Choosing buck converters and LDOs for miniature industrial automation equipment: what to consider


As the factory automation and control equipment market evolves, shipments of equipment with sensors such as field transmitters, machine vision and position sensors are increasing. As a result, the demand for feature-rich power integrated circuits (ICs) that could power these devices is also growing.

Figure 1 shows a block diagram of a temperature transmitter. The nonisolated power-supply subsystem (highlighted in red) consists of a low dropout regulator (LDO), a DC/DC converter or a power module. In an earlier technical article, “Powering tiny industrial automation control equipment with high-voltage modules: how to ensure reliability,” my colleague Akshay Mehta explained how to power miniature industrial automation control equipment with high-voltage modules. In this article, I’ll take a look at how to use buck converters and LDOs for the same purpose.


Figure 1: Temperature transmitter subsystem

High input voltage, higher stakes

There are a number of ways to regulate the input DC voltage in factory automation and control equipment. You can use an LDO, a DC/DC converter or a power module. LDOs such as the TPS7A47 are commonly used in sensor power supplies due to their simple design and ability to attenuate input noise and deliver a ripple-free output voltage. DC/DC converters are a good choice for applications operating at lower output voltages, higher input voltages or higher output currents. For example, the LMR36503 and LMR36506 DC/DC converters enable a low shutdown current specification of 1 µA and an operating quiescent current specification of 7 µA. For loads with low output currents – less than 20 mA – these performance specifications ensure higher efficiency for 4- to 20-mA loop applications. Figure 2 shows the efficiency and thermal performance of the LMR36506 converter.


Figure 2: Efficiency and thermal performance at 24 VIN, 5 VOUT, 2.1 MHz at 0.6A
Big challenge, small solution
 
Most field sensors are small, which constrains the size of the printed circuit board (PCB). For instance, ultrasonic sensors with M12 housing need a PCB width less than 9 mm. Incorporating power-supply components on a small PCB in a subsystem – as shown in Figure 1 – becomes very challenging for hardware designers.
 
A power module with an integrated inductor such as the TPSM265R1 can address this challenge, since DC/DC converters require that you use additional components such as an inductor to maintain high frequencies. However, if you prefer to use a DC/DC converter, you can reduce the total solution size by choosing converters that operate at higher frequencies, which reduces the size of the inductor and capacitor, or by selecting a device with integrated external components.
 
For example, the LMR36503 and LMR36506 come in 2-mm-by-2-mm packaging with a 55-ns minimum controllable on-time, which ensures that the converter can operate a 2.1-MHz switching frequency with a direct step-down from 24 VIN to 5 VOUT. The high switching frequency enables you to use an ultra-small inductor and output capacitor, while the internal loop compensation and fixed 5-V/ 3.3-V output options reduce the overall external component count. It is possible to optimize the total solution size further, as shown in Figure 3.
 

Figure 3: LMR36506 example solution size
Lowering EMI, raising the standard
 
All switching power supplies generate electromagnetic interference (EMI) by virtue of the fact that they switch the input voltage using fast rise and fall times. An EMI filter and metal shielding can help resolve EMI issues, as Figure 4 illustrates. However, a multiple-stage EMI filter could reduce the efficiency of your application while increasing solution size and design costs. To combat this reduced efficiency, use DC/DC converters designed to provide lower EMI.

Figure 4: An EMI filter structure for DC/DC converters
 
The LMR36503 and LMR36506 are designed with a flip-chip on-lead (FCOL) technology, which eliminates power device wire bonds that might result in higher package parasitic inductance. As shown in Figure 5, the IC is flipped upside down, and copper posts on the IC are soldered directly to a patterned leadframe. This reengineered construction enables a small solution size and a low profile, as each pin attaches directly to the leadframe. In addition, the flip-chip package lowers package parasitic inductance versus traditional wire-bond packages, resulting in much lower ringing and noise generation during switching transitions.
 

 
Figure 5: Wire-bond quad flat no-lead and FCOL packages
Conclusion
 
As the field sensor housing gets smaller, the constraint on PCB size becomes more of a challenge for board designers to provide power to the sensors. In this case, the LMR36506 is an option for meeting this challenge.
 
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