There is a wide variety of linear voltage regulator ICs with pin counts from 3 to 14. All are series regulators because the series regulator is more efficient than the shunt regulator.
Some IC regulators are used in special applications in which external resistors can set the current limiting, the output voltage, and so on. By far, the most widely used IC regulators are those with only three pins: one for the unregulated input voltage, one for the regulated output voltage, and one for ground.
Available in plastic or metal packages, the three-terminal regulators have become extremely popular because they are inexpensive and easy to use. Aside from two optional bypass capacitors, three-terminal IC voltage regulators require no external components.
Types of Voltage Regulator ICs
Most IC voltage regulators have one of these types of output voltage: fixed positive, fixed negative, or adjustable.
IC regulators with fixed positive or negative outputs are factory-trimmed to get different fixed voltages with magnitudes from about 5 to 24 V.
IC regulators with an adjustable output can vary the regulated output voltage from less than 2 to more than 40 V.
IC regulators are also classified as standard, low-power, and low dropout.
Standard IC regulators are designed for straightforward and noncritical applications. With heat sinks, a standard IC regulator can have a load current of more than 1 A.
If load currents up to 100 mA are adequate, low-power IC regulators are available in TO-92 packages, the same size used for small-signal transistors like the 2N3904. Since these regulators do not require heat sinking, they are convenient and easy to use.
The dropout voltage of an IC regulator is defined as the minimum headroom voltage needed for regulation. For instance, standard IC regulators have a dropout voltage of 2 to 3 V. This means that the input voltage has to be at least 2 to 3 V greater than the regulated output voltage for the chip to regulate to specifications.
In applications in which 2 to 3 V of headroom is not available, low dropout IC regulators can be used. These regulators have typical dropout voltages of 0.15 V for a load current of 100 mA and 0.7 V for a load current of 1 A.
On-Card Regulation versus Single-Point Regulation
With single-point regulation, we need to build a power supply with a large voltage regulator and then distribute the regulated voltage to all the different cards (printed-circuit boards) in the system.
This creates problems. To begin with, the single regulator has to provide a large load current equal to the sum of all the card currents. Second, noise or other electromagnetic interference (EMI) can be induced on the connecting wires between the regulated power supply and the cards.
Because IC regulators are inexpensive, electronic systems that have many cards often use on-card regulation. This means that each card has its own three-terminal regulator to supply the voltage used by the components on that card.
By using on-card regulation, we can deliver an unregulated voltage from a power supply to each card and have a local IC regulator take care of regulating the voltage for its card. This eliminates the problems of the large load current and noise pickup associated with single-point regulation.
Load and Line Regulation Redefined
Manufacturers of fixed IC regulators prefer to specify the change in load voltage for a Regulated Power Supplies 979 range of load and line conditions. Here are definitions for load and line regulation used on the data sheets of fixed regulators:
- Load regulation = 5 ΔVout for a range of load current
- Line regulation = 5 ΔVout for a range of input voltage
For instance, the LM7815 is an IC regulator that produces a fixed positive output voltage of 15 V. The data sheet lists the typical load and line regulation as follows:
Load regulation = 12 mV for IL = 5 mA to 1.5 A
Line regulation = 4 mV for Vin = 17.5 V to 30 V
The load regulation will depend on the conditions of measurement. The foregoing load regulation is for TJ = 25°C and Vin = 23 V. Similarly, the foregoing line regulation is for TJ = 25°C and IL = 500 mA. In each case, the junction temperature of the device is 25°C.
LM78XX Series Voltage Regulator ICs
The LM78XX series (where XX = 05, 06, 08, 10, 12, 15, 18, or 24) is typical of the three-terminal voltage regulators. The 7805 produces an output of +5 V, the 7806 produces +6 V, the 7808 produces +8 V, and so on, up to the 7824, which produces an output of +24 V.
Figure 1 shows the functional block diagram for the 78XX series. A built-in reference voltage Vref drives the noninverting input of an amplifier. The voltage regulation is similar to our earlier discussion.
