size of the 10-pin MSOP used in the older LTC6902.

Unlike many of the competing devices, the LT6902 and LTC6908 provide multiple phase-shifted outputs for synchronizing multiple switching regulators, Zimmer said. A primary difference between the two parts is the way the outputs are configured. The LTC6902 has four outputs, which can be configured in various phase modes. The LTC6908 takes a more simplified approach that meets the needs of most applications by offering two outputs with two choices of phase offset, Zimmer said. “Most applications will prefer the ease of use of the LTC6908 to the flexibility of the LTC6902,” he said.

The LTC6908 is said to be very easy to use. The user programs only the main frequency. If SSFM is enabled, the user selects one of three filter modulation rates. The filter modulation scheme provides an intelligent way to implement SSFM by using a proprietary internal tracking filter. This filter allows the designer to scale back the highest frequencies when the bandwidth of the switcher is being exceeded by the highest frequency content. “Essentially, this allow for SSFM to be used up to its maximum potential to provide the most effective EMI reduction,” Zimmer said.

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