AN4102 datasheet(5/16 Pages) STMICROELECTRONICS

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AN4102

Application description

Doc ID 023129 Rev 1

5/16

The resistor is placed in parallel to a high-sense operational amplifier (A) with fixed gain that

amplifies the voltage (V) present on the resistors. A sample-and-hold stage is then inserted

and connected to an analog input of the MCU (PF0 IDD measurement) that finally converts

the resulting voltage, which reflects the consumption current.

In low-power modes only, a counter enabled by the STM8AL (PC4 pin) manages the

measurement timing while the microcontroller is idle. The microcontroller is woken-up after a

150 ms delay through the K2 switch.

While the microcontroller is in one of the power-saving modes, a capacitor (C) is able to

store the measurement charge so that the microcontroller can later give the value of the low-

power mode current consumption during the microcontroller’s wake-up phase (in the first 50

ms). Switch S is opened at the device start-up so as to keep the charge collected in the

capacitor (C) intact while the microcontroller is in low-power mode.

The current measurement precision is enhanced by taking into account the I bias current of

its own operational amplifier. When JP1 is placed in the OFF position (IDD measurement

circuitry disabled), a special test invoked by the USER2 push-button at the device start-up

measures this current value and stores it in the non-volatile memory. Once this value is

stored in the device, it is deducted from the next IDD measurement to compensate errors

due to I bias current (see Section 1.5.2: Bias current record).

For additional information related to the IDD measurement feature, refer to the

STM8A-DISCOVERY user manual (UM1574).

1.5

Getting started with the application

Two modes can be run within the STM8AL low-power application example.

●

IDD measurement in different power modes.

●

Bias current record.

The IDD measurement is available when the application is powered-up with the USER1

push-button pressed while the bias current record is available when the application is

powered-up with the USER2 push-button pressed.

However, for best performances, it is recommended to control and record the I bias current

before starting the IDD measurement mode.

1.5.1

IDD measurement

Firstly, check that JP2 is set to the ON position. Before applying power to the STM8AL board

through the USB cable (or through an external power supply), press the USER1 button and

maintain it pressed while the USB cable is connected. The IDD mode is selected and the

green LED LD4 is set. The "** TEST **" message is displayed. You are entering the IDD

measurement sequence. Once this mode is launched, the different steps automatically

unfold as described in the diagram below. If one of the measurements is outside the range

of the specification, the sequence ends with an error message related to the failing

measurement.

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