IC 723 is a general-purpose adjustable output voltage regulator that is used in positive or negative output power supplies as series, shunt and switching regulator.
Figure below shows the internal schematic arrangement of IC 723. As we can see from the figure, it is a series-pass linear regulator and comprises a temperature compensated reference, an error amplifier, a series-pass transistor and a current limiter with access to remote shutdown.
Internal schematic arrangement of IC 723
Figure below shows an IC 723 based circuit for building low positive output (2-7V). Recommended value of R3 is (R1 × R2)/(R1 + R2)] and RSC = 0.6/ISC, where ISC is short-circuit limiting value.
Regulated output voltage is given by VREF × [R2/(R1 + R2)].
Low positive output voltage regulator using IC 723
Figure below shows an IC 723 based circuit for building high positive output (7-37V). Recommended value of R3 is (R1 × R2)/(R1 + R2)] and RSC = 0.6/ISC, where ISC is short-circuit limiting value.
Output voltage is given by VREF × [(R1 + R2)/R2].
High positive output voltage regulator using IC 723
Popular fixed positive output voltage three- terminal regulators include LM/MC 78XX-series and LM 140XX/340XX-series.
. LM117/217/317 is a common adjustable positive output voltage regulator type number. It is available in current ratings of 500 mA, 1000 mA and 1500 mA. The output voltage is adjustable from 1.2 V to 37 V. In the high-voltage version of this series of regulators, designated as LM 117HV/217HV/317HV, the output voltage is adjustable from 1.2V to 57 V.
Popular fixed negative output voltage types include LM/MC 79XX-series and LM 120XX/320XX-series three-terminal regulators.
LM137/237/337 is a common adjustable negative output voltage regulator type number. It is available in current ratings of 500 mA, 1000 mA and 1500 mA. The output voltage is adjustable from −1.2 V to −37 V. In the high-voltage version of this series of regulators, designated as LM 117HV/217HV/317HV, the output voltage is adjustable from −1.2 V to −47 V.
A two-digit number in place of ‘XX’ indicates the regulated output voltage.
Dropout voltage is an important specification of three-terminal regulators. It is the minimum unregulated input to regulated output differential voltage required for the regulator to produce the intended regulated output voltage. It is in the range of 1.5 V to 3 V and is lower for regulators with lower load current delivery capability and lower regulated output voltage value. For example, a 5V regulator has a dropout voltage specification of 2V against 3V for a 24V regulator for the same current delivery capability. Also, a 100mA output regulator has a drop voltage specification of 1.7V against 3V for 1500 mA regulator for the same regulated output voltage.
Figure below shows the basic application circuit of LM/MC 78XX-series three-terminal regulators. Here, C1 and C2 are decoupling capacitors. C1 is generally used when the regulator is located far from the power supply filter.
Typically, a 0.22 ⋃F ceramic disc capacitor is used for C1. Capacitor C2 is typically a 0.1 F ceramic disc capacitor.
Basic application circuits using three-terminal regulators
In this case the regulated output voltage in that case would be greater than the expected value by a quantum equal to the voltage applied to the common terminal.
Figure below shows the application circuit of a fixed output three-terminal regulator as a constant current source.
Three-terminal regulator as a constant current source
The load current in this case is given by (VREG/R + IQ), where IQ is the quiescent current.
Figure below shows the application of LM 117/217/317 as an adjustable regulator. Here C1 and C2 are decoupling capacitors; CADJ provides ripple rejection. CADJ of 10 ⋃F provides typically 80 dB rejection. The output voltage is given by
Or
VREF and IADJ are typically 1.25 V and 100 ⋃A, respectively.
Basic application circuit using LM 117/217/317
Figure below shows the application of LM 137/237/337 as an adjustable regulator.
Basic application circuit using LM 137/237/337
Here C1 and C2 are decoupling capacitors; CADJ provides ripple rejection. CADJ of 10 ⋃F provides typically 80 dB rejection. The output voltage is given by
Here VREF and IADJ are typically −1.25 V and 100 ⊔A, respectively.
The power dissipated in the regulator is the product of load current and input–output differential voltage. As the power dissipation increases beyond the rated value, the internal series-pass transistor becomes non-conducting, thus allowing the device to cool down. This mode is referred to as the thermal shutdown mode.
Figure below shows the typical circuit where an external transistor is used to boost the load current delivery capability of a positive output regulator
Use of external transistor to boost current delivery capability of a positive output regulator
Figure below shows the typical circuit where an external transistor is used to boost the load current delivery capability of a negative output regulator.
Use of external transistor to boost current delivery capability of a negative output regulator
In both the circuits, as long as VBE (Q1) remains below its cut-in voltage, the regulator functions in its usual manner as if there were no external transistor. As the VBE (Q1) attains the cut-in voltage due to an increasing load current, Q1 conducts and bypasses part of load current through it
The magnitude of load current allowed to go through the regulator equals VBE (cut-in)/R. Rest of the current passes through the external transistor.