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Electronics => Beginners => Topic started by: biker.josh07 on September 15, 2014, 01:35:13 pm

Title: UC3843 PWM Controller T symbol
Post by: biker.josh07 on September 15, 2014, 01:35:13 pm

I'm building a PWM control circuit using the UC3843 IC. I'm trying to understand the operation of the IC from the block diagram given.

I'm just curious as to what the T symbol is with the clock input. Also I'm pretty sure that the SR latch is the NAND type rather than the NOR type but confirmation would be helpful.

Thanks,

Josh.

Title: Re: UC3843 PWM Controller T symbol
Post by: bktemp on September 15, 2014, 01:42:54 pm

It is a toggle flipflop that gates every second clock pulse: The duty cycle is limited to <50% and the frequency is halved compared to a 3842.

Title: Re: UC3843 PWM Controller T symbol
Post by: Simon on September 15, 2014, 01:47:32 pm

If you referring to Rt and Ct T stands for timing. the clocking circuitry is internal but you use an external resistor and capacitor to set the timing.

Title: Re: UC3843 PWM Controller T symbol
Post by: T3sl4co1l on September 16, 2014, 05:53:30 am

Nah, it's only used in the -44 and 45.  The -43 and -45 are low voltage versions (UVLO 5V or something like that).

So, important to note that the RtCt value used with the 44/45 must be half that for the 42/43 to achieve the same output frequency! ;)

For more ideas, check out the UC3808: it's the same basic thing, but it has alternating outputs.  It's simply the same logic, but with the opposite phase from the T f/f brought out through logic to a pin driver.

Tim