Linear LT8570 sync?

[Richard Crowley]

I have been looking at the Linear LT8570 DC/DC converter chip for a couple of projects:

1) Providing modest current (<500mA) + and - 15V rails for some audio projects with any old random DC wall wart basic power source. The objective being to have a module that will provide a modest but clean split power supply for some op-amp-based audio circuits.  But enabling the user to source a wall-wart that suits their local mains power without being too picky about the output voltage. 

Offering a sensibly-priced bipolar power supply for modest-size audio projects seems to be something that I'm not finding readily available.  Most of the kits one finds require a center-tapped AC transformer input which isn't too terribly difficult to find, but it rather eliminates any possibility of using inexpensive external wall-warts. And I don't want to get into the business of offering kits that require mains power connectivity.  Even singie-output AC wall-warts aren't all that readily available in suitable voltages (18-22 VAC)

2) Generating the low-current but relatively high voltage (48V) required for microphone phantom power. Since the power is delivered through the spec 6K81 resistors, it requires worst-case of 15mA per microphone circuit.
Ref design: http://cds.linear.com/image/32410.png

My question is: There is a "SYNC" pin shown on the data sheets, but in all cases it is shown unused (tied down to ground). The descriptions mention feeding an external frequency into it, so I must presume that it does PWM to maintain the voltage regulation.  I am worried that if I have two or three of these circuits together in a project I want to avoid them hetrodyning against each other and producing an audible beat frequency.  So I thought it would be good to use the SYNC function. 

I thought it would be as easy as connecting the sync pins together to cause the interconnected chips to operate at the same frequency. But it appears that it isn't as simple as that. I must generate some frequency externally to drive the chips.  Perhaps I could just use a 555 or something to generate some arbitrary super-sonic frequency to force all the chips to operate together.  These are generally all-analog projects without digital circuitry or microcontrollers or clock frequencies already available.

[MagicSmoker]

Typically, you drive the SYNC pin with an external clock (3.3V logic level suffices) and set the on-chip oscillator to a slightly lower frequency, but it's easy enough to double-check this in LTSpice.

[Jay_Diddy_B]

Hi,

On most of the LTspice models, including the LT8570, the sync function is not modelled.

The main purpose of the syncing them is to avoid beat frequencies. If you at had one 400 kHz and another at 402 kHz you could get some 2kHz currents flowing in your input filter, which would be annoying.

There is some guidance on the datasheet about the sync frequency and the oscillator requirements.

Parts like the LTC6902 are used to provide the sync signals.

Regards,

Jay_Diddy_B