SLAAEN0 September   2024 MSPM0L1227 , MSPM0L1227-Q1 , MSPM0L1228 , MSPM0L1228-Q1 , MSPM0L2227 , MSPM0L2227-Q1 , MSPM0L2228 , MSPM0L2228-Q1

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Introduction
  5. 2Low-Frequency Subsystem Introduction
    1. 2.1 Resetting LFSS IP Using VBAT
    2. 2.2 Power Domain Supply Detection
      1. 2.2.1 Start-Up Sequences
      2. 2.2.2 LFSS IP Behavior
    3. 2.3 LFXT, LFOSC
    4. 2.4 Independent Watchdog Timer (IWDT)
    5. 2.5 Tamper I/O
      1. 2.5.1 IOMUX Mode
      2. 2.5.2 Tamper Mode
        1. 2.5.2.1 Tamper Event Detection
        2. 2.5.2.2 Timestamp Event Output
        3. 2.5.2.3 Heatbeat Generator
    6. 2.6 Scatchpad Memory (SPM)
    7. 2.7 Real-Time Clock (RTC)
    8. 2.8 VBAT Charging Mode
  6. 3Application Examples
    1. 3.1 Tamper I/O Heartbeat Example
    2. 3.2 RTC Tamper I/O Timestamp Event Example
    3. 3.3 Supercapacitor Charging Example
    4. 3.4 LFOSC Transition Back to LFXT Example
    5. 3.5 RTC_A Calibration
      1. 3.5.1 Peripheral ADC 12
      2. 3.5.2 RTC_A

2.8 VBAT Charging Mode

Table 2-1 shows the simple charging circuit can provide current-limited charging of a supercapacitor at VBAT from the primary VDD supply when VDD is greater than VBAT, while also allowing VBAT to start at time zero when VBAT is at 0V. The charging circuit needs to be enabled by software when the SUPERCAPEN bit inside the SYSTEMCFG register is set. Make sure the VBATGOOD bit in the SYSSTATUS register is set, this bit is set only when VBAT Power Domain is valid.

Table 2-1 Specifications
PARAM VALUE UNIT
VDD TYP 3.3 V

RSWITCH

TYP

1400

Ω

RSWITCH

MAX

2700

Ω

CSCAP

TYP

0.33

F

VBAT

MIN

1.62

V
Maximum ICHARGE (Smallest REXT)
PARAM VALUE UNIT

REXT

3300

Ω
ICHARGE(T0, MAX)

1.6

mA
Charging Times
PARAM VALUE UNIT
Tau 1980 s
Full charge time (5T) 9900 s
165 min
2.8 hrs
Run time
PARAM VALUE UNIT
VRANGE (VDD-VBAT,MIN) 1.68 V
IDIS 1.5 µA
TDIS (Discharge life) 369600 s
6160 min
102 hrs
4.2 days

REXT and RSWITCH together set the time zero maximum charge current ICHARGE(T0,MAX). REXT can be increased to reduce ICHARGE(T0,MAX) at the expense of charge time, but REXT must meet a minimum value of REXT(MIN) to make sure the VBAT pin detects VBOR+; therefore, the backup island starts at T0 and does not need to wait for charging.

Equation 1. V B A T ( T 0 )   =   ( V D D - V C A P ) R E X T ( R E X T + R S W I T C H )
Equation 2. I C H A R G E   =   ( V D D - V C A P ) ( R E X T + R S W I T C H ) + I L E A K
Equation 3. R E X T   =   ( V B A T ) ( R S W I T C H ) ( V D D - V B A T )
Equation 4. I C H A R G E ( T 0 , M A X )   =   V D D ( V B A T ) ( R S W I T C H ) ( V D D - V B A T ) + R S W I T C H

Charging is enabled by software and is automatically disabled when VDD drops below VBAT. The discharge life is driven by VBAT burn (IDIS), REXT is minimal.

MSPM0L2228 Supercapacitor Charging CircuitFigure 2-10 Supercapacitor Charging Circuit