UM1521
Hardware layout and configuration
Doc ID 022868 Rev 1
25/56
Figure 10.
STM32L152D-EVAL I
DD
Low power modes measurement timing diagram
shows how the counter and T1 ensure that, 150 ms after IDD_CNT_EN falling
edge, the shunt resistor R81 is connected between VDD_MCU and the power supply in
order to reduce the measurement range to 30 uA full scale when VDD = 1.8 V.
Then after another 150 ms for current stabilization, R81 is shorted, the I
DD
measurement is
stored in C82, and the MCU is woken up. After wakeup the MCU can measure the I
DD
current corresponding to the Low power mode stored in C82.
2.20.3 Ibias
current
measurement procedure
In Low power mode the bias current of operational amplifier input (U19 pin 4) is not
negligible compared to I
DD
current (typical Ibias is ~240 nA). To obtain a reliable MCU I
DD
measurement it is possible to subtract the bias current from the I
DD
low power measurement
since this current is not sinked by the MCU. The procedure for accurate I
DD
measurement is:
1.
Set jumper JP10 on pin 1 and pin 2.
2.
Follow the Low power mode procedure to measure I1 = I bias.
3.
Remove jumper JP10 from pins 1 and 2 and place it on pins 2 and 3.
4.
Follow the Low power mode procedure to measure I2 = I
DD
+ I bias.
5.
Calculate actual I
DD
: I
DD
= I2 - I1.
If JP10 jumper is in pins 2-3: STM32L is powered through I
DD
measurement circuit (default).
If JP10 jumper is in pins1-2: STM32L is powered directly by +3V3, I
DD
measurement circuit
is bypassed. Refer to
for detail.
Note:
1
When jumper JP10 is removed the current consumption of the STM32L can be measured by
connecting an ammeter between jumper JP10, pins 1 and 2.
2
RDS(on) typical value of analog switch T1= 50 MOhm, so to improve measurement
accuracy it is recommended to take into account the RDS in series with R62 (1 Ohm) in the
I
DD
Run mode current calculation.
3
To avoid current injection from MCU to components on the board during I
DD
measurement it
is strongly recommended to keep VDD_MCU
≤
3.3 V. Some components on the board are
powered by 3.3 V (for instance the ST-LINK) so if VDD_MCU is higher than 3.3 V a current
can be in injected on signals like T_NRST (PB0 of U21) which disturbs the measurement in
low power mode.
MCU mode
Run
Low power
Wake-up
Clear CNT
IDD_CNT_EN (PC13)
Q12 = LOW_POWER_EN
(T1 pin 3)
Q13 = IDD_WAKEUP (PA0)
Q13n = disconnect filter
0 ms
150 ms
300 ms
event
event
Wake-up
period
I
DD
measurement
3
4
5
6
(U18 pin 4)
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