LM7800 SERIES

LM7800 SERIES

LM7800 SERIES

FIXED REGULATOR

• Functional Block diagram

• Theory

LM7800 SERIES

Three terminal voltage regulator

78XX series XX=05,06,07,08,10,12,15,18,24

7805 - OUTPUT - +5 V

7806 - OUTPUT - +6 V

7808 - OUTPUT - +8 V

7824 - OUTPUT - +24 V

FUNCTIONAL BLOCK DIAGRAM

Built-in reference voltage V_ref

_{Drives the non-inverting input of the amplifier}

_{Voltage Divider}

_{Samples the output voltage and returns the feedback voltage to the inverting input of a high gain amplifier}

_{Output Voltage - R1'+R2'/R1' (Vref)}

Reference voltage is equivalent to the zener voltage

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 15 V) in the 78XX series.

The tolerance of the output voltage is ±4 percent.

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

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.

FIXED REGULATOR

Pin 1 - input,

Pin 2 - output

Pin 3 - ground.

Output voltage of +5 V ; Maximum load current over 1 A.

Load regulation - 10 mV for a load current between 5 mA and 1.5 A.

Line regulation is 3 mV for an input voltage of 7 to 25 V.

Ripple rejection of 80 dB, which means that it will reduce the input ripple by a factor of 10,000.

Output resistance - 0.01 W,

LM7805-Very stiff voltage source to all loads within its current rating.

When an IC is more than 6 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. 8.9(b)].

To improve the transient response of the regulated output voltage, a bypass capacitor C2 is sometimes used on Pin 2.

Typical values for either bypass capacitor are from 0.1 to 1 mF.

The data sheet of the 78XX series suggests 0.22 mF for the input capacitor and 0.1 mF for the output capacitor.

Any regulator in the 78XX series has a drop-out 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.