LP2957 Working and Applications

Voltage regulators such as LP2957 are used in circuits to get a constant output even when input is fluctuating. These devices usually contain Op-amps, which acts as a differential amplifier. So, to have the proper working of these devices, a minimum voltage difference between input and output voltage has to be maintained. This voltage difference is called Dropout value.

There are voltage regulators with different dropout values. A given voltage regulator can detect and correct the changes of input voltage only up to the value equal to its dropout level. Low dropout level devices can detect change even when input is almost equal to the output voltage. One of such devices is LP2957.

What is LP2957?

LP957 is a 5V low dropout voltage regulator, manufactured by Texas Instruments. It is a micropower voltage regulator. It has electronic shutdown, error flag and a very low quiescent current value of 150μA.

This device has a very low dropout voltage of 470mV at 250mA load current. Micropower applications usually use LP2957. It has a 5 pin configuration.

Here, for snap on or snap off operations, the output can be wired to eliminate transition voltage states where micro-operation may be unpredictable.

Block Diagram

LP2957 is a low dropout fixed output linear voltage regulator. Its output is fixed to 5V. Input given to this device can range from -20V to 30V. The internal circuit has three differential amplifiers.

These differential amplifiers are usually supplied with reference voltages to control the output voltages. The output generated is given as feedback to these amplifiers where the error in output voltage is calculated.

LP2957 block diagram
LP2957 block diagram

Based on the value of error, the output voltage is adjusted so that the difference between the reference voltage and the output voltage is zero. Being low dropout value regulator, LP2957 can detect the error of even 470mV and corrects the output voltage.

So, LP2957 is the best choice for micropower applications and in the applications where such a minor error in the output voltage cannot be tolerated.

How to use LP2957?

Lp2957 circuit
Lp2957 circuit

External Capacitor

To ensure stability, a 2.2μF and greater capacitor are required between the output pin and ground. This capacitor value can be increased without limit. Lower capacitance is used for stability at lower values of output current.

If there is more than 10 inches wire between input and battery a 1μF capacitor should be placed from the input pin to ground. This capacitance can be increased if the regulator is wired for snap-in/snap-out output and the source impedance is high.

Shutdown Input

When logic LOW is applied to shutdown pin, the signal will shut off the regulator output. If the input is driven from open-collector logic, a pull-up resistor must be connected from shutdown input to the regulator input.

Dropout Voltage

The minimum input-output voltage difference required for the output voltage to stay within100mV of the output voltage measured with 1V differential.

Heat Sink Requirement

A heat sink is chosen for the IC depending upon the maximum power dissipation and the maximum ambient temperature of application. If the regulator is powered from a transformer connected to the AC line, the maximum specified AC voltage and maximum load current must be considered.

When the value of power dissipation is 600C/W and higher, the regulator can be used without an external heat sink. If the value is below 600C/W, an external heat sink is required.

Snap-on/Snap-off Operation

LP2957 snap-in and snap-out operation
LP2957 snap-in and snap-out operation

Using three external resistors this device can be wired for snap-on/snap-off operation. The shutdown input holds the regulator off until the input voltage reaches the turn-on threshold, at which point the output snaps on. The output voltage snaps off when the input voltage decays below the turn-off threshold.

LP2957 Pin Configuration

LP2957 is a 5 pin IC manufactured by Texas Instruments. It is available as 5 lead TO-220 and DDPAK/TO-263 packages. The 5 pins of LP2957 linear voltage regulator are  (Error,) ̅(Shutdown) ̅, Ground, Output and Input.

LP2957 pin configuration
LP2957 pin configuration

LP2957 works with the input voltage range of -20V to 30V and gives an output current of 250mA. When shutdown pin is activated, a 50mA of pull-down current which acts as output crowbar will bring down the output quickly.

This regulator has a tight line and load regulation and low output temperature coefficient of 20ppm/0C typically. TO-220 package has bent staggered leads. TO-263 is a plastic surface mount package.

LP2957 Specifications

Specifications of LP2957 are as follows-

Operating Conditions

  • LP2957 has a 5V fixed output voltage.
  • It has 1.4% accuracy.
  • LP2957 gives 0.3%  of line regulation.
  • Minimum input given to the IC is -20V.
  • It has a load regulation of 0.4%.
  • Quiescent current of LP2957 is 150μA.
  • Minimum operating temperature is -400C.
  • The maximum operating temperature of LP2957 is +1250C.
  • Maximum input voltage is 30V.
  • LP2957 has a maximum output current of 250mA.
  • Output voltage driven from this IC can be 4.93 to 5.07V.
  • Reference voltage given to the IC is 1.23V.

Device Specifications

  • Dimensions of the TO-220 IC is 10.16×4.99×4.57mm.
  • Easy programming of Snap-on or Snap-out output operations can be done using shutdown pin.
  • This device has Reverse battery protection.
  • This device also has thermal limiting.
  • When the output of LP2957 is out of regulation it has error flag signals.
  • Accuracy of LP2957 can be ensured at room temperature levels as well as at all the operating temperature levels.
  • These devices have limited built-in ESD protection.
  • There is an internal limitation of Power dissipation.
  • Storage temperature range is -650C to +1500C.
  • Lead temperature for soldering is 2600C.
  • The junction-to-ambient thermal resistance of TO-220 without heat sink is 600C/W and for DDPAK/TO-263 package is 730C/W.

Applications

The applications of LP2957 are as follows-

Alternative IC

Some of the IC’s available in the market which can be used as an alternative to LP2957 are TPS732 family, TPS795, TPS718XX, TPS719XX, UA78MXX, LP7805, REG102 series.

For low dropout level applications and micropower applications LP2957 is extensively used. The datasheet provided by Texas instruments gives various electrical characteristics at different operating temperatures. How useful was LP2957 for your application?

Image Resources: Texas Instruments

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