LMS8117A Low Dropout Linear Regulator

A voltage regulator is an important device used in electronic and electrical applications. These are used for providing a regulated voltage supply. Both positive and negative voltage regulators are available. These circuits make use of differential amplifiers to provide a regulated output voltage from fluctuating input voltage. LMS8117A is a series of such low dropout voltage regulators.

Dropout value of voltage regulators is the minimum voltage difference between input and output voltage that the device can regulate. Low dropout voltage regulators are known as LDO. The dropout voltage of such regulators is usually in the range of mV. Dropout value of LMS8117A is 1.2V.

What is LMS8117A?

LMS8117A is a 1A low dropout linear voltage regulator. To get an output voltage accuracy of 1% the device contains a Zener trimmed bandgap reference. LMS8117A is a positive voltage regulator. It is versatile in applications. This device has a wide temperature range and tight line and load regulation.

Block Diagram

Protection Circuits

To protect the device against the overload and damage from operating in excessive heat, current limiting and thermal protection circuit is provided to the device.

Programmable Feedback

The easy output voltage or output current programming in adjustable versions can be done using ADJUST pin. A single resistor with a resistance of 1.25Ω and power rating greater than 1.252 watts must be used for current regulation.

There are four functional modes for the device. They are Normal operation, operation with the low input voltage, operation with light loads, and operation in self-protection.

Normal Operation

In adjustable version to provide output regulation, the OUTPUT pin sources current necessary to make the output voltage 1.25V greater than the reference voltage. For the fixed version, the OUTPUT pin sources current, to provide output regulation, the output voltage is made greater than the ground voltage.

Operation with Low Input Voltage

For proper working of LMS8117A, the minimum voltage difference between input and output voltage should be 1.25V. When the value is less than 1.25V, the output voltage will be equal to the input voltage and dropout voltage and no regulation will be given.

Operation in Self-protection

The device gets shut down when an overload occurs. This is done to reduce the output current to prevent the device from damage. The device will reset automatically from overload. Until the overload is removed the output will be reduced.

Circuit Diagram of LMS8117A

LMS8117A requires less number of components while used in applications. While using adjustable version it requires two resistors for forming voltage divider circuit and an output capacitor for load regulation.

If the regulator is situated far away from power supply filter a 10-µF tantalum capacitor is required at the input. To improve PSRR a bypass capacitor can be used across R2. The values of these capacitors are the same for almost all applications.

Output Capacitor

The minimum value of the output capacitor for LMS8117A is 10µF. Load stability and transient response are merely increased if the value of the capacitor is increased. The ESR of the output capacitor should be in the range from 0.5Ω to 5Ω.

Load Regulation

Degradation of Load regulation occurs when the load voltage is less than the output voltage by the sum of voltage drops along with the line resistances. To prevent this load must be tied directly to the OUTPUT pin on the positive side and to the ground pin on the negative side.

Protection Diode

There is an internal diode between INPUT and OUTPUT pin of LMS8117A that can withstand microsecond surge current of 10A to 20A. But when extremely large output capacitor is used and input is instantaneously shorted to ground, the regulator may get damaged. So, to prevent this an external diode is used between INPUT and OUTPUT pins.

Pin Configuration

The pin configuration of LMS8117A low dropout linear regulator is shown below.

LMS8117A is available as a space-saving IC in the form of SOT-223 and T0-252 packages. Both are 3-pin packages. The Pin configuration of both of these packages is also similar.

Pin-1 acts as an adjustable pin for an adjustable output version (ADJUST). For fixed voltage versions this pin acts as a ground pin –GND.Pin-2 is the output voltage pin- OUTPUT of the regulator. It also acts as a TAB.

Pin -3 is the input voltage Pin of the regulator- INPUT. The input capacitor should be placed as close as possible to the input pin and the system ground.

Based on the maximum power dissipation and the maximum ambient temperature of the device, a heat sink can be required for proper operation.

The heat sink is not required if junction-to-ambient thermal resistance’s maximum allowable value is greater than or equal to 61.40C/W for SOT-223 package, or greater than or equal to 56.10C/W for TO-252 package.

Specifications

The specifications of LMS8117A are as follows-

• LMS8117A is a low-dropout positive voltage regulator.
• LMS8117A is available as 1.8V, 3.3V fixed output voltage versions.
• This regulator is also available in an adjustable voltage version.
• It has a dropout value of 1.2V at a load current of 1A.
• Recommended input voltage for all versions is 15V.
• For the adjustable version, the voltage can be set from 1.27V to 13.8V.
• Available as two types of packages – SOT-223 and TO- 252.
• It requires out capacitor for stability.
• Thermal shutdown circuit is provided to save the device from damage due to overheating.
• The device also has a current limiting circuit.
• The maximum line regulation of this regulator is 0.2%.
• This device gives a maximum load regulation of 0.4%.
• The maximum input voltage for this device is 20V.
• Power dissipation for LMS8117A is internally limited.
• LMS8117A has a maximum junction temperature of 1500C.
• Storage temperature range for this device is from -650C to 1500C.
• Maximum quiescent current for fixed voltage versions is 10 when the input voltage is less than 15V.
• The output capacitor is also used to improve transient response.
• LMS8117A has the same pinout as LM317.

Applications

Applications of LMS8117A low- dropout linear regulator are as follows-

• This device is used as a post regulator for switching DC-DC converters.
• LMS8117A is a series of High-efficiency linear regulators.
• Because of their smaller size, LMS8117A are used in portable battery powered instrumentations.
• LMS8117A is also applied in battery chargers.
• LMS8117A is applied in programmable output regulation as well as on-card regulation.
• This device can also function as a precision current regulator.
• Using a bypass capacitor, high ripple-rejections can be achieved.
• LMS8117A is also used as microprocessors supplies.

Alternative ICs

IC that can be used as an alternative to LMS8117A and TLV767 and LP38690. With the use of a small external circuit consisting of two resistors and a capacitor, the adjustable version of LMS8117A can be used for various applications.

Further information on various electrical characteristics under different operating conditions is available in the datasheet given by Texas Instrumentations. In which of your applications was LMS8117A helpful?

Image credit: Texas Instrumentation