VOLTAGE REGULATORS

Types of Voltage Regulators• Zener Diode Regulators• Series Transistor Regulators• Low Dropout (LDO) Regulators• Packaged Regulators

Headroom and Dropout Voltage• Headroom

• The difference in voltage between the input voltage and the output voltage of the regulator.

• Dropout Voltage• The minimum allowable headroom. When the difference between

the input and output voltage is less than the dropout voltage, the output voltage is no longer regulated or pinned to a specific voltage.

LDOs• A typical dropout voltage is 2V. LDOs operate with very

small headrooms, on the order of 0.2-0.5 V.• LDOs tend to have higher output resistances than non-LDO

regulators.• LDOs have less stable operating windows and may start to oscillate.

• To reduce the chance of oscillation, LDOs should have a capacitor connected across the output. The value and type of capacitor should be specified in the datasheet.

• LDOs are fabricated using pMOSFET.• This reduces current consumption in steady states as the gate

current is zero.• VDS can be less than VCE(sat) when the MOSFET is in the triode/non-

saturation region. This allows the dropout voltage to be very small.• RDSon of the MOSFET can be made very small by adjusting VGS, again

lowering the dropout voltage of the regulator.

Electronic Design Project• You have plenty of headroom, given that a 9V battery will

be used and the ICs need at most 5V.• Everyone will use a regulator, part L78S05C. Read the datasheet

to determine what extra components are needed to properly operate the regulator.• You should be able to answer the following questions about the

regulator:• What capacitor is required on the output?• What range of input voltages should be used?• What happens if the component is connected backward (input is the output

and visa versa)?• What is the dropout voltage at 100 mA?• What happens if the input voltage drops too low?• What is the maximum output current?• How much power is dissipated?• What is meant by the term “ripple rejection”?

HEAT SINKS

Temperature is a killer• The properties of all materials change with temperature.

The characteristics of semiconductors are very temperature sensitive.• This will be discussed further in Electronic Devices in Year 3.

• The temperature of a semiconductor IC will change as the power dissipated in the component changes.• To keep semiconductor ICs operating properly, heatsinks are

placed on the packages. In addition, active cooling (fans, heat pipes, water cooling) may be used, though passive cooling (natural convection) is preferred for increased reliability and reduced power consumption.

Is a Heatsink Needed for the Regulator

• Headroom • 9V – 5V = 4V

• Current• Quiescent Current: 8 mA• Current under load: 100 mA

• Maximum power dissipation• 4V(100 mA) = 0.4 W of power

Junction Temperature• This is the temperature of the semiconductor component inside of the

package. • It must stay below the maximum rated temperature of the component for

the component to work properly and have reasonable lifetime.• Note that some of the operating parameters change with temperature so it

may be necessary to stay below the maximum rated temperature for a particular application.

Calculation of Junction Temperature• Ambient: Temperature of the environment around the IC• Case: Temperature of the IC package• Thermal resistance: A measure of how easy heat can

flow from the semiconductor out to the case/ambient.

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Electronic Design Project• Luckily, there is no need to use a heatsink on the

regulator for this project as the increase in temperature of the regulator isn’t enough to cause sufficient changes in the operation of the regulator to affect this application.• There is an application note posted that provides information on

how to design (select) a heatsink if one is needed.