LM337 Adjustable Negative Voltage Regulator 1.25-37V/1.5A

Description

The LM377 is an adjustable negative voltage linear regulator that can output -1.25 to -37V at up to 1.5A current with an input voltage range of -3 to -40V.

PACKAGE INCLUDES:

• LM337 Adjustable Negative Voltage Regulator

KEY FEATURES OF LM337 ADJUSTABLE NEGATIVE VOLTAGE REGULATOR:

• Adjustable Negative voltage linear regulator
• -3 to -40V input voltage range
• -1.25 to -37V output voltage
• 1.5A continuous current with 2.2A surge capability
• TO-220 package

The LM337 is the complementary adjustable negative voltage version of the popular LM317 adjustable linear regulator.  Input voltage can range from -3 to -40V and the output can be adjusted from -1.25V to -37V  with an output current of up to 1.5A.  They have built-in current limiting and over temperature protection and tend to be fairly robust devices.

The LM337 can be used to replace a number of different fixed voltage regulators when used for prototyping purposes.  They can can also be easily wired up to make a simple low cost variable power supply for use in prototyping.  When coupled with the LM317, it can be part of an adjustable dual polarity power supply.

Basic Operation

The LM337 is a 3-terminal floating regulator and does not have a ground pin as is found with most regulators. That allows for potentially very high voltages to be regulated as long as the maximum input-to-output voltage rating of 40V is not exceeded.

In place of a Ground pin, it has an Adjustment pin which uses a resistor divider network between the output terminal and ground to set the output voltage.  These can be two fixed resistors if a fixed output voltage is desired, or one of the resistors can be variable potentiometer to allow the output to be adjusted over a range.

Unlike typical 78XX type regulators, the LM337 does require a minimum load current in order to fully regulate.  This is typically less than 10mA so it is not typically an issue for most applications.  A small load resistor could be placed on the output to guarantee a 10mA draw if it was an issue.

The basic system does not necessarily require an input bypass capacitor, but if using it farther than 4″ from the supply filter caps that is providing the input voltage, then a 1uF  tantalum or 10uF aluminum electrolytic input filter capacitor should be added. An output filter cap on the other hand is needed for stability.  This can be a 1.0uF Tantalum or 10uF aluminum electrolytic capacitor.

A basic circuit for hooking the LM337 up with an adjustable output is shown below.

Power Dissipation

Linear regulators have less ripple on their outputs compared to DC-DC converters that can be used for the same basic purpose, but the trade-off is that the linear regulators also tend to dissipate more heat in the process.  The reason is that the linear regulator uses a series pass transistor on its output to drop the excess voltage.

The power dissipation of a linear regulator is dependent on the difference between the input voltage (Vin) and the output voltage (Vout) along with the amount of current that is being drawn from the regulator.  The larger the voltage difference is between Vin and Vout, the higher the power dissipation will be which limits how much current can be drawn from the device.

The power dissipation of the LM337 device is easily calculated as Power Dissipation = (Vin – Vout) * Iout.

If the LM337 input is -15V and the output is adjusted to -10V and it is providing 1A of current, then Power Dissipation = (15V – 10V) * 1A = 5W.  The LM337 TO-220 package will need to dissipate 5W of power.  Under typical conditions, the device can dissipate about 1 – 1.25W before a heat sink becomes necessary, so in our example here, the device would definitely need a heat sink. Maximum output current without a heat sink in this case would be limited to about 250 – 300mA and the device will be running in the range of 85-95°C.

If you instead ran the LM337 off a -12V input, Power Dissipation = (12V – 10) * 1A = 2W.  Still pretty warm, but much more manageable than 5W.  With no heatsink you could draw 500-700mA.

As a general rule, you always want to use as low of an input voltage as possible to minimize power loss though the device and maximize the output current available.

Notes: 

1. The LM337 has a different pin-out than the LM317, so pay attention to that if working with both.
2. The tab of the LM337 is common with the Input pin.
3. Under high current loads or with large input to output voltage differentials, the device can get very hot, so use care when handling.

Technical Specifications