AN2559 Datasheet, PDF (23/35 Pages) STMicroelectronics – System power supply board for digital solutions

AN2559
6
Measurements
Measurements
The performance and properties of each part of the board is indicated in the measurements
below. These measurements were performed for the PM6680A and L5971AD blocks
independently.
6.1
6.1.1
PM6680A block - measurements
The performance measurements of the PM6680A part focus mainly on efficiency, light load
consumption, output ripple and transients.
Efficiency and light load consumption modes
Since the device consists of three power parts (two controllers and one LDO) it makes
sense to measure total efficiency. Figure 17 displays how efficiency depends on input
voltage level at full load output (V_CORE 2.5 V / 4 A, V_I/O 3.3 V / 2 A).
Figure 17. Efficiency of the dual step-down converter at full load
Efficency (%)
100.0
90.0
80.0
70.0
60.0
50.0
40.0
30.0
20.0
10.0
0.0
0
5
10 15
20 25
30
35 40
Vin (V)
AI12696
The efficiency is in the range of 83 - 91%. It should be noted that the total efficiency strictly
depends on the performance of each component. The System Supply board was designed
to satisfy a wide input voltage range. Therefore, 60 V MOSFETS are used on the board. If
the input voltage of the end application is less (up to 30 V for instance), efficiency can be
improved by using lower RDS_ON 30 V MOSFETs in the same package. The expected
efficiency gain is about 3 - 4%.
The PM6680A can work in several modes with regard to light load. These options are mainly
used for battery applications where relatively high consumption at light load can drain the
battery even when no power is requested. The PM6680A allows three modes (see 3.0.2):
PWM, No Audible Noise and Skip. Figure 18 shows the consumption of the board for
different modes of the PM6680A. There is no load on the output and other parts of the
SMPS are disabled.
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