Looking for Flyback/PWM controller with external oscillator only (no internal)

[avihaio]

Hi again,

I'm looking for a PWM/flyback controller which does not have an internal oscillator in it.
The motivation is that I want to provide my own oscillator signal and don't want to be limited by the "SYNC" pin restrictions compared to the internal oscillator frequency, so I can be flexible with the frequency that I choose (and maybe change in the future). I don't want another frequency in my converter.

So the bottom line is that I need a controller with something like an "oscillator-in" pin.

Googling "external oscillator" and similar refers to controllers with "SYNC" pin which does not meet my expectations.
BTW, maybe a PWM/flyback controller is not what I need and if you have another idea, it's also OK.

Thank you!
Avihai

 

[Easy peasy]

make a whole PWM controller from discrete chips - not too hard and you get to control the freq, 4046 can be used as a VCO allowing freq control over a 50kHz to 250kHz range say ...

 

[KlausST]

Hi,

Why does the SYNC functionality not meet your requirements?

If you need a wider range of frequency, then you probably need to consider to adjust feedback loop (filter, frequency, gain...) to your frequency.

Klaus

 

[avihaio]

I have only this clock signal that comes from somewhere else which I can use, so for example if I have an EN pin I must connect it to Vin in order to have the output voltage working, but I still want to have the ability to disable the output when stopping this clock.
External logic is not an option, I want to try to do this as simple as possible (if possible).

Thanks

 

[KlausST]

Hi,

adjusting clock for synchronizing is different to "disable the output".
I don´t recommend to simply stop the clock. One can not predict what happens then.

Klaus

 

[treez]

you can sync the ucc28c4x range of pwm controllers (EG UCC28C43) with a square wave into the RC pin. I think it may be via an inversion, but its still what you want ultimately.
There is also a microchip pwm controller which specifically takes a pwm input from a micro.....i forget its name now, MCP1530 ???.....something like that.

 

[dick_freebird]

Might look at emerging "digital power" controllers which
don't have/need a ramp (but these seem to often be
PFM, hysteretic rather than fixed frequency PWM (which
I understand you want to vary (why, I do not) but not
in-the-moment (?)

As mentioned, gross changes to frequency can require
corresponding changes to loop filter and to the output
filter (esp. inductor value needs to be in the sweet
spot, not so low that you can saturate and not so high
that you can't slew to follow load, smartly). So you may
want to think harder about "nice to have" vs "gotta have"
(why go for a universal bare clock when you can't change
by more than 2X without having to whip out the soldering
iron?).

 

[avihaio]

you can sync the ucc28c4x range of pwm controllers (EG UCC28C43) with a square wave into the RC pin. I think it may be via an inversion, but its still what you want ultimately.
There is also a microchip pwm controller which specifically takes a pwm input from a micro.....i forget its name now, MCP1530 ???.....something like that.

The MCP1630 seems like exactly what I need, but unfortunately I can't find a spice model for it. Do you know other similar products with this option?

 

[treez]

the lt1241 is a spice model for the ucc28c43.....lt1241 is in ltspice....so you can use it there...and put a square wave into the RC pin.

I will send you an ltspice also of a discrete controller ...coming soon

- - - Updated - - -

here it is

 

[avihaio]

the lt1241 is a spice model for the ucc28c43.....lt1241 is in ltspice....so you can use it there...and put a square wave into the RC pin.

I will send you an ltspice also of a discrete controller ...coming soon

- - - Updated - - -

here it is

This is very helpful.

Concerning the LT1241, I see that in simulation it actually stops when the square wave stops. How does the duty cycle of the clock affects the controller? Does it limit the the duty cycle of the internal oscillator like it does in the MCP1630?

Thanks a lot!

 

[avihaio]

This is very helpful.

Concerning the LT1241, I see that in simulation it actually stops when the square wave stops. How does the duty cycle of the clock affects the controller? Does it limit the the duty cycle of the internal oscillator like it does in the MCP1630?

Thanks a lot!

And BTW, are there more controllers like the LT124X (or its TI twin)? I prefer something with a minimum input voltage of 3V

 

[KlausST]

Hi,

Did you check the selction guides at the manufacturer´s internet pages?

Klaus

 

[avihaio]

Hi,

Did you check the selction guides at the manufacturer´s internet pages?

Klaus

Of course. The problem is that they don't differ between the normal SYNC option (that uses a configured oscillation frequency when there's no sync signal) and the one I'm looking for, like in the LT1241 and MCP1630) where you can stop the regulation when the external signal stops.
I didn't find a "keyword" to find only the second option, which is the one I need.

Thanks

 

[mtwieg]

This is very helpful.

Concerning the LT1241, I see that in simulation it actually stops when the square wave stops. How does the duty cycle of the clock affects the controller? Does it limit the the duty cycle of the internal oscillator like it does in the MCP1630?

Thanks a lot!

I'm assuming that duty cycle of the external clock shouldn't matter because only the timing of one edge (rising/falling) matters. Note that these are both current mode controllers. If you need voltage mode control, then this won't work.

Anyways as was advised above, gating the clock is a very risky idea unless you know exactly what's going on inside. My recommendation would be to have a simple external clock detector circuit (a diode, resistor, and capacitor) whose output is connected to the controller's enable/disable pin. When the clock disappears, it will free-run according to its internal oscillator for a little bit until it is shut down by the clock detector.

 

[treez]

This is very helpful.

Concerning the LT1241, I see that in simulation it actually stops when the square wave stops. How does the duty cycle of the clock affects the controller? Does it limit the the duty cycle of the internal oscillator like it does in the MCP1630?

Thanks a lot!

ill take a look...but i think you can set the max duty cycle with the square wave duty cycle that you put into it.....i think its an inversion in there....ie when your square wave input is low, the gate goes high, or something....if you experiment with it in ltspice you will see....there is only a current source inside it, (plus obviously a comparator) so you are ok putting a square wave into the clock pin of the lt1243 or the ucc28c43.

 

[dick_freebird]

I've been slogging through old Motorola databooks looking
at (or, for) PWM guts and ran across some older MCxxxx
parts that are highly pinned out, to where you might be
able to connect your own sawtooth to the rest of a
modulator.

Last DC-DC design I did, we had a tri-mode clock that
used a clock detector to decide whether to use incoming
clock (100kHz - 5MHz) or pin DC level for 500K / 1MHz
internal oscillator settings. Clock detector has to be way
slower than the lowest valid clock frequency to detect,
and this attribute is probably going to be sloppy as it's
sort of a go / no-go, barely care about values, type of
feature.