# L298 dual H-Bridge RDS(on)

I am working on a project that mandates the use of a particular Arduino motor driver shield, which makes use of an L298 dual H-bridge (datasheet here)

I need to perform some power calculations in order to size up my power supply, and ensure that I keep the driver's self-heating manageable. If my understanding is correct, in DC operation, an H-bridge will dissipate $P = I^2(RDS_{on(H)} + RDS_{on(L)})$

Unfortunately, the datasheet doesn't mention $RDS_{on}$. Is there some other way to calculate the power dissipated in a H-bridge that I am missing?

This is a bipolar bridge, not a FET bridge, so instead of acting resistive the transistors reach a somewhat constant saturation voltage when they are on. So if you look in the data sheet for the saturation voltage at your operating current, each transistor will dissipate Vsat times the collector current when on.

If you look at the data sheet you'll find this diagram: -

There is a H curve and there is a L curve. H is for high transistor and L is for low transistor. High and Low refer to their positions in the H bridge diagrammatically.

At 1A on the L transistor it'll drop 1.2V implying a series resistance of 1.2 ohms. It gets a bit better at 2A - it drops about 1.8 volts implying a series resistance of about 0.9 ohms. These aren't mosfets or relay contacts - they are BJTs.

You should be able to work power out now.

• I'm a bit confused by these numbers. At 2A the bridge will drop a combined ~3.7V (nominal) according to figure 1 (confirmed by the table of electrical characteristics), which gives 7.4W dissipated in the h-bridge alone, which seems unreasonably high. If this is correct it will dissipate more power than the motor it is driving. – bhillam Mar 19 '14 at 12:42
• @bhillam Your calculations are correct. The L298 is very inefficient in this regard. – m.Alin Mar 19 '14 at 12:49
• Its basically a repackaged L293 aka SN754410 and I don't know why people keep using them. – Andy aka Mar 19 '14 at 13:36