Power rating for a 2 ohm resistor on the input power rail of a 5V DC circuit

Question

I'm working with a surface mount IC in an SOP-8 package. The IC handles LIPO battery charging at up to 2.1A. It also provides an integrated boost converter with 5V, 2.4A output. Below is an image of a prototype I built that works fine using through-hole components.

Now, I want to rebuild this circuit with all surface mount components which are as small as possible on a PCB. In the example circuit shown in the IC datasheet includes a 2 ohm resistor in series with a capacitor on the USB charger input power rail, the resistor is shown as R1 in the schematic. The "VIN" in the schematic represents the IC power input at the #1 pin.

The question is around the resistor R1. Firstly, I'm not sure I even need to use it? Is the purpose of this resistor just to drain the bypass capacitors? Secondly, if best practice would be to use the 2 ohm resistor here, what is the appropriate power rating which I must choose?

The voltage source will be a standard consumer-grade 5V ac/dc converter with 1A to 3A output, going to a micro-USB input.

In the prototype shown in the picture, I used the large through hole resistor rated at 3W, and it seemed to work fine, at least in the runtime testing as long as 8hrs, it never got hot to touch.

In moving to surface mount components, 2ohm resistors with such high wattage ratings are both large and expensive. I'd prefer to eliminate it due to PCB size constraints and cost.

^{simulate this circuit – Schematic created using CircuitLab}

Answer 1

There will be no DC current through that resistor, so unless there is something terribly wrong with the circuit or the supply powering this circuit, it will never heat up.

The purpose of the resistor is not to drain the bypass caps, as there is no DC path to drain them. The purpose of the resistor and capacitor is to dampen transient voltage spikes that might happen when devices are hot-plugged together, for example there is an USB cable connected to a powered 5V source and then this charger is plugged in. The cable has inductance so the inrush current taken by the caps can cause large voltage spikes that are maybe enough to destroy or degrade the chrager IC, so it is better leave the RC damper in the circuit than remove it.

Since the resistor is just a part of an RC filter, it means there will ever be max 5V over the resistor, and thus max 2.5A current spike running through the resistor, which means max power dissipation is 12.5 Watts. However, the capacitor will be approximately fully charged in 0.1 milliseconds, so the surge is very short.

So technically, a resistor that can handle the large surge is needed. In practice, any standard SMD resistor could be used, as long as it can handle the momentary high current for extremely small amount time without acting as a fuse.

The pulse load ability depends on resistor model, for example it might require a 1206 resistor that can normally handle 0.25W to handle a short pulse load of 12.5W.