Design power supply for SIM800L Core Board

Key topics : Powering SIM800L Module, SIM800L Core Board, Linear Regulator, Buck Converter, SMPS, Arduino

Introduction

As it seems most of the issues related to SIM800L core board is due to inappropriate power supply. If you want refer this SIM800L hardware design where manufacturer clearly specifies the suitable power supply for SIM800L module. 

Refer the power supply specifications 

• Voltage : 3.4 V to 4.4 V (4.0 V recommended, Some of the latest versions may work directly with 5V power supply)
• Current Rating : 2 A (To cater the demand at transmit bursts)

If you provide this voltage and observe the current through the Ammeter you will hardly notice a current of 2 A but few milli Amps. It will be using 2A at transmit bursts for less than a milli second at a time. Therefore it is really important to provide a power supply which can provide 2A with a very minimum voltage drop.

Most of the time issues related to power supply is because of people using linear regulators which cannot supply adequate power as well as not the suitable voltage. (Even in Arduino boards 5V power supply cannot provide more than 500 mA and 3.3V Power supply cannot provide more than 100 mA, Also they are not in the range of required voltage level) So directly connecting SIM800L module with Arduino will never fulfill the requirements. 

We'll see what data sheet recommends to power SIM800L module,

Manufacturer recommends to have the following arrangement as much as closer to the Vin pin of the SIM800L module. 

SIM800L Power Input

If you observe closely you can see that some of those capacitors are already included in the core board. However importance of those components are that,

• Zenor diode will be removing any voltage sure if present protecting the sensitive electronics
• 33pF and 10pF capacitors will remove any high frequency interferences
• 100uF and 1uF are also useful to make the input power supply smooth as well as to help with high current bursts during transmits. Also it is much more effective to use Tantalum capacitors due to low ESR (Equivalent Series Resistance) 

Since we cannot work with any of the commonly available power supplies or Arduino power outputs we'll see what are the other options we can have.

1) If you are using a 5 V Power supply manufacturer recommends to use a LDO (Linear Drop-Out) regulator which would be a great and efficient solution in such case. Here in this case they've recommended MIC29302A which can supply up to 3A with a 1% tolerance. 

MIC29302 Power Supply for SIM800L

Lets see the output voltage it is designed,

2) Since this seems little bit complicated we can easily achieve the same performance by using a diode with a forward bias voltage drop of 0.7V and a rating of 2A. (Consider forward bias voltage when selecting the diode)

Diode Power Supply SIM800L

3) If you are using a power supply above 5V using an LDO or another type of linear regulator would be very much inefficient. So the best option becomes using a Buck Converter (Switch Mode Power Supply) For such applications LM2596 based buck converters would be the ideal solution which is commonly available as well as less expensive.

LM2596 Adjustable Buck Converter Schematic

This is the schematic of most commonly available. Below image shows a commonly available commercial solution of this circuit.

Commercially available LM2596 Converter Module

However if you refer the above datasheet of SIM800L module, manufacturer recommends a slightly different setup with addition of another inductor and couple of capacitors which would deplete the amount of noise this produces. Normally buck converters tends to have a little bit high amount of noises compared to linear regulators. But this works fine with SIM800L module. Even though it is better to add a 100uF capacitor and a 100nF capacitor closer to the SIM800L module when using this.

4) Since most of the Li-Ion Cells are of 3.7 V -v4.2 V and it is in the recommended range we can directly use a Li-Ion cell to power up a SIM800L Module. However be careful when using small batteries as they may not be able to provide the adequate voltage at 2A current burst due to internal resistance. 

LM2596 Buck Converter Vs LM7805 Linear Voltage Regulator

Keywords : LM7805, LM2596, Linear Regulators, Buck Converters, Switch Mode Power Supply (SMPS)

Recap on 7805 Linear Voltage Regulator

We've had a previous discussion on Linear Voltage Regulators and Linear Drop Out Regulators. You can find the article here. Today we will discuss on the the difference between the 7805 Linear Regulator and the LM2596 Buck Converter.

IMG 01 : 7805 Schematic

IMG 02 : Circuit on a breadboard

IMG 03 : LM7805 output from an oscilloscope

Now lets have a look at the LM2596 based buck converter module and then we'll go for the comparison.

IMG 04 : LM2596 Buck Converter Schematic

IMG 05 : LM2596 Buck Converter Output from an Oscilloscope

Commercially available LM2596 Converter Module

Now we'll see the comparison between LM7805 and LM2596.

Comparison of Specifications

IMG 06: LM7805 and LM2596 Comparision

LM7805 is a Linear Regulator where LM2596 is a Switch Mode Power Supply (Buck Converter). The biggest disadvantage of LM7805 over LM2596 is the inefficiency. To understand this we'll do a simple calculation. We'll take an application of a 20V input and a 10 Ohms Load resistor.

IMG 06: LM7805 Efficiency Calculation

Then,
Total Power Supplied = 20 V x 0.5 A = 10 Watts  (Resistive Loads)

Resistive Load Power Consumption = 4.98 V x 0.5 A = 2.5 Watts

Efficiency

What happens to the rest of 7.5 Watts ? It is dissipated through the Linear Regulator. So higher the input Voltage and Current higher the power dissipation of regulator. Also, when power dissipation of the regulator increases, efficiency becomes lower. This is the biggest disadvantage of Linear Regulators. We'll be needing bigger heat-sinks when this power dissipation becomes higher. But in the case of Buck converters, they are very efficient and requires very minimum of additional heat sinks. Also when power consumption is critical (ex: battery powered applications) Buck converters would be really handy.

Compactness and Cost

However linear regulators are much more compact and cost effective compared to Buck converters. Therefore they are widely used when efficiency is not a big concern. Also when the input voltage is very close to output voltage we can use linear dropout voltage regulators which may be efficient as much as buck converters due to lower voltage difference from input to output.

Noise handling Capacity

See the Image 03 and Image 05 I've posted above. You will see that there is a small noise (voltage notch)  present in buck converter output. It is said that there could be even higher noise in buck converter and may cause issues for some sensitive micro-controller applications. So in that case Linear Regulator seems to be slightly better.