6.2.1 Buck Mode Output Current Capability
First, note that some portion of the power generated by the DC-DC is internally supplied to the PMC (power management circuit)
and required to supply the other VDDs, that means less than total maximum output power of the DC-DC is capable of being
provided to external circuitry. A simple way to estimate the amount of power required by the device is to refer to the data sheet
section “Power consumption operating behaviors”, where there are several typical scenarios already characterized.
In case it is needed to measure the power consumed by microcontroller, the most accurate method is to measure the input
power-on VDDx and VDD_RFx pins when application is configured in bypass mode, considering the same voltage levels as
generated by DC-DC.
If there is no availability to modify the hardware for measuring the current in bypass mode, another method is to multiply the
VDCDC_IN current and voltage to calculate the input power with no loading and then multiply by 90 %, which is the typical DC-DC
efficiency, thereby obtaining the approximate microcontroller required power. For example, consider the case where the MCU is
configured for Buck mode and a current of 4.8 mA is consumed while VDCDC_IN = 3.0 V. Power IN = 4.8 mA x 3.0 V = 14.4 mW.
So, the power required by the microcontroller in that configuration is 12.96 mW (Power IN x 90 %). Leaving a total of 112 mW
(assuming a 125-mW maximum power output, 125 mW – 12.96 mW) available to power the RF portion and other circuits.
There is a maximum capacity for VDD_1P8 to provide current, even if RF circuit is off, it is not possible to provide all power though
VDD_1P8 pin. Below is the maximum IDD_1P8_Buck curve varying with VDD_1P8 voltage:
VDD_1P8 (V)
Buck Mode Maximum Current Capability on VDD_1P8
1.6
3.2
2.4
2.8
2.0
50
20
25
30
35
40
45
45
27
IDD_1P8_Buck
(mA)
Figure 17. Maximum IDD_1P8_buck versus VDD_1P8 voltage
The above graph, Maximum IDD_1P8_buck versus VDD_1P8 voltage, is representative of most devices in this
series. However, some devices deviate from these limits and you should refer to the device-specific data sheet
as to the actual limits at 1.8 V and 3.0 V. Also note that other conditions, such as VDCDC_IN voltage may affect
these limits.
NOTE
6.2.2 Boost Mode Output Current Capability
The method to calculate the required energy to supply the microcontroller in Boost mode is similar to the methods explained in the
Buck mode topic. The only difference for boost mode is that when using Boost DC-DC conversion, the maximum output current
capable of being supplied by VDD_1P8 varies according to VDCDC_IN.
Below is the maximum IDD_1P8_Boost current varying as a function of VDCDC_IN in two scenarios, when VDD_1P8 = 1.8 V and
when VDD_1P8 = 3.0 V.
NXP Semiconductors
Current estimation and efficiency report
MKW4xZ/3xZ/3xA/2xZ DC-DC Power Management, Rev. 3, 04 June 2021
Application Note
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