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PWM Ramp Voltage of SG3525

TheDannia

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Hi,

I'm trying to calculate a feedback compensator for SG3525 IC. I need pwm ramp voltage for this IC but it's not given in datasheet. How can I find or calculate it?

Thank you.
 

KlausST

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Hi,

Do you mean an internal signal?
Or a signal on a pin? Which pin? (Use the number or the name as in the datasheet.)

Klaus
 

d123

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Hi,

You don't mean the section called 'PWM Comparator' in the datasheet electrical specifications, do you? The bit that says input threshold zero duty cycle and maximum duty cycle which is 0.7 to 3.6V?
 

TheDannia

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Hi,

Do you mean an internal signal?
Or a signal on a pin? Which pin? (Use the number or the name as in the datasheet.)

Klaus
I mean the ramp voltage of sawtooth wave in voltage to pwm comparator inside the IC, I need it for compensation calculations
 

TheDannia

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Hi,

You don't mean the section called 'PWM Comparator' in the datasheet electrical specifications, do you? The bit that says input threshold zero duty cycle and maximum duty cycle which is 0.7 to 3.6V?
so the difference between them: 2.9V is the ramp voltage of sawtooth wave in pwm comparator is 2.9V ??
 

d123

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Hi,

I wouldn't like to say anything misleading... It might mean that the error amplifier output voltage can go from 0.7V up to 3.6V, so the sawtooth would need to be sized in amplitude to fit that range (for the expected duty cycle, presumably). It might not mean that...

I think EA output should be about 1/3rd to 1/2 ramp peak voltage. Some converter app notes have a formula for it, I think it's buck converter that is: EA to duty cycle transfer function = 1/Vp (Vp is peak-to-peak value of sawtooth).

Hope what I've said is right.
 

d123

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Hi,


Seems app notes for 3525 are for the monolithic or paleolithic IC, or whatever term is more appropriate, SG1524.

ST, ST-AN250 (cd00003778) - 'A Second-Generation IC Switch Mode Controller Optimized for High Frequency Power MOS Drive'
p.7: f = 1/(CT x (0.07 RT + 3 RD)), and a bit more oscillator info.

Microsemi, PS-2-SG1524 (an_ps_2_sg1524_rev2) - 'Simplifying Converter Design with a New Integrated Regulating Pulse Width Modulator'
p.2: Oscillator info.
p.3: Comp. pin function/use.

Silicon General, Application Notes - SG1524B (PS-5_SG1524B_AppNote) - 'Converting 1524 Switching Power Supply Designs to the SG1524B'
p.3: pin 9 EA formulas for frequency compensation component values and poles and zeroes.

ST, ST SG2525A SG3525A datasheet
p.3: EA output low and high levels, and so on.
p.4: fOSC formula
p.6: figure 1 and figure 2 are oscillator charge and discharge times with some component values.

It seems RD goes from pin 5 (ramp) to pin 7 (discharge), if used.

To me, it looks like you have to pay attention to the EA and the PWM Comparator electrical specifications about min. and max. in and out levels, use figure 1 and 2 and fOSC formula to set your linear ramp. Nice puzzle... In case it helps to guesstimate ramp peak at '? t':
delta V/delta t = I x C. Linear isn't quite same as RC charging constant to calculate V at '? t'.

Hope not answering things you didn't ask, sorry if have.
 

FvM

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The bit that says input threshold zero duty cycle and maximum duty cycle which is 0.7 to 3.6V?
To calculate the modulator gain, you better refer to the typical rather than marginal threshold values, 0.9 - 3.3V.
 
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