DocID027351 Rev 3
55/100
UM1855
Hardware layout and configuration
99
that communicate with the MCU through I/O ports. Voltage exceeding 3.3 V on the MCU
output port may inject current into 3.3 V-supplied peripheral I/Os and false the MCU current
consumption measurement.
2.30.1
IDD measurement principle - analog part
The analog part is based on measuring voltage drop across a shunt resistor, amplified with a
differential amplifier. The STM32L476ZGT6 microcontroller supply current is shunted, by
jumper settings, to flow through the measurement 1
Ω
resistor R135: JP11 terminals 1 and 2
are to be open, terminals 3 and 4 closed. When the transistor T2 is in conductive state, the
MCU supply current is proportional to the voltage across R135. When T2 is in high-
impedance state, the MCU supply current is proportional to the voltage across the series of
R135 and R123. The former state is used for measuring the current consumption in dynamic
run mode, the latter in low-power mode.
The differential amplifier uses three stages U15B, U15C, U15D of quadruple operational
amplifier device U15, TSZ124. The gain is set to 50, so every 1 mA of supply current is
represented by additional 50 mV at the U15C output, terminal 8 of U15.
The resistance formed with the series of R135 and R123, when T2 is in high-impedance
state, is of 1001
Ω
. It makes the voltage on terminal 8 of U15 increase by approximately
50 mV for every µA of MCU power consumption. The full-scale range, with VDD at 1.8 V is
about 30 µA.
Even with precision resistors R136, R125, R129, R132 to set the gain of the differential
amplifier, the output voltage may theoretically become negative. To avoid the need of
negative power supply, a positive offset of about 220 mV is created at the output, at zero
current consumption of the MCU. This offset does not need to be precise. Any dispersion is
compensated through a calibration procedure detailed in
Section 2.30.4
.
For allowing the IDD measurement, the jumper in the JP11 header must be placed such as
to short its terminals 3 and 4.
