Risk is something that all of us encounter quite frequently in life. We take risks when we decide to get behind the wheel of a car, make an investment, or take the first step out of bed. Virtually every decision we make has some component of risk. And usually the amount of risk we take is somehow tied to a reward at the end.
With all that risk swirling around us every day, why take risks when developing a motor drive solution? Reward yourself by considering an integrated solution and all the advantages it offers.
Take a look at the discrete implementation of an H-bridge in Figure 1 below. The H-bridge allows you to control a DC motor in both a forward or reverse direction. The freewheeling diodes provide a current path during deadtime, or the coast state, when all the inputs are zero. When the inputs are opposite polarity, current flows through TR1 and TR4 (input A = HIGH) or through TR3 and TR2 (input B = HIGH). Deadtime is established by carefully tuning the biasing resistors in the circuit to ensure TR1 and TR2 or TR3 and TR4 are never on at the same time causing a short between power and GND.
This is a low cost solution when you look at it on the surface and consider only individual component cost. But, when you consider things like engineering time to properly tune it, board area consumed by the discrete components, assembly cost of individually placing the components, and the cost of handling product returns with unprotected transistors, it quickly looks less attractive to today’s design engineer.
Figure 1: Discrete H-bridge
Compare this to an integrated solution, like the DRV8837 shown in Figure 2 below, which combines everything you need into a single 2x2mm package. Other than the dramatic improvement in solution size, a couple of things in the block diagram offer significant advantages. The first is the integrated temperature protection. During start-up and stall conditions, the current in a motor exceeds transistor current ratings resulting in damage, or hot spots, on a PCB leading to reliability and safety concerns. Additionally, over-current protection protects the output transistors from inadvertent output shorts during assembly or by the end user that may be tinkering with the final product. For the discrete implementation, a fuse is required to add this same level of protection. Finally, a sleep state disables the charge pump and other logic to reduce the quiescent current down to 30 nanoamps. This significantly improves battery life over the discrete solutions that have off-state leakage currents in the milliamp range.
Besides the protection features, there are other often overlooked improvements in an integrated solution like very low deadtime. Due to component variability, and to reduce risk of shoot-through between high-side and low-side transistors, dead time in a discrete solution is typically >1us. Deadtimes on an integrated solution with optimized internal gate drive circuits are typically less than 100ns. The lower dead times allow you to switch at higher frequencies with more PWM resolution. This results in a more linear output for precise motor drive applications.
|Figure 2: DRV8837 Integrated motor drive block diagram|
With every decision we make, it is always a risk vs. reward tradeoff. Consider that the next time you need a motor drive circuit and enjoy the reward of more time, quicker time to market, less cost, and product reliability that will help you get a better night of sleep.