UM2243 Rev 2
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UM2243
Dynamic current measurement principle
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10.1
Dynamic current block diagram description
In
Figure 17: Dynamic current block diagram
, VDD is generated from a programmable
voltage regulator (see
Figure 2: Hardware block diagram
). VDD is set from 1.8 V to 3.3 V by
the user to supply the target VOUT. Two shunt resistors called ‘LOshunt’ and ‘HIshunt’, are
in the series path from the voltage source VDD to the output, VOUT.
‘LOshunt’ has a value of typically 1000 ohms and the related differential amplifier, A1, deals
with the measurement of ‘low’ currents. The second shunt, ‘HIshunt’, of typically 2 ohms and
the related differential amplifier, A2, handle the measurement of ‘high’ currents. Without
defining ‘low’ and ‘high’ current limits, it is sufficient to note that there is a threshold between
‘low’ and ‘high’ currents.
Inside the dashed rectangle, the voltage stabilization loop injects a current at point B in
order to maintain the output voltage at a given value, VDD-Vdrop. Vdrop is the maximum
voltage drop acceptable for the highest current of the range, and is typically 30 mV. Diode D
guarantees that if the output voltage is higher than the limit VDD-Vdrop, the loop has no
effect (there is no leakage current).
10.2
Behavior for dynamic current measurements
The boundary between low and high currents is the output current to give a voltage drop of
typically 30 mV (Vdrop) in the LOshunt resistor (1000 ohms), that is, 30
μ
A.
For output currents from 100 nA to 30
μ
A, the stabilization loop does not inject current at
point B; the embedded MCU measures the current by reading the output of amplifier A1. For
currents higher than 30
μ
A, the stabilization loop injects current at point B and acts as a
voltage regulator to ensure that the output voltage does not drop by more than 30 mV
compared to the target VDD current value. The embedded MCU reads the output of the
amplifier A2.
The LOshunt resistor can be shorted by a power switch controlled from the MCU (the
ONE_SHUNT signal in
) when the common current measurement method by one
shunt and without a stabilization loop is required by the system.
For further details regarding dynamic current measurement data acquisition pre-processing
and post-processing, please refer to the X-NUCLEO-LPM01A PowerShield firmware user
manual