A voltage divider consisting of R1’ and R2’ samples the output voltage and returns a feedback voltage to the inverting input of a high-gain amplifier. The output voltage is given by:
The primes attached to R1’ and R2’ indicate that these resistors are inside the IC itself rather than being external resistors. These resistors are factory-trimmed to get the different output voltages (5 to 24 V) in the 78XX series. The tolerance of the output voltage is ±4 percent.
The LM78XX includes a pass transistor that can handle 1 A of load current, provided that adequate heat sinking is used. Also included are thermal shutdown and current limiting.
Thermal shutdown means that the chip will shut itself off when the internal temperature becomes too high, around 175°C. This is a precaution against excessive power dissipation, which depends on the ambient temperature, type of heat sinking, and other variables. Because of thermal shutdown and current limiting, devices in the 78XX series are almost indestructible.
Figure 2a shows an LM7805 connected as a fixed voltage regulator. Pin 1 is the input, pin 3 is the output, and pin 2 is ground.
The LM7805 has an output voltage of 15 V and a maximum load current over 1 A. The typical load regulation is 10 mV for a load current between 5 mA and 1.5 A. The typical line regulation is 3 mV for an input voltage of 7 to 25 V.
It also has a ripple rejection of 80 dB, which means that it will reduce the input ripple by a factor of 10,000. With an output resistance of approximately 0.01 Ω, the LM7805 is a very stiff voltage source to all loads within its current rating.
When an IC is more than 6 inches from the filter capacitor of the unregulated power supply, the inductance of the connecting wire may produce oscillations inside the IC. This is why manufacturers recommend using a bypass capacitor C1 on pin 1 (Fig. 2b).
To improve the transient response of the regulated output voltage, a bypass capacitor C2 is sometimes used on pin 3. Typical values for either bypass capacitor are from 0.1 to 1 µF.
The data sheet of the 78XX series suggests using a 0.33 µF or larger tantalum, mylar, or other capacitor with low internal impedance at high frequencies for the input capacitor and 0.1 µF for the output capacitor.
Any regulator in the 78XX series has a dropout voltage of 2 to 3 V, depending on the output voltage. This means that the input voltage must be at least 2 to 3 V greater than the output voltage. Otherwise, the chip stops regulating.
Also, there is a maximum input voltage because of excessive power dissipation. For instance, the LM7805 will regulate over an input range of approximately 8 to 20 V. The data sheet for the 78XX series gives the minimum and maximum input voltages for the other preset output voltages.
LM79XX Series Voltage Regulator ICs
The LM79XX series is a group of negative voltage regulators with preset voltages of -5, -6, -8, -10, -12, -15, -18, or -24 V. For instance, an LM7905 produces a regulated output voltage of -5 V. At the other extreme, an LM7924 produces an output of -24 V.
With the LM79XX series, the load-current capability is over 1 A with adequate heat sinking. The LM79XX series is similar to the 78XX series and includes current limiting, thermal shutdown, and excellent ripple rejection.
Regulated Dual Supplies
By combining an LM78XX and an LM79XX, as shown in Fig. 3, we can regulate the output of a dual supply. The LM78XX regulates the positive output, and the LM79XX handles the negative output. The input capacitors prevent oscillations, and the output capacitors improve transient response.
The manufacturer’s data sheet recommends adding two diodes to ensure that both regulators can turn on under all operating conditions.
An alternative solution for dual supplies is to use a dual-tracking regulator. This is an IC that contains a positive and a negative regulator in a single IC package. When adjustable, this type of IC can vary the dual supplies with a single variable resistor.
Adjustable Voltage Regulator ICs
A number of IC regulators (LM317, LM337, LM338, and LM350) are adjustable. These have maximum load currents from 1.5 to 5 A.
For instance, the LM317 is a three-terminal positive voltage regulator that can supply 1.5 A of load current over an adjustable output range of 1.25 to 37 V. The ripple rejection is 80 dB. This means that the input ripple is 10,000 smaller at the output of the IC regulator.
