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Boost functionality of industrial DC/DC converters

Posted: 07 Apr 2016 ?? ?Print Version ?Bookmark and Share

Keywords:DC/DC converter? 2 A buck converter? TPS62097? HotRod? QFN?

When employing a step down DC/DC converter in industrial applications, the ideal device can be used across several platforms and applications. This is especially true for a multipurpose 2 A buck converter. There are a number of 2 A buck converters on the market, but the choice becomes very limited when the overall solution needs to be small. One of the main obstacles is the limited pin count with small packages facilitating all device features. This article will outline key application features of a TPS62097, a 2 A buck converted. We will discuss package construction of the new HotRod (QFN), as well as take a look at general reflow soldering guidelines for this new package.

HotRod (QFN, DFN) packages allow higher converter efficiency
Quad Flat No-lead (QFN) and Dual Flat No-lead (DFN) packages have been around for quite some time. The next evolution is the advanced construction of the HotRod QFN or HotRod DFN, which has better electrical characteristics achieving higher efficiency.

A DC/DC switching converter achieves higher efficiency when switching losses are minimised. Switching losses are minimised when rise and fall time of the switch is fast in relation to the switching period. This leads to a higher switching speed with higher switching frequencies of the converter. Today's rise and fall times of switching DC/DC converter are in the lower nanosecond range. This requires low parasitic lead inductance of the package in order to minimise over and undershoots of the switch node. Switch node over and undershoot should not exceed the process voltage rating and should be kept low in order to minimise EMI (Electro Magnetic Interference). Figure 1 shows the traditional QFN/DFN package.

Figure 1: QFN package (Quad Flat No-lead).

The cross section of the QFN package in figure 1 shows bond wires connecting the die to the leadframe and device pins. The bond wire inductance will always be present and sets a limit for the switching speed of the DC/DC converter. The HotRod (QFN) package in figure 2 does not require bond wires thus minimising parasitic inductance.

Figure 2: TPS62097 HotRod (QFN) Package minimises parasitic inductance

In the cross section of figure 2 the die is flipped, or upside down on top of the leadframe. Copper posts contact the die to the leadframe and pins. The outer pins of the package are usually the I/O pins, whereas the pins in the middle are high current contacts for the power MOSFETs of the die. Such a construction has lower parasitic inductance and lower DC resistance compared to the traditional QFN package of figure 1. This results in greater efficiency levels as shown in figure 3.

Figure 3 shows the conversion efficiency for a 2 A step down converter. All devices in this example operate with an input voltage of 5 V and generate 1.8 V. TPS62065 and TLV62084 are packaged in traditional QFN packages. The TPS62097 packaged in the HotRod (QFN) / FlipChip (QFN) package achieves the highest efficiency especially at full load current. Higher efficiency allows the use of small package outlines without excessive package temperature.

Figure 3: High efficiency with the FlipChip QFN of TPS62097.

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