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Choosing the optimal LPDDR4 power supply

Low-power, double-data-rate (LPDDR4) memory is a popular standard for mobile devices such as smartphones, tablets and notebooks. Both LPDDR4 and its successor, LPDDR4x, require lower supply voltages – 1.1V and 0.6V, respectively – which place demanding requirements on the power converters supplying them.

Meeting very tight output-voltage regulation targets requires a fast transient response with little under- and overshoot and high output-voltage accuracy. Additionally, since all applications are battery-powered, the efficiency at both light and full loads is critical to the system’s battery life. Finally, solution size is an important factor for many power supplies, especially in mobile applications.

In this article, I’ll compare three Texas Instruments (TI) power solutions for LPDDR4 portable applications, comparing their output-voltage accuracy, efficiency and solution size to obtain an optimal solution for a wide variety of applications.

Output voltage accuracy

The first requirement of any power supply is to regulate the output voltage. The DC/DC converter efficiently converts the battery voltage down to a lower output voltage and must keep it within a specified regulation window, typically around ±5%, to ensure the correct functioning of the LPDDR4 memory. Output voltage accuracy has two main contributors: DC accuracy and AC accuracy.

DC accuracy refers to the steady-state output voltage accuracy compared to its nominal set point, which is typically programmed with two resistors connected to the feedback (FB) pin of the power supply. This accuracy is a function of the accuracy of the power supply’s FB pin as well as the accuracy of the resistors used to set the output voltage, as explained in [1]. A power supply with a highly accurate FB pin, such as 1% over the operating temperature range, allows worse AC accuracy while still meeting the overall accuracy target.

AC accuracy refers to the transient variation in the output voltage due to a sudden change in either input voltage or load. Large load changes occur very often in LPDDR4 applications, as memory is written over and over again. For a short time, when the load changes, the output voltage varies from its nominal value before the power supply brings it back to the regulation voltage. Having a power supply with a fast transient response is thus key to minimizing AC accuracy and meeting the system’s accuracy requirement.

Figure 1 shows the relative AC accuracy of three 3A LPDDR4 solutions by comparing their transient response to the same 300mA (10% of rated load) to 2.7A (90% of rated load) load step. The TI TLV62585 uses a simpler current-mode control topology for cost-sensitive applications. The TPS62085 and TPS62088 use TI’s fast DCS-Control topology to achieve the best output-voltage regulation [2].

Figure 1

Transient response comparison of 3A LPDDR4 power supplies

Transient response comparison of 3A LPDDR4 power supplies

Efficiency

Efficiency at light, standby and full loads is critical to the power supply’s performance, especially for portable applications. An inefficient power supply at light loads drains the battery too quickly when the portable device is not in use, while an inefficient power supply at heavy loads gets too hot for users. Furthermore, newer LPDDR4 memory can have standby currents down into the microampere current range, which extends the light-load efficiency requirement down to even lighter loads. The power supply’s quiescent current (IQ) is critical to maintaining high efficiency in these cases [3].

Figure 2 compares the efficiency of the same 3A LPDDR4 power supplies when converting a 3.6V input voltage to LPDDR4’s 1.1V output voltage. I used the same size and height inductor for each measurement, changing the inductance value to meet each power supply’s requirements. The TLV62585 has the lowest efficiency at both light and heavy loads due to its higher IQ and the on-resistance of its internal MOSFET. Because it uses the lowest switching frequency and the largest inductance, which reduces switching losses, its medium load efficiency is highest.

The TPS62088 has the best efficiency at light loads due to its lowest 4 μA quiescent current. However, efficiency at the full-rated 3A load is highest with the TPS62085, which has very low on-resistance MOSFETs combined with a medium switching frequency.

Figure 2

Efficiency comparison of 3A LPDDR4 power supplies

Efficiency comparison of 3A LPDDR4 power supplies

Solution size

A power supply’s solution size is generally proportional to the switching frequency and package used. A higher switching frequency allows smaller filtering components, inductors and capacitors, while ball-grid-array (BGA) packages (also known as wafer-chip-scale packages [WCSPs]) enable very tight placement of the power supply’s components and the absolute smallest size.

Table 1 shows the switching frequency, inductor value, package type and package size of the 3A LPDDR4 power supplies. The TPS62088 and its BGA package operate at the highest frequency and thus achieve the smallest size.

Table 2 summarizes the various performance metrics of these three LPDDR4 power-supply designs, including the total solution size achieved.

Table 1

 Size and package comparison of 3A LPDDR4 power supplies

Size and package comparison of 3A LPDDR4 power supplies

Table 2

Overall comparison of 3A LPDDR4 power supplies

Overall comparison of 3A LPDDR4 power supplies

LPDDR4 memory requires a good power supply to function properly, although the power supply needs optimizing for each specific application and system.

Cost-sensitive systems such as notebooks may not require the best transient response or smallest size. The latest smartphones require excellent efficiency at both light and heavy loads, as well as the absolute smallest size, and therefore benefit from a power supply in a BGA package. For the same reasons, camera modules in smartphones have similar power-supply requirements.

Some LPDDR4 memories require the best output-voltage regulation or the highest full-load efficiency to minimize temperature rise. These applications accept a larger solution size to optimize these parameters.

Finally, supporting future memory technologies like LPDDR4x requires a lower output voltage of 0.6V with tight DC and AC accuracy, which all power supplies may not be able to provide. Balancing these trade-offs is critical to optimizing an LPDDR4 power-supply design.

References

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