How Can You Effectively Manage Heat in A3959 DMOS Full-Bridge Drivers?

High-current motor drivers get hot during operation. If your A3959 driver overheats¹, your system will shut down unexpectedly. This ruins production schedules. You need a solution to keep temperatures down immediately.

The A3959 handles currents up to 3A, but heat is its main enemy. Effective thermal management² requires using the exposed thermal pad³, optimizing PCB copper areas⁴ for heat spreading, and selecting low-resistance sense resistors⁵s](https://nexcir.com/how-do-you-match-awg-wire-gauges-with-anderson-or-xt60-connectors/)%%%FOOTNOTE_REF_6%%% to minimize power loss and prevent thermal shutdown.

I have seen many engineers struggle with this specific chip. It looks robust on the datasheet, but heat kills it quickly in real-world applications. The chip is small, and the power is high. This combination creates a challenge. I want to share my insights on how to keep this driver cool. Let me show you exactly how to fix this.

Why Does the A3959 Generate So Much Heat Under Load?

You chose the A3959 for its 50V and 3A rating. But during testing, the case temperature spikes dangerously high. This risks component failure and expensive product recalls.

The heat comes from the internal On-Resistance (Rds(on))⁷ of the DMOS output transistors. When high current flows, power is lost as heat. Without a proper path for this heat to escape, the junction temperature rises rapidly towards the shutdown limit.

I want to explain why this happens. The A3959 is a DMOS device⁸. DMOS stands for Double-diffused Metal-Oxide-Semiconductor. It is better than older bipolar drivers because it has lower resistance. However, it is not perfect. The resistance is not zero.

When you push 3A of current through the motor, it also goes through the driver. The internal switches have resistance. We call this $R_{DS(on)}$. For the A3959, the typical source driver resistance is about 270 mΩ. The sink driver is similar. This might look small. But heat generation follows a square law.

The Physics of Heat Generation

You can calculate the power loss using a simple formula: $P = I^2 times R$. If your motor draws 3 Amps, the math looks like this:

• Current ($I$): 3A
• Resistance ($R$): 0.54Ω (Source + Sink combined)
• Power Loss ($P$): $3 times 3 times 0.54 = 4.86$ Watts

Almost 5 Watts of heat is a lot for a small plastic package. If you do not move this heat away, the chip temperature rises. The A3959 has a thermal shutdown feature⁹. It turns off at 165°C. This protects the chip, but it stops your machine.

Key Thermal Parameters

I always tell my clients to look at these numbers in the datasheet:

At Nexcir¹⁰, we see this problem often. Engineers design the circuit correctly electrically. But they forget the physical heat. The chip is too small to cool itself with just air. You must help it. The next section explains how to use your circuit board to cool the chip.

How Should You Design the PCB for Maximum Heat Dissipation?

A bad board layout makes the best chip fail. You cannot rely on air cooling alone. Poor copper design leads to hotspots and system instability.

You must solder the A3959's exposed thermal pad³ directly to the PCB ground plane¹¹. Use multiple thermal vias¹² to transfer heat to the bottom layer. Increase the copper area connected to the power and ground pins to act as a heatsink.

The PCB layout is the most important factor for the A3959. I treat the circuit board as a component. It is not just a holder for parts. It is a heatsink. The A3959 package has a metal pad on the bottom. We call this the "thermal slug¹³." This slug is the main exit door for heat.

The Exposed Thermal Pad

You cannot leave the thermal pad floating. You must solder it to the PCB. I recommend connecting it to the ground plane¹¹. This provides the lowest thermal resistance. If you do not solder this pad, the thermal resistance increases significantly. The chip will overheat even at low currents.

Using Thermal Vias

Heat needs to move away from the chip. The top layer of the PCB is usually crowded with traces. The bottom layer often has more space. You need a bridge between the top and bottom. We call these "thermal vias¹²."

• Placement: Place vias directly under the thermal pad of the A3959.
• Quantity: Use as many as possible. I suggest 4 to 8 vias.
• Size: A diameter of 0.3mm is standard. It prevents solder from wicking away too much.

Copper Area and Thickness

The copper on your board acts like a radiator. More copper equals better cooling. I advise my customers to use large ground plane¹¹s. Extend the copper from the ground pins and the thermal pad.

Also, consider the thickness of the copper. Standard boards use 1oz copper. For high-power motor drivers, 2oz copper is better. It is twice as thick. It spreads heat twice as fast.

Layout Checklist for A3959

I use this checklist for every design review:

1. Is the thermal slug¹³ soldered? It must connect to the PCB.
2. Are there vias under the chip? They connect the top ground to the bottom ground.
3. Is the ground plane¹¹ large? Maximize the area on both sides of the board.
4. Are traces wide? Power traces should be wide to reduce extra resistance.

If you follow these rules, the board absorbs the heat. The chip stays cool. The motor keeps running.

Which External Components Help Reduce Thermal Stress?

Wrong resistor choices add unnecessary heat. If your sense resistors⁶ are too high, you waste power. This lowers efficiency and heats up the surrounding board area.

Select current sensing resistors with very low resistance values and high power ratings. Using a lower voltage reference allowing for smaller sense resistors⁶ reduces power loss ($I^2R$). Also, ensure low-ESR bulk capacitors¹⁴ are placed close to the supply pins to stabilize current.

The A3959 does not work alone. It needs external parts. The most critical parts for heat are the current sense resistors⁶. These resistors monitor the current going to the motor. The current flows through them. This means they also generate heat.

Choosing the Right Sense Resistor

The A3959 measures voltage across this resistor. If the voltage hits a certain level, it limits the current. You might think any resistor works. That is wrong.

If you choose a high resistance value, you create high voltage drop. High voltage drop means high power loss. For example, if you use a 0.5Ω resistor for 2A current: $P = 2^2 times 0.5 = 2$ Watts. That is 2 Watts of extra heat right next to your driver chip. This adds to the thermal load of the PCB.

I suggest using a lower resistance value. You can adjust the reference voltage ($V_{REF}$) on the A3959 to match. If you use a 0.1Ω resistor: $P = 2^2 times 0.1 = 0.4$ Watts. You just saved 1.6 Watts of heat. This makes a huge difference.

Resistor Technology Matters

Do not use standard carbon film resistors. They cannot handle the pulse currents. I recommend:

• Metal Strip Resistors¹⁵: They handle high power and heat well.
• Low Inductance: This prevents voltage spikes.
• Wide Terminal Types: These dissipate heat into the PCB better.

Capacitor Placement

Capacitors stabilize the voltage. The A3959 switches high current very fast. This pulls hard on the power supply. Without good capacitors, the voltage ripples. This causes the chip to work harder and get hotter.

Place a bulk electrolytic capacitor close to the power pins ($V_{BB}$). Also, place a ceramic capacitor right next to the pins. The ceramic capacitor handles the high-frequency noise. The electrolytic handles the large energy gulps.

Summary of Component Strategy

At Nexcir¹⁰, we help customers find these specific parts. We know which brands are reliable. We ensure you get the original components that match your thermal design.

Conclusion

To keep your A3959 cool, you must solder the thermal pad, use plenty of copper vias, and choose low-resistance sense components. Nexcir¹⁰ supports your design with authentic parts and expert advice.