LM1875T/NOPB IC

Technical Specifications of LM1875T/NOPB

Datasheet	LM1875T/NOPB datasheet
Category	Integrated Circuits (ICs)
Family	Linear – Amplifiers – Audio
Manufacturer	Texas Instruments
Series	–
Packaging	Tube
Part Status	Active
Type	Class AB
Output Type	1-Channel (Mono)
Max Output Power x Channels @ Load	25W x 1 @ 8 Ohm
Voltage – Supply	16 V ~ 60 V, ±8 V ~ 30 V
Features	Short-Circuit and Thermal Protection
Mounting Type	Through Hole
Operating Temperature	0°C ~ 70°C (TA)
Supplier Device Package	TO-220-5
Package / Case	TO-220-5 Formed Leads

LM1875T/NOPB Description

The LM1875 is designed to be stable at a closed-loop gain of 10 or more. However, the LM1875 can oscillate under specific circumstances like any other high-current amplifier. These typically involve the printed circuit board layout or the output/input coupling. The printed circuit board must have the proper layout, which must be verified. The LM1875 will operate without a hitch when mounted on a circuit board similar to this datasheet. Still, it is frequently necessary to modify the layout to meet the unique physical requirements of a particular application. The load ground, the output compensation ground, and the low-level grounds (feedback and input) must all return to the circuit board ground point via separate paths when designing a new layout. This is necessary to avoid interference between the different grounds.

In its absence, input voltages on a ground conductor will serve as signals due to the high currents flowing there, leading to high-frequency oscillation or excessive distortion. It is advised that the output compensation components and the 0.1 F supply decoupling capacitors be positioned as physically close to the LM1875 as possible to reduce the effects of PCB trace resistance and inductance. Because of this, it’s important to minimize the length of the paths these parts take to get back to the ground. Sometimes, when the source impedance is high, or the input leads are too long, the current in the amplifier’s output leads, which also act as antennas, can be coupled through the air to the amplifier’s input, causing the amplifier to oscillate at a high frequency.

A small capacitor (between 50 and 500 pF) placed across the circuit input will fix the problem and render further action unnecessary. The LM1875 struggles to drive highly capacitive loads efficiently, much like most power amplifiers. If there is no series resistance between the LM1875’s output and the capacitor, ringing will be present in the square wave response if the capacitance is greater than 0.1 microfarads. Although driving load capacitances of up to 2 microfarads can prevent oscillation in the amplifier, doing so is not advised. If the load is anticipated to be highly capacitive, a resistor with a value of at least 1 should be connected in series with the LM1875’s output. To shield amplifiers from low impedances at high frequencies, it is common practice to couple to the load through a 10 resistor in parallel with a 5 H inductor. The following sentence describes this technique.

LM1875T/NOPB Distortion

Additionally, the earlier advice on grounding circuit boards will be beneficial in lowering the amount of excessive distortion in audio applications. Maintaining physical separation between the traces and wires of the power supply and the traces and wires connected to the inputs of the LM1875 is crucial for achieving a low THD. As a result, the LM1875 inputs are protected from inductive coupling by the large and nonlinear power supply currents.

It is advised that the power supply wires be kept apart from the circuit board and twisted together. These wires must be positioned on the board so that they are perpendicular to the plane of the board for at least a few inches at the locations where they will be soldered. Total harmonic distortion (THD) levels with 10W output into an 8 load should be less than 0.05% at 20 kHz and less than 0.02% at 1 kHz if the physical layout is done correctly.

Current Limit and Safe Operating Area (SOA) Protection

The output transistors of a power amplifier can be damaged by an excessively high applied voltage, a high current flow, or a high-power dissipation. While the exterior power supply design determines the maximum voltage that can be applied to the amplifier, the amplifier’s internal circuitry typically limits the maximum current that can flow through the amplifier’s output devices to a predetermined value. Due to their typically unrestricted short-term power dissipation, monolithic audio power amplifiers can be problematic when driving reactive loads, which can result in high currents being drawn by reactive loads even when high voltages appear at the output transistors. This might be a problem when trying to reactive power loads.

In addition to limiting current to about 4A, the LM1875 reduces the current limit value when an output transistor has a high voltage across it. This allows the LM1875 to protect electronic components more effectively. It is not recommended to use protection relays on motors or loudspeakers if they are to be driven with nonlinear reactive loads. When an amplifier’s output is wired to a load, the terminal voltage of the load could potentially try to outweigh the power supply voltages being applied to the amplifier. This can occur when an amplifier is connected to a load whose terminal voltage is connected to a load with integrated protection relays. This may eventually cause the output transistors to degrade or the entire circuit to fail catastrophically. The typical form of protection for this type of failure mechanism is a pair of connected diodes placed between the output of the amplifier and the supply rails. When driving standard reactive loads, it is not necessary to add these components externally to the LM1875 because they are already part of its internal circuitry.

Thermal Protection

The LM1875 features an advanced thermal protection scheme, which helps to prevent the device from being subjected to prolonged thermal stress. The LM1875 will power down once the temperature on the die reaches 170 degrees Celsius. It resumes regular operation when the temperature of the die falls to approximately 145 degrees Celsius; however, if the temperature increases, it will shut down at only 150 degrees Celsius. Because of this, if the fault condition is only temporary, the device can heat up to a relatively high temperature; however, if the fault condition is sustained, the maximum die temperature will be limited to a lower value.

Because of this, the IC is less likely to fail under constant fault conditions, as thermal cycling stresses are greatly reduced. Because the heat sink has a direct influence on the temperature of the die, selecting a heat sink with a thermal resistance that is low enough to ensure that the die will not experience thermal shutdown while the device is running normally is essential. The long-term reliability of any power semiconductor device can be significantly improved by utilizing the best heat sink that can be obtained, given the system’s constraints regarding cost and space.

Conclusion

When using a single supply, the LM1875 device can most effectively transfer heat away from the device when mounted directly to the heat sink with the tab at ground potential. As a result, an insulator made of mica or another material is no longer required.

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