# *SG3525: Pin Connections

Discussion in 'General Electronics Chat' started by JUAN DELA CRUZ, Oct 20, 2008.

1. ### JUAN DELA CRUZ Thread Starter Active Member

May 27, 2008
121
0
Hi guys.. I've a trouble using SG3525 PWM controller IC..

Here are my questions..

1. What is the formula needed to calculate the Osc. frequency (not shown in the datasheet attached) ?

2. The frequency of the OUTPUT A (pin 11) & OUTPUT B (pin 11) is ½ the OSC. Output (pin 4) frequency ?

3. How can shutdown the SG3525? ...by holding SHUTDOWN (pin 10) "high" ?

Thank you

File size:
36.3 KB
Views:
920
• ###### SG3525.pdf
File size:
212.8 KB
Views:
249
Last edited: Oct 21, 2008
2. ### JUAN DELA CRUZ Thread Starter Active Member

May 27, 2008
121
0
Hi guys.. I've a trouble using SG3525 PWM controller IC..

Here are my questions..

1. What is the formula needed to calculate the Osc. frequency (not shown in the datasheet attached) ?

2. The frequency of the OUTPUT A (pin 11) & OUTPUT B (pin 11) is ½ the OSC. Output (pin 4) frequency ?

3. How can shutdown the SG3525? ...by holding SHUTDOWN (pin 10) "high" ?

Thank you ^_^

3. ### jpanhalt AAC Fanatic!

Jan 18, 2008
5,699
909
The datasheets do not show a simple formula to use. You have to go to Figures 1 and 2 from which oscillator timing for various resistances and capacitors can be determined graphically.

Pin 10 is shutdown; alternatives are described on page 2.

The datasheets for the Linear equivalent are the same.

John

4. ### JUAN DELA CRUZ Thread Starter Active Member

May 27, 2008
121
0
Thank you sir for th info... ^_^

One more thing Sir ....is the output frequency of pin 11 (OUTPUT A) & pin 14 (OUTPUT B) is ½ the OSC. OUTPUT (pin 4) ? Thank you

Apr 5, 2008
15,806
2,390
Hello,

The flip-flop will divide the clock frequency by 2.
The latch will deliver the pulse width.

Greetings,
Bertus

6. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
I don't have much to add (I was asked offline) but page 4 on your pdf file was interesting, looks like a practicle guide to use. Interesting chippie though, I could see trying to use it myself.

7. ### JUAN DELA CRUZ Thread Starter Active Member

May 27, 2008
121
0

Thank you for both of you.. ^_^

How about the exact/ right formula in calculating the output frequency ?

...I need a 50HZ/ 60Hz output 

Last edited: Oct 22, 2008

Apr 5, 2008
15,806
2,390
Hello,

In the datasheet the shutdown is done by connecting the shutdown-pin to Vref using a 2K resistor.
An other possibility is holding the softstart pin low via a 5 K resistor.

Greetings,
Bertus

9. ### JUAN DELA CRUZ Thread Starter Active Member

May 27, 2008
121
0
Thank you sir.. ^_^

1. How can I do that "automatically".. I mean to SHUTDOWN the SG3525 at a voltage for about approx. 11.8V ? Do I need an op-amp voltage comparator to detect the battery voltage (Sg3525 supply) variation from 13.8V to 11.8V ?

2. How about the exact/ right formula in calculating the output frequency ?
...I need a 50HZ/ 60Hz output ƒ @ OUTPUT A & B (pins 11/14)
..and a 100Hz / 120Hz @ OSC. Output (pin 4)

Last edited: Oct 22, 2008
10. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
The problem is there are no formula's. I don't know if this means it isn't predictable enough, or if it was just too complex to list. If the freq is X2 whatever you actually need then you'll probably need to figure out a charge time and discharge time from Figure 1 and Figure 2 that will give you the freq you need. In other words, use the charts to calculate values, and it won't be a symetrical square wave.

You'll probably have to breadboard one to pin the values down for real, and even then use variable parts (such as variable resistors).

