Chapter 1

Design Lm2596 is a multipurpose IC we are using it as a buck converter. Here is the circuit

diagram.

1.1 Circuit Diagram

Figure 1.1: Buck converter circuit

1.2 Design The following is the design approach.

1.2.1 Variable Resistor

For adjustable output voltage levels, we have connected a variable resistor of 5k at the feedback pin. This pin is connected to internal error amplifier which controls the output voltage, the other pin is at Vref=1.23V. So, when the voltage across the feedback pin gets equal to Vref output is set to a certain fixed voltage depending upon the resistance ration (R2/R1) of variable resistor.

���� = ���� �1 +��

���………………………………………………… . . (1.1)

R1 + R2 = 5k ………………………………………………………………….… (1.2)

Solving these two equations

3.3 V : R1=1.88k , R2=3.12k 5 V : R1=1.25k , R2=3.75k 12 V : R1=0.50k , R2=4.50k

1.2.2 High frequency Diode

Diode is present to provide the return path when switch is off. Since internal

oscillator is operating at 150 kHz so we need a high frequency diode. Currently

we are using 1N5822 shotkey diode (2A).

1.2.3 Input Capacitor Input capacitor is there to smooth output voltage. Its value should be large. We

selected the value of 470uF. As we increase the value of Capacitor our input

voltage will be smoother.

1.2.4 Output Capacitor Output capacitor is present to filter the high frequency ripple in output Voltage. As

per data sheet of LM2596

∆���� = ∆� ∗ ��� ∗ � ∗ ����…………………… ..(1.3)

∆���� = 5��∆� = 250����� ≤ 0.02 Capacitor with high voltage and

capacitance has low ESR value so selecting 470uF (50V).

1.2.5 Inductor Inductor is used in combination of output capacitor to filter the square pulse

output of switch to DC. Since frequency is very high (150 kHz) we assume the

inductor current to be linear.

��� = ∆� ∗�

���− ���� − ��

Where Vs=1.16V (Saturation voltage)

���� =∆� ∗ �

�� + ����

Where Vd = 0.5V

1

��= ��� + ����

After Solving

� = (��� − ���� − ����) ∗�� + ����

�� ∗ ∆� ∗ (��� − ���� − ����)

���� = 12�∆� = 250��� ≥ 167��

1.3 Modes of Operation LM2596 have two modes of operations.

1.3.1 Continuous mode In this mode the current through inductor is continuous. The switch output in this

mode is simple pulse shaped. Usually large load current exist in this mode. The

design values are same as above.

1.3.2 Discontinuous mode In this mode the current through inductor drops to zero for some time. We can

bring the LM2596 in discontinuous mode by either inductor value below critical

value or decrease load current. When inductor current goes zero neither switch

nor diode conducts, hence a small amount of energy circulate between inductor

and diode. Stray capacitance causes ringing effect at output pin. Overall design

remains same but the value of inductor decreases to one third.

1.4 Under Voltage Lockout This is the feature which keeps the IC off until the input voltage reaches a certain

threshold level. LM2596 ON/OFF pin serves this purpose. It remains ON if Vin is

less than 1.6V. During OFF state it consumes only 80 uA.