Industrial DC/DC converters require more functionality
When using 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, like the new TPS62097. 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 making the TPS62097 ideal for use across several application platforms. We will discuss package construction of the new HotRod (QFN), highlighting its advantages, as well as taking a look at general reflow soldering guidelines for this new package.
HotRod (QFN, DFN) packages allow higher converter efficiency
QFN (Quad Flat No-lead) and DFN (Dual Flat No-lead) 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 minimized. Switching losses are minimized 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 minimize 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 minimize 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 minimizing parasitic inductance.
Figure 2. TPS62097 HotRod (QFN) Package minimizes 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. High efficiency with the FlipChip QFN of TPS62097
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.
The package offers further advantages in terms of thermal resistance and pin count. The copper post connections minimize thermal resistance compared to bond wire connection. For high current pins longer lead fingers are used. The lead pad design can vary from package to package in terms of length, width and position. This is due to internal die layout, package leadframe design and copper post placement. It is unlikely you would find pin to pin compatible devices on the market unless they are designed to be pin to pin compatible right from the outset. The advantage for small packages is the increased pin count since more device features can be integrated. TPS62097 is a good example of a feature rich device coming in a small 2×2 HotRod (QFN) package.
TPS62097 takes full advantage of package pin count and small dimensions
The device has all the necessary functions for a stand-alone converter or operated in conjunction with other point of load converters where power up and power down sequencing are key.
Figure 4. TPS62097 can be used to power a processor or provide the reference for an ADC
This is done using the power good signal (PG) and the SS/TR (Softstart/Tracking) pin. The output voltage follows the voltage signal applied to the SS/TR pin for simple sequencing. The device achieves high efficiency at full load current and at light load current using the Power Save Mode. For industrial applications and EMI sensitive applications the device has a Mode pin. Leaving this pin open or with a resistor connected sets the device switching frequency and operates with forced PWM (Pulse Width Modulation) resulting in limited switching frequency variation. In this mode the output voltage has a voltage tolerance of +/-1% and can be used as a reference supply for ADCs as an example. Last but not least the package assembly requirements are not different to any other traditional leadless packages.
HotRod (QFN, DFN) packages have standard assembly requirements
QFN/DFN packages and HotRod or FlipChip (QFN/DFN) packages are used in industrial applications because of superior electrical advantages and small physical dimensions. The assembly and solder reflow requirements are similar to standard leaded packages. Good results can be achieved with an optimized reflow soldering, assembly and optical inspection process. One of the important package features is the self-alignment of the package to the land pattern due to surface tension during the solder reflow process. As a result, it is very unlikely that the package will become mis-aligned. If mis-alignment occurs it is typically by an entire pad. This effect allows gross visual alignment check after reflow soldering. For pin inspection it is useful when a side fillet formation is present, although this is not required since the side termination is not plated. The exterior fillet formation may vary based on assembly manufacturing process. It is possible to achieve good fillet formation as shown in Figure 5 using an optimized process.
Figure 5. TPS62097 HotRod (QFN) shows good fillet formation
Conclusion
HotRod (QFN/DFN) demonstrates good thermal and electrical performance. The assembly process is the same as for standard QFN/DFN packages. The TPS62097 device takes full advantage of such a small package. The rich feature set makes the device an ideal multi-purpose point of load regulator suitable for industrial and consumer applications across multiple platforms and applications.
References:
- Texas Instruments Application Report SLUA271A; QFN/SON PCB Attachment
- Texas Instruments Application Report SNOA401R; AN-1187 Leadless Leadframe Package (LLP)
- Texas Instruments Datasheet TPS62067 2A High Efficiency Step Down Converter with iDCS Control
About the author
Oliver Nachbaur has many years’ experience in the semiconductor industry working as an Application Engineer and System Engineer with Power Management and Analog Products. Nachbaur is a Senior Member of Technical Staff at Texas Instruments in Germany. Presently Nachbaur is working as a Technical Business Development Manager for the Low Power DCDC group at Texas Instruments. Nachbaur received a degree in Electrical Engineering in 1996 in Germany
