Re: Any beefy coil driver chips?

Winfield Hill wrote:
On Aug 2, 5:26 pm, Joerg <inva...@xxxxxxxxxxxxxxx> wrote:
Winfield Hill wrote:
On Aug 2, 12:21 pm, Joerg wrote:
Winfield Hill wrote:
On Jul 31, 3:38 pm, Joerg <wrote:
Nico Coesel wrote:
A GAL16V8 or similar? I still think an NE555 solution is the easiest.
Yeah, no gas guzzler devices. A 555 type is nice but there are some buck
and boost chips that have more analog on board and could possibly find
an unorthodox use here.
Joerg, I suggest you look at IR's IRS2153 or 21531 self-oscillating
high-voltage half-bridge ICs. These generate their own low-voltage
supply from the coil supply, and they have a 555-like oscillator that
you can easily trick into continuous on or off states, besides PWM.
Thanks, Win. Very interesting chip. However, for an initial continuous
on state of the upper FET the boost cap won't come up because for
some reason they just hung that onto the drive for the lower FET. But
there may be options with connecting the coil to V+. Makes current
sense on it a bear though.
I'm assuming you'd drive the coil to ground. That way you start
out with the output low and with the coil off, and at that time the
hi-side flying cap gets charged, ready for your first turn-on.
Actually I must ramp the coil current to the hilt the instant the rail
comes up. In cases like this the rail is the switched line and also
the only power source. Wanting an extra DC power line to come in
is almost like renting a backhoe and starting to trench :-)

Hah! No, this isn't a problem. Use a small mosfet, with Ciss
no more than 100pF, so the flying cap, Cboot, need not be
larger than say 470pF. Use a small cap across the IRS2153
internal 15V zener, say 2.7nF. If the input-power resistor is
680 ohms (nominal 10 to 15mA of current), then the start-up
time, including charging Cboot, is under about 3us. This
means that you can very quickly assert the ON state.


That is a nice trick. It doesn't even have to be that fast. Several ten usecs for start-up would be ok. Compared to SMPS converters this stuff is all relaxingly "slow".


Hmm, I wonder, might there be any bounce in the 30V
input drive, better add a diode in series with the 680 ohms.

Note, you can run the rest of your control circuitry from the
15V power tap of the 2153 chip. Hah, a cmos 555 still has
a chance to play a role, for your 1 to 5ms high-power timing.
With a diode, its output can hold up the CT pin.


Abs max says 20mA so even at half that value there should be plenty to spare. For some reason they only give a typical supply current but with Iq max at 1mA plus whatever the oscillator causes that leaves plenty for some CMOS logic. Of course they stated the typical current at a highish timing resistance where you can't get it above 20kHz. If you ever need something like this with more nifty power supplies the HV9901 has a 2nd regulator in there that can be set to the levels of modern uC and is nicely tied to a bandgap reference in there. Only 1mA but that suffices for a little MSP430 at a low clock speed. But the comparator offset specs are horrid, I am wondering if they just were ultra-conservative.


Well, it's fun to talk about these things. But if we get down
to brass tacks, and get serious, I have to say I'm not happy
with the IRS21531 chip's long 0.6us dead time during PWM.
While it may be appropriate for a 200 to 600V application,
it's far too long at low voltages, and means the lower power
FET's body diode will always be conducting during the dead
time, with its ugly reverse-recovery-time snap-off issue. OK,
add a Schottky diode across the lower mosfet. That'll fix it.


That's a common trick in synchronous bucks but so far I always got away without the Schottky.

--
Regards, Joerg

http://www.analogconsultants.com/

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