linear integrated circuits-specialised IC and applications-btech semester five

1. By Anita a thattil S5 ec alpha Rajagiri school of engineering,kakkanad IC VOLTAGE REGULATORS

2. INTRODUCTION Integrated ICs with improved capabilities are appearing in an ever increasing numbers. Often ,the use of specialized IC produces a simpler and more accurate circuits. Examples: MF5(National ‘s switched capacitor filter), SE/NE 565(Signetics’Phase locked loop-PLL) , timer SE/NE 555 etc Applications of specialized ICs: *universal filters *timers *Phase locked loop(PLL) *power amplifiers *voltage regulators *switching regulators *voltage references

3. Topics to be dealt 78XX series 79XX series 317 variable regulators 1723 switching regulators

6. Multi-pin type, e.g. LM723C

7. 3-pin type, e.g. 78/79XX

8. Multi-pin regulators are less popular but they provide the greatest flexibility and produce the highest quality voltage regulation

10. Adjustable output voltage regulators: positive (LM317) and negative(LM337)

11. Switching regulators:motorola ‘s MC1723

13. Rectifier:converts AC to DC, but the DC output is varying

14. Filter: smooths the DC from varying greatly to a small ripple.

17. Load regulation: defined as the change in output voltage for a change in load current.

18. Temperature stability or average temperature coefficient of output voltage(TCVO): defined as the change inoutput voltage per unit change intemperature and expressed in millivolts per degree celsius.

19. Ripple rejection: is the measure of regulator ‘s ability to reject ripple voltages.it is usually expressed in decibels.

20. Quiescent current Iq : current that flows into the regulator and does not include current drawn by load or internal resistor networks.

22. Basic building block of +ve regulators

26. 3 terminal positive voltage regulator with seven voltage options

27. High Output Current - typically 1.5A

28. Short circuit current limit - 750mA at 5v

29. Internal thermal overload protection

30. Low quiescent current - 6mA

31. Max input voltage = 35v

33. These IC's are similar to the 78 series, but operating on negative voltage, and providing a regulated negative output voltage.

34. The capacitors connected at the input and output sides are used to provide additional filter circuits of regulator circuits.

36. They are available in same seven voltage options with two extra voltage options,-2V and -5.2V

38. Advantages of adjustable voltage regulators:

39. * improved system performance

40. * improved overload protection

41. *improved system reliability

43. Circuit of LM317 Resistors Rx and R2 set the output to any desired voltage over the adjustment range (1.2 to 57 V) When configured as shown in figure LM317 develops a nominal voltage of 1.25V referred to as reference voltage Vrefbetween output and adjustment terminal. This reference voltage is impressed across resistorR1,since voltage is constant current I is also constant Since R1 sets current I its called current set or program resistor

44. Referring to figure,the output voltage Vo is Vo = R1 + R2 (I1 +Iadj)………….(1) Where I1= Vref/R1……….(2) R1 =current set resistor R2 =output set resistor Iadj= adjustment pin current Substituting (2) in (1) we get :Vout = VREF [1+ R2/R1] + IADJR2 where VREF = 1.25 V, the reference voltage between the output and adjustment terminals the current IADJ is very small (100 micro Amperes) and constant. So the voltage drop across R2 due to IADJ is also very small and can be neglected so that equation can be written as Vout = 1.25 [1+ R2/R1] The current set resistor Rx is usually 240 Ohms, and to achieve good load regulation it should be tied directly to the output of the regulator rather than near the load. The load regulation is 0.1 percent while the line regulation is 0.01 percent per volt.

45. LM317 with capacitors,protection diodes No capacitors are needed unless LM317 is situated far from the power supply filter capacitors in which case a bypass capacitor Ci is used. Output capacitor Co can be added to improve the transient response. When external capacitors are used protection diodes are also used to prevent capacitors from discharging through low current points into the regulator.

47. They are negative adjustable voltage regulators

49. Voltage source Vin

50. Switch S1

51. Pulse generator Vpulse

54. Since this switching mechanism allows the conversion similar to transformers, the switching regulator is referred to as dc transformer.

55. Vo is expressed as ;

56. Vo = ton*Vin

57. T

58. Case 1: if time period T is constant,V0 is directly proportional to the on time ton , for a given value of Vin. This method of changing the output voltage by varying ton is called pulse width modulation

62. When the transistor is off(switch is open) , no current flows,therefore no power dissapation.

63. When the transistor is on,(switch is closed), high current flows but Vce becomes low and therefore power dissapation is less.

64. Basic circuit of switching regulator

65. Bridge rectifier-switch-transformer-rectifier

66. Bridge rectifier converts ac input signal to unregulated dc

67. Switch : closed implies currrent is allowed to flow

68. open implies no current is allowed to flow

69. Transformer: the output of the switching network is coupled to the rectifier through the transformer

74. Delivers load current of about 150mA without an external pass transistor.

75. Output voltage range 3Vdc to 37 Vdc

76. .01%line regulation

78. It is a general purpose regulator and can be used as fixed ,variable and switching regulator.

79. The regulator requires an external transistor and a 1mh choke.

80. To minimize its power dissipation during switching, external transistor used must be switching power transistor.

81. The 1 mH choke smooths out current pulses to the load