hello Everyone,

i'm using TL494 for PWM of a buck DC-DC converter

its data sheet has an example of how to connect such a circuit , but some of the components aren't clearly explained ..

check the pictures and please tell me :

1) what is the function of the 47ohm and 150ohm resistors at the base of the transistor ?

2) how to calculate them ?

3) what is the function of the two 5.2kohm connected to pins 1 & 2 ? isn't their error amplifier has infinite input impedance ? so why the limiting ?

thanks in advance

 

The images you posted are so small that I can't read them.

Here is a search on Alldatasheet.com that brings up datasheets of TL494's from several manufacturers:
http://www.alldatasheet.com/view.jsp?Searchword=TL494

TI's datasheets are usually pretty good. OnSemi and Freescale are going to have the same datasheet, as they're offshoots from Motorola.

 

here are the pictures in a larger size

thanks for trying to help but i already have those datasheets

 

Well, the 47 Ohm resistor keeps the TIP32 shut off; it pulls the base of the PNP transistor to V+. The 150 Ohm resistor limits current through the base when it is pulled down towards ground by the connections to pins 8/11.

But you're on to something - those numbers just aren't right. The "47" resistor should be at least 470, perhaps 4.7k or even 47k. A 47 Ohm resistor there would cripple operation of the circuit, and would waste a LOT of power, along with blowing a lot of their IC's.

Since you have provided no other information than a part number and couple of schematics that you might've obtained from literally anywhere, I suggest that you contact the manufacturer of the particular IC that you purchased, and ask that one of their engineers explain it to you, and point out the obvious error that you discovered in the 47 Ohm resistor on the base.

But before you do that, see if they have provided an updated datasheet with corrections and clarifications. If they've done that, it's a moot point.

 

If you go to the TI web site and do a search on the TL494 you should get to this page.
At the bottom of the page there are links to all sorts of useful technical documents and applications notes.

 

I think the 47 ohm resistor is about the right value. The current through that resistor would be around 12-20mA with total drive current of around 70-250mA across input voltage range.

Considering that most of the power dissipated in switching occurs during turning-off, it is appropriate to sacrifice a little bit of the total drive current during the on state through that resistor. My usual rule of thumb for medium frequency BJT switching with this configuration, the turn-off base current is about 10 percent of the total drive current, leaving about 90 percent for the on state base current and increasing this appropriately with frequency.

As for the OP questions, the calculation of 150 ohm resistor is from the desired maximum collector current, the gain of transistor at that current, Vbe, the supply voltage and the voltage drop across the IC (pin 8 & 11 to ground) plus some safety margin.

Ib = Ic/hFE

Add around 25 percent for turn-off and safety margin:

Idrive = Ib + 0.25Ib

Rb = (Vsupply - Vbe - Vdrop) / Idrive

Calculating the turn-off resistor, assuming 10 percent from Idrive, whichever:

Roff = Vbe / (0.1*Idrive)

This is just a quick and rough calculation to get to a good starting point, the real test is seeing how fast the turn-off is compared to the turn-on with a scope and adjusting the Roff accordingly. Detailed calculation can be carried out from the base charge during saturation (on state) and Vbe curve by calculating how long it would be required to discharge the Qb through Roff with varying Vbe.

 

okay thanks a lot n9352527 i think i got it

still i dont understand the function of the 5.1k ohms at pin 1 and 2

 

They are input resistors for the error amplifier. The one on the negative input, in conjunction with the resistor on pin 3, sets the gain of the error amplifier and feeds the reference signal. The one on the positive input feeds in the error signal.

http://www.allaboutcircuits.com/vol_3/chpt_8/5.html

 

They are input resistors for the error amplifier. The one on the negative input, in conjunction with the resistor on pin 3, sets the gain of the error amplifier and feeds the reference signal. The one on the positive input feeds in the error signal.

http://www.allaboutcircuits.com/vol_3/chpt_8/5.html

thank you thingmaker3