They are the 2012-size LQM21FN100M80 and LQM21PNR47MGH that use ferrite cores of multilayer construction. When a smaller part is required for 4 MHz, LQM18PNR47NFR is recommended.
Later in our descriptions, we will explain the reason for recommending these parts for use with BLE.
The required voltage for BLE is made available by the DC-DC converter built into the BLE IC.
Under these circumstances, the power inductor for voltage conversion will be connected externally, but depending on the performance of the power inductor used here the power conversion efficiency of the DC-DC converter will vary.
Since the matter of power saving is highly regarded in BLE, the external connection of a power inductor offering excellent power conversion efficiency is tied in with the power saving performance of BLE.
Here, consideration is given to the necessary performance of power inductors suited to BLE, and actual products are introduced.
Investigation of the effects that inductor differences have on BLE power supply efficiency
We investigated the effects that inductor differences have on BLE power supply efficiency.
Here, attention is paid to the DC losses and the AC losses that exist in inductors. DC losses are losses that are attributable to the DC component of the current flowing in the inductor, and are expressed below using the DC resistance (Rdc) and the DC component of the current (Idc).
On the other hand, AC losses are losses that are attributable to mainly the AC component of the current flowing in the inductor, and are expressed below using the AC resistance (Rac), which is the apparent resistance due to the winding resistance plus the core losses, and the AC component of the current (Irms).
The AC component (Irms) expresses the size of the current amplitude, and this value can be reduced by increasing the switching frequency or increasing the inductance of the power inductor.
Currently, as of 2018, the DC-DC converters built into the BLE ICs are available with a switching frequency on the order of 1 MHz or 4 MHz. Accordingly, we conducted a simulation to see to what degree the proportion of DC loss and AC loss would become when using inductors with an inductance of 1.5 µH and 10 µH at each of the respective frequencies. (Fig. 1)
Although the frequency conditions and other factors also have an influence, Rac increases more than Rdc in the power inductor. For this reason, in the small area in which the load current (equal to Idc) is 50 mA or less as shown in Fig. 1, we see that the DC loss becomes very small and the AC loss occupies a large portion.
At the lower right of Fig. 1, inductance (L) is 10 µH, and the frequency is 4 MHz; under these conditions, the smaller current amplitude, due to the high inductance and high frequency, suppresses the AC loss and so the proportion of the DC loss increases. Even so, because the load current in BLE is very small, on the order of 10 mA, the AC loss over the entire region can be thought to be dominant.
From these results, in order to reduce the inductor loss in BLE applications, the inductance, which affects AC loss, and the Rac become important parameters.
At low Rac it is ideal to obtain high inductance; however, in trying to obtain inductance the winding resistance and core losses will be increased. For this reason, the balance between inductance and Rac becomes important. In regard to Rdc, in the BLE environment where there is a small proportion of DC loss, the degree of influence may be considered to be low. However, when Rdc becomes overly large it cannot be ignored and attention must be paid to this.
Fig. 1 Proportion of AC loss to DC loss of a power supply IC for BLE
As causes of DC-DC converter power losses, in addition to inductor losses, other losses that can be considered include switching losses of the IC, and IC continuity losses.
Concerning power inductors intended for BLE, since the size of the inductance may influence the switching loss, this point must also be considered in the selection.
Fig. 2 is a simulation of the current waveforms that flow in the inductor. In order to raise the power efficiency when operating under a low load like BLE, generally operation is with a control mode called PFM (pulse frequency modulation) control. In PFM control, rather than perform switching continuously, the cycles are reduced to raise efficiency. The left chart of Fig. 2 shows the current waveform when the inductance has been changed at the same frequency.
Larger inductance will keep down the size of the current amplitude; however, the number of triangular waves generated will increase to adjust the total current amount. This signifies an increase in the number of switching cycles, and there will be an increase of the switching loss. Similarly, the right chart of Fig. 2 shows the current waveform when the frequency has been changed with the same inductance. Higher frequencies will keep down the size of the current amplitude; however, it is understood that the number of switching cycles will increase.