Again, manufacturers redefine the load and line regulation to suit the characteristics of the IC regulator. Here are definitions for load and line regulation used on the data sheets of adjustable regulators:
- Load regulation = Percent change in Vout for a range in load current
- Line regulation = Percent change in Vout per volt of input change
For instance, the data sheet of an LM317 lists these typical load and line regulations:
- Load regulation = 0.3% for IL=5 10 mA to 1.5 A
- Line regulation = 0.02% per volt
Since the output voltage is adjustable between 1.25 and 37 V, it makes sense to specify the load regulation as a percent.
For instance, if the regulated voltage is adjusted to 10 V, the foregoing load regulation means that the output voltage will remain within 0.3 percent of 10 V (or 30 mV) when the load current changes from 10 mA to 1.5 A.
The line regulation is 0.02 percent per volt. This means that the output voltage changes only 0.02 percent for each volt of input change. If the regulated output is set at 10 V and the input voltage increases by 3 V, the output voltage will increase by 0.06 percent, equivalent to 60 mV.
Figure 4 shows an unregulated supply driving an LM317 circuit. The data sheet of an LM317 gives this formula for output voltage:
In this equation, Vref has a value of 1.25 V and IADJ has a typical value of 50 A.
In Fig. 4, IADJ is the current flowing through the middle pin (the one between the input and the output pins). Because this current can change with temperature, load current, and other factors, a designer usually makes the first term in above equation much greater than the second. This is why we can use the following equation for all preliminary analyses of an LM317:
The ripple rejection of an IC voltage regulator is high, from about 65 to 80 dB. This is a tremendous advantage because it means that we do not have to use bulky LC filters in the power supply to minimize the ripple. All we need is a capacitor-input filter that reduces the peak-to-peak ripple to about 10 percent of the unregulated voltage out of the power supply.
For instance, the LM7805 has a typical ripple rejection of 80 dB. If a bridge rectifier and a capacitor-input filter produce an unregulated output voltage of 10 V with a peak-to-peak ripple of 1 V, we can use an LM7805 to produce a regulated output voltage of 5 V with a peak-to-peak ripple of only 0.1 mV.
Eliminating bulky LC filters in an unregulated power supply is a bonus that comes with IC voltage regulator.
Parameters of Voltage Regulator ICs
Following Table lists some widely used IC regulators.
The first group, the LM78XX series, is for fixed positive output voltages from 5 to 24 V. With heat sinking, these regulators can produce a load current up to 1.5 A. Load regulation is between 10 and 12 mV.
Line regulation is between 3 and 18 mV. Ripple rejection is best at the lowest voltage (80 dB) and worst at the highest voltage (66 dB). The dropout voltage is 2 V for the entire series. Output resistance increases from 8 to 28 mV between the lowest and highest output voltages.
The LM78L05 and LM78L12 are low-power versions of their standard counterparts, the LM7805 and LM7812. These low-power IC regulators are available in TO-92 packages, which do not require heat sinking.
As shown in above Table, the LM78L05 and LM78L12 can produce load currents up to 100 mA.
The LM2931 is included as an example of a low-dropout regulator. This adjustable regulator can produce output voltages between 3 and 24 V with a load current up to 100 mA. Notice that the dropout voltage is only 0.3 V, which means that the input voltage need be only 0.3 V greater than the regulated output voltage.
The LM7905, LM7912, and LM7915 are widely used negative regulators. Their parameters are similar to those of their LM78XX counterparts. The LM317 and LM337 are adjustable positive and negative regulators that can deliver load currents up to 1.5 A.
Finally, the LM338 is an adjustable positive regulator that can produce a load voltage between 1.2 and 32 V with a load current up to 5 A.
All the regulators listed in Summary Table 22-1 have thermal shutdown. This means that the regulator will cut off the pass transistor and shut down the operation if the chip temperature becomes too high.
When the device cools off, it will attempt to restart. If whatever caused the excessive temperature has been removed, the regulator will function normally.
If not, it will shut down again. Thermal shutdown is an advantage that monolithic regulators offer for safe operation.