# How to choose correct value of inductor in a circuit

I have a PCB where SIM900 is used. As SIM900 needs at least 2amps of current during transmission burst so I have used LM2576 for its power. Below is its schematic:

In the circuit, I am using 200uH inductor. During a call, SIM900 stops suddenly and inductor is blown. This happens for the first time so I replaced it and again I tried and the same thing happened. Inductor starts heating up and then it get blown. What value of inductor should I use with this circuit.?

• What's with all those little plus signs in the schematic? It looks like just a lazy screen shot from Eagle. If you don't care about your project, there's no reason anyone else should care about it either. Also, you can't expect everyone to know what a SIM900 or LM2567 is off the top of their heads. – Olin Lathrop Dec 27 '16 at 12:34
• Please provide a description (what kind of chip/function?) and reference links (datasheet ideally) for SIM900 and LM2576. As Olin said, we don't necessarily know what they are. – Laogeodritt Dec 27 '16 at 12:43
• Also, is your inductor rated for the peak current through it? (Not necessarily equal to the average/DC current which I infer is 2A.) – Laogeodritt Dec 27 '16 at 12:43
• Yeah we need the datasheet for the inductor, as well as the 2576 and the SIM900. – Brian Drummond Dec 27 '16 at 12:52
• @OlinLathrop When we give names to the net in eagle, it comes with the little plus sign. – S Andrew Dec 27 '16 at 12:54

It sounds like the inductor is not rated for the current. You want 2 A average out, so the inductor must be rated for more than that. Purchasing a component from a local market without a datasheet is no way to source parts.

The LM2567 is a rather old part requiring high inductance. I've only looked at the first page of the datasheet, but it says right up front that it only runs at 52 kHz fixed frequency. That requires a large inductor.

You need to actually read the datasheet. Right on page 1 it shows a Schottky diode. Your 1N4004 is totally inappropriate here. It's long reverse recovery time is probably significantly stressing the LM2576. That may actually be the real cause of the failure. The inductor appears to be the problem after the LM2567 output fails shorted to the input.

Especially when you don't really understand how something works, you need to follow the directions carefully.

You should assess if you really need a continuous 2A from your power supply.

GSM modules may take a peak current of up to 2A when they transmit, but the transmission only lasts 577us, and it only transmits every 4.615ms. That gives an average current of 2A/8, or 250mA. Also, the 2A figure is a worse case. You will only see 2A when you are in an area of poor GSM signal strength, and using a poor or badly matched antenna.

Usually a power supply rated for a continuous 500mA is adequate, but you need to ensure that the voltage on VBAT doesn't droop during the transmission. According to the datasheet you are allowed a 0.3V droop, so using a 1000uF low ESR capacitor connected to VBAT close to the GSM module should hold up VBAT during the transmission. I often use 2x or 3x 470uF low ESR tantalum capacitors here.