In this article we will try to understand the construction of a 1.23 V to 50 V adjustable switching power supply circuit using the IC LM2576.
The LM2576 family of regulators is a monolithic integrated circuit that performs all of the active operations of a step-down (buck) switching regulator. It offers exceptional line and load stabilization and can handle loads upto 3 amps.
These ICs can be configured to generate 3.3 V, 5 V, 12 V, 15 V fixed output voltages. Additionally this chip can be also wired like a variable voltage power supply, with a maximum output range of 1.25 V to 50 V.
Remember, there are different versions of the LM2576 IC for generating the above mentioned specific fixed output voltages, and the adjustable output voltage.
Meaning, a 5 V version can be used for generating only a 5 V fixed voltage output, a 12 V version for generating a fixed 12 V output, and so on.
Likewise, for getting an adjustable output voltage, you will have to specifically select the adjustable (ADJ) version of the LM2576 regulator IC, and configure it according to the given circuit diagram.
Why the Adjustable Version of LM2576 is more Efficient
The adjustable version of the LM2576 is indicated by the letters ADJ on the device, as shown in the above figure.
The adjustable version of the LM2576 appears to be more efficient because of the following reasons:
This chip can be configured as an adjustable switching regulator, simply by configuring a potentiometer across its feedback pin.
Additionally, the adjustable version can be also used as fixed output voltage regulator by replacing the potentiometer with a resistive divider across its feedback pin.
Switching Regulator Vs Linear Regulator (What's the Difference?)
So, what's so special about using a LM2576 based switching regulator, instead of a linear regulator such as an LM338 based regulator?
The main advantage of using an LM2576 regulator is that, it uses a switching PWM across an inductive buck converter stage. The switching PWM across an inductor causes the output voltage regulation by controlling the back EMF from the inductor. This causes the output regulation to be very efficient, with minimum heat dissipation.
Since the heat dissipation is minimum, the power loss is minimum at the output. Meaning, in a switching regulator the output V x I is very near to the input V x I.
On the contrary, linear regulator ICs like LM338 or LM317 or L200 regulate their output voltage by dissipating a lot of heat through their body. The temperature dissipated by these ICs is dependent on the load current and the difference between the input voltage and the output voltage. As this difference increases the heat dissipation also increases. This makes linear regulators extremely inefficient, unless the output regulated voltage is nearly equal to the input voltage.
Functional Block Diagram
The following diagram shows the functional block diagram and the internal configuration of the IC LM2576. The diagram also indicates how the various pinouts of the IC needs to be configured with the external components to produce the intended regulated output voltages.
The above block diagram shows the basic set up configuration which can be used for all the fixed voltage version of the LM2576 IC.
The functions and designations of the IC LM2576 pinouts is explained in the following points.
Pin#1 (VIN): This is the supply input pin which is connected to the collector pin of the internal high-side transistor. This pin should be connected to the power supply and the CIN input bypass capacitors. Make sure to use the shortest possible link between the VIN pin, the high frequency bypass CIN and GND.
Pin#2 (Output): This is the internal power transistor's emitter pin, which is a switching node. We connect the cathode of the external diode and an inductor to this pin.
Pin#3 (Ground): This functions as the Ground pin. The connection reaching CIN should be kept as short as possible.
Pin#4 (Feedback): This pin performs as the Feedback sense input pin. It is to be linked to the junction of feedback divider resistors, to fix VOUT for the ADJ (adjustable) version. Alternatively, this pin could be also hooked up straight with the output capacitor for the fixed output voltage version IC.
Pin#5 (ON/OFF): This pinout works as the Enable input to the voltage regulator. A High on this pin causes the IC to switch OFF and a low on this pin allows the IC to remain switched ON. This pinout can be simply connected with the ground line to keep the regulator in the enabled mode. Never keep this pinout open or unconnected.
IC Tab: This terminal is supposed to be connected with the GND. Being the tab of the IC this must be screwed to a suitable heatsink for thermal dissipation.
How to Build an Adjustable LM2576 Switching Power Supply Circuit
- R1 = any resistor between 1 K an 4.7 K (1/4 watt 5%)
- R2 = 47 K Potentiometer
- Cin, C1= 100 uF/63 V Electrolytic
- Cout = 2200 uF/63 V Electrolytic
- D1 = 1N5822 Schottky Diode
- IC = LM2576HV-ADJ
- L1 = 150 uH Inductor 5 amp
- L2 = 20 uH 5 Amp
The above diagram shows a simple 1.2 V to 50 V switching power supply circuit using the LM2576HV-ADJ IC, which can produce a maximum output current of 3 amps.
The various switching parameters involved with the above circuit can be learned from the following points:
An unregulated 55 V DC input is applied across pin#1 which is the VIN pin of the IC and pin#3 which is the ground pin of the IC.
The capacitor Cin is installed close to the above pinouts to ensure effective ripple rejection across the input DC pins of the IC.
As soon the IC LM2576HV-ADJ is powered as explained above, its internal PWM oscillator becomes active.
The PWM oscillator internally starts generating a calculated amount of PWM. The duty cycle of the PWM depends on the feedback voltage applied to pin#4, via the resistive divider pot R2 and R1.
This calculated PWM is supplied to the external buck converter stage comprising of L1, D1 and Cout via the output pin#2 of the IC.
The L1, D1 and Cout appropriately respond to the PWM to produce an optimized DC output voltage, reduced to the desired level (between 1.2 V and 50 V).
It is important to know that the current will be 3 amps at the maximum 35 V or 50 V outputs. This means, for lower output voltages the current will be proportionately higher.
Pin#5 is the ON/OFF or the shutdown pinout of the IC LM2576HV-ADJ.
As long as this pinout has a potential of less than 1.2 V DC, the IC remains functional and active.
However, if the potential on pin#5 exceeds 1.4 V, the IC LM2576 goes into a shutdown mode. This causes the output voltage to instantly shut off.
Despite of superb output voltage and current regulation, there might be some ripple DC content at the output.
To counter or eliminate this ripple content, you can add the "optional output ripple filter" stage at the output of the circuit, as indicated in the circuit diagram.
Using the Adjustable Version to get Fixed Output Voltages
As discussed earlier, the adjustable version of LM2576 IC can be also configured to get fixed voltage outputs, simply by replacing the R2 pot with a fixed calculated resistor.
An example of this design can be witnessed in the following diagram:
R2 can be calculated using the following formula:
R2 = R1 ( VOUT / VREF - 1 )
where VREF = 1.23 V, R1 can be any value between 1 k and 5 k