Our world is becoming automated. We see a strong initiative for more automation in our everyday lives, from smarter homes (AC, lighting and white goods) to easier and better travel in automobiles. This requires a lot of processors and logic devices! But how is logic controlling all of those motors, LEDs and relays? Peripheral, motor and low-side drivers are integral parts in making this happen. You may already know about a very standard driver used in most applications, the Darlington transistor. But as we strive to build innovative, better solutions, I feel compelled to ask: how can we make the standard even better?
What does the standard driver look like?
The simplest, yet the most common peripheral driver today is the Darlington transistor array. This low-side driver enables logic devices to drive or control a device with a higher power demand (shown in Figure 1):
Figure 1: Darlington low-side driver
In current systems, designers use arrays consisting of multiple Darlington pairs for control of a full system. This type of system typically allows logic devices with TTL or 5V CMOS to drive devices with up to 50V and 500mA per channel. Whenever current demand is too high for a single channel to drive, paralleling the channels helps distribute the current load evenly (shown in Figure 2).
Figure 2: Darlington Array Driver
However, using this type of architecture has its own tradeoffs and constraints. One of the biggest problems is the increased board size whenever most, if not all, channels of the peripheral driver are overloaded. This then requires the use of an additional driver to divide the current demand among these. Another setback is the increased power dissipation this device adds to your system. The voltage in the low-side of the output for this device is increased, due to the stacked NPN transistors, by about 0.7V. The dissipated power by this system will now look like:
PD = VOL*IO
PD = (~0.7V + 2Ω*IO) * Io
How to make the standard better?
One solution for these tradeoffs is to use an NMOS transistor instead of the Darlington pair. This low-side driver architecture has reduced dissipated power and can support inputs for all GPIO levels, from 1.8V to 5V.
Figure 3: NMOS low-side driver
This configuration allows us to drive peripherals the same way as with a Darlington pair with significantly lower power dissipation:
PD = VOL*IO
PD = (2Ω*IO) * Io
TI’s new peripheral driver, the TPL7407L, is a seven-channel, NMOS low-side driver array that replicates this architecture. This device allows us to replace any standard seven-channel Darlington based driver, while keeping dissipated power lower than the standard solution. This device also has an increased current support, allowing higher current demands to be allotted to a single channel or fewer than the standard device.
Figure 4: 7CH NMOS Low-Side Driver
Peripheral driving is used heavily in high-voltage applications such as white goods, HVAC, automobiles and building automation. If you have or are designing a system that uses a Darlington transistor array as a peripheral driver, improve your system without having to go through an extensive redesign with this device. This simple change can take your design to a whole new level and make your system that much better!
Want to learn more?
- Talk directly with engineers on TI’s E2E technical forums