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I am using LM3524D IC configured like Figure 8:

Figure 8

I am supposed to get a 10V output with a 5V input using 20kHz Fosc. I am pretty sure about my connections and the values but I can't seem to reach the 10V output it only gives me 4.7V. I used TIP48 transistors and 1N4001 diodes. Do you have any ideas on how to troubleshoot this?

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Any reason why you don't use an integrated boost converter? It would save you the troubles of selecting the right transistor. Many switchers only need the coil, diode and capacitor as external components. – stevenvh Aug 29 '12 at 15:45
my prof was specific with the LM3524d and the configuration. so yeah, i am stuck with that – Dennis Aug 29 '12 at 15:49
Aha, then selecting the right transistor was probably part of the assignment! :-) Nice schematic by the guys at National: the transistors don't even have a refdes, let alone a type specification. :-( – stevenvh Aug 29 '12 at 15:54
What value did you use for R1? It's possible that you're simply not driving the switching transistor hard enough. – Dave Tweed Aug 29 '12 at 16:03
i used 220ohms for R1. – Dennis Aug 29 '12 at 16:06
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2 Answers

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Both the TIP48 and the 1N4001 are poor choices for a switcher: they're not fast enough. Switching has to occur fast if you want the coil's voltage to swing high when you cut off the current. A Schottky diode is better than the 1N4001, first because Schottky's don't have a reverse recovery time, and also they have a lower voltage drop, which improves efficiency.
The TIP48 has a transition frequency of only 10 MHz, what you need there is a switching transistor.

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can you suggest what transistor should i use? i have also tried IRF530 transistors i did not work either. – Dennis Aug 29 '12 at 15:41
@Dennis - Hey, don't get impatient, I'm looking for one! :-) – stevenvh Aug 29 '12 at 15:43
ok noted. i also tried to increase the Fosc up to 30kHz, but still no luck. i was hoping that at least i might be able to get an increased voltage. do you have any idea why it just decreased the voltage instead of increasing it? i have also tried replacing the IC itlself – Dennis Aug 29 '12 at 15:57
@Dennis - 20 kHz and 30 kHz are exactly the same. Strange that the IRF530 didn't do anything. The 5 V may be a bit at the edge, but it should do something. Do you have oscilloscope pictures for both IRF530 and TIP48? I'm particularly interested in the voltage at D1's cathode and D2's anode. – stevenvh Aug 29 '12 at 16:03
i don't not have the pictures. but from what i remembered it was a pulse ranging from 0 to 1.7v i am not pretty sure. how do i exactly get the value for R1 – Dennis Aug 29 '12 at 16:28
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up vote 1 down vote

First, fix your switching compoments as Steven has recommended. The boost diode needs to be ultrafast, preferably with soft recovery characteristics.

I would also connect the SD pin to ground instead of leaving it floating, to prevent spurious shutdowns of the IC.

Second, check your reference voltage. \$ V_R \$ with respect to GND should be 5V, and \$V_{NI}\$ will obviously be half of that if you're following Figure 8.

Third, check that your feedback divider (\$R_F\$ and the \$5k\Omega\$ resistor going to \$V_{INV}\$) are sized to give you \$V_{NI}\$ when the output is 10V.

Next, using a scope, check that your desired switching frequency can been seen at \$C_T\$ or at OSC.

Next, see if there's any duty cycle being generated at CB.

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ok noted. thanks. Last time i checked, the frequency at OSC was right. but i am not pretty sure about the value of R1 for the second transistor, i used 220ohms.i am not really sure how my partner arrived and gave me that value. how do i exactly get the right value for R1? – Dennis Aug 29 '12 at 16:25

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