In this way, in PFM control, by increasing the inductance or the frequency and making the current amplitude smaller, on one hand the AC losses can be reduced, but there is also the aspect of increasing the switching losses.
Fig. 2 Current waveform when the inductance value or the switching frequency is changed
Well then, about how much inductance would be good to use?
In Fig. 3, a simulation was used to look at the power supply losses when the inductance value was changed at both the 1 MHz and the 4 MHz switching frequencies. (The characteristics of the LQM21P-GH series were used as a base, and Rac was also changed in conjunction with changes of the inductance value for the computation).
The graphs display the respective power supply total losses and the losses of only the inductors.
In the case of the low 1 MHz switching frequency, it can be seen that as the inductance became larger the inductor loss dropped, and accompanying this the total losses also dropped. On the other hand, in the case of 4 MHz, as the inductance became larger the inductor loss dropped, but the result was that the total losses increased. The reason for this is that as the inductance became larger, the switching losses increased as a result.
In the case of 4 MHz compared to 1 MHz, because the number of switching cycles is large, the proportion of switching loss is large, and so it can be said that the increase of switching loss appeared remarkable due to the change of the inductance.
From these results, the value on the order of 10 µH is suited for 1 MHz, and 0.47 µH to 1.0 µH is suited for 4 MHz.
Fig. 3 Relationship between inductance value and power supply losses
Summary regarding inductors suited for BLE
Tables 2 and 3 show a summary of inductors suited for use with BLE.
As explained above, it can be said that inductance on the order of 10 µH is suitable for a switching frequency of 1 MHz, and inductance from 0.47 µH to 1.0 µH is suitable for 4 MHz.
Although not specifically touched upon here, there are guidelines for DC resistance and core losses, and inductors having specifications as indicated in Tables 2 and 3 may be sought.
Table 2 Recommended inductor specifications for a switching frequency of 1 MHz
■For a switching frequency of 1 MHz,
L = 10 µH or more, and low R core losses are important. The effects of Rdc are small.
*Examples of specific inductors: LQM21FN100M80
Table 3 Recommended inductor specifications for a switching frequency of 4 MHz
■For a switching frequency of 4 MHz,
L = 0.47 µH to 2.2 µH, and low R core losses are important. The effects of Rdc are small.
*Examples of specific inductors: LQM21PNR47MGH、LQM18PNR47NFR
Performance of inductors selected for use with BLE
The performance when using specific inductors for BLE that were introduced in the previous section was compared with that of regular inductors.
Even with inductors of the same inductance specifications, differences in the DC resistance and the magnetic material brought results that changed. First, the case of the 1 MHz switching frequency.
In this case, the multilayer type inductor LQM21FN100M80 was selected as the inductor for BLE. This type of inductor was installed on commercially available BLE equipment and the results of the evaluation are shown in Fig. 4. It was observed that by using LQM21FN100M80, the power consumption dropped.
Fig. 4 Comparison of power consumption with BLE at a switching frequency of 1 MHz
Operation conditions: ：Vin=3.30V、Vout=1.68V、FSW=1MHz、Idc=10mA
Fig. 5 is the case of BLE equipment with a switching frequency of 4 MHz. In this instance, an LQM21PNR47MGH inductor with an inductance of 0.47 µH was used. Also in this case, by replacing the inductor with one offering appropriate performance, the power consumption could be reduced successfully.
Fig. 5 Comparison of power consumption with BLE at a switching frequency of 4 MHz
Operation conditions ：Vin=3.30V、Vout=1.80V、FSW=4MHz、Idc=4mA
Summary
The power inductors that are used in BLE have an influence on the power supply efficiency and because of this they are key parts that affect the low power consumption that is a feature of BLE.
The specifications sought in an inductor will differ depending on the switching frequency of the power IC used in BLE. At 1 MHz, the value will be on the order of 10 µH, and at 4 MHz, on the order of 0.47 µH to 1.0 µH.
However, even with the same form and the same inductance value, the efficiency of the power supply will differ depending on the series of inductor; therefore, it becomes necessary to select an inductor series suited to BLE.
Here are the optimum power inductors for BLE that were introduced