The shutdown as I interpret it assumes a ground, a high voltage level shuts it down. I'm not sure of it though.

11. ### JUAN DELA CRUZ Thread Starter Active Member

May 27, 2008
121
0
How can I calculate the values of RT & CT needed to have a 100Hz/ 120Hz output from the Pin 4 (Osc. output) using these charts.

I need some assistance here 'coz I'm an undergraduate... didn't know the relation between the charge/ discharge time to the output frequency ^_^

Thank you

File size:
47.4 KB
Views:
481
File size:
48.9 KB
Views:
467
12. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
My best guess (and it is a guess) is to break it down. The charge time plus the discharge time should equal the period.

If you want 100Hz the period is 10ms, 10000 μs. Figure 1 shows using a .1μF cap this should be around 150KΩ to 180Ω. The discharge is going to be small no matter what, so go for 470Ω for 180μs.

10000μs + 180μs = 10180μs, which should work out to 98Hz.

Again, this is a guess. Never used the chippie.

13. ### JUAN DELA CRUZ Thread Starter Active Member

May 27, 2008
121
0
Thank you Mr. Bill ^_^

Hmmmm... I'm quite confuse. How about for 120Hz Osc. output?

Apr 5, 2008
15,806
2,390
Hello,

The cycletime = 1 / frequency.
For 120 Hz => 1/120 = 8.333 mSec = 8333 μSec.
So look in your graphics for the resistor value with a capacitor of 0.1 μF.

Greetings,
Bertus

15. ### JUAN DELA CRUZ Thread Starter Active Member

May 27, 2008
121
0
I've noticed that the discharge time is affected by RD (DISCHARGE resistor) shown in the second chart. That is, I think should be connected to CT & DISCHARGE pins. So, the discharge time and the charge time is independent to each other. That is, the charge time is controlled by RT & CT... and th DISCHARGE time is dependent to the value of RD (Discharge resistor) and CT. What do you think ?

Apr 5, 2008
15,806
2,390
Hello,

I think you should add the charge- and discharge- time to calculate the cycletime.
I think it is a sawtooth generator with seperate charge and discharge resistors.

Greetings,
Bertus

17. ### JUAN DELA CRUZ Thread Starter Active Member

May 27, 2008
121
0
I think the formula in calculating the Osc. output ƒ is 1.44 / RT (in ohm) x CT (in Farad)...
So, for OUTPUT A & B ƒ = 0.7 / RT x CT

I based it in a given tested oscillator output frequency in the SG3525A datasheet. Here is the portion of the SG3525A datasheet.

File size:
34.8 KB
Views:
452
18. ### JUAN DELA CRUZ Thread Starter Active Member

May 27, 2008
121
0
So, I think using this formula..

 = 1.44 / RTxCT

RT = 120kΩ, & CT = 0.1µF ..for 120Hz Oscillator output frequency

RD (discharge resistor) must have a low resistance for faster discharge..
I chose 100Ω (for approx. 25µS discharge time) using the same value of CT (0.1µF)

What do you think sir?

Apr 5, 2008
15,806
2,390
Hello,

I think your calculations are correct.
Keep in mind that a capacitor can have a tolerance up to 20 %.
The resistor also takes part in the tolerance question, depending on the type you use.

Greetings,
Bertus

20. ### SgtWookie Expert

Jul 17, 2007
22,183
1,729
I think that you can attempt to derive a formula all that you wish, but at best "your mileage will vary".

I looked at a datasheet I downloaded from ST Microelectronics. It doesn't have a formula either; two separate charts for charge and discharge times.

The recommended oscillator frequency range is from 100Hz to 400kHz. You are attempting to operate this IC at the extreme lower end of the recommended range.

I suspect that the charge/discharge response curves will tend to vary significantly over temperature when you're running the IC at nearly minimum or nearly maximum frequencies. You will have to verify operation by testing.

It wasn't designed to be a precision oscillator; merely oscillate in a ballpark range.

It sounds to me like you're trying to build a UPS or inverter. That isn't what the IC was designed for. It was designed for use as a building block in a DC-DC switching power supply.