
SWCS037G
– MAY 2008 – REVISED APRIL 2011
4.2
Power References
The bandgap voltage reference is filtered (resistance/capacitance [RC] filter) using an external capacitor
connected across the VREF output and an analog ground (REFGND). The VREF voltage is scaled,
distributed, and buffered in the device. The bandgap is started in fast mode (not filtered), and is set
automatically by the power state-machine in slow mode (filtered, less noisy) when required.
Table 4-12 lists the voltage reference characteristics.
Table 4-12. Voltage Reference Characteristics
Parameter
Test Conditions
Min
Typ
Max
Unit
Output Load Condition
Filtering capacitor
Connected from VREF to GNDREF
0.3
1
2.7
μF
Electrical Characteristics
VIN
Input voltage
On mode
2.7
3.6
4.5
V
Internal bandgap reference voltage
On mode, measured through TESTV terminal
1.272
1.285
1.298
V
Reference voltage (VREF terminal)
On mode
0.749
0.75
0.77
V
Retention mode reference
On mode
0.492
0.5
0.508
V
IREF NMOS sink
0.9
1
1.1
μA
Ground current
Bandgap
25
μA
IREF block
20
Preregulator
15
VREF buffer
10
Retention reference buffer
10
Output spot noise
100 Hz
1
μV/√Hz
A-weighted noise (rms)
200
nV (rms)
P-weighted noise (rms)
150
nV (rms)
Integrated noise
20 to 100 kHz
2.2
μV
IBIAS trim bit LSB
0.1
μA
Ripple rejection
<1 MHz from VBAT
60
dB
Start-up time
1
ms
4.3
Power Control
4.3.1
Backup Battery Charger
If the backup battery is rechargeable, it can be recharged from the main battery. A programmable voltage
regulator powered by the main battery allows recharging of the backup battery. The backup battery charge
must be enabled using a control bit register. Recharging starts when two conditions are met:
Main battery voltage
> backup battery voltage
Main battery
> 3.2 V
The comparators of the backup battery system (BBS) give the two thresholds of the backup battery charge
startup. The programmed voltage for the charger gives the end-of-charge threshold. The programmed
current for the charger gives the charge current.
Overcharging is prevented by measurement of the backup battery voltage through the GP ADC.
Table 4-13 lists the characteristics of the backup battery charger.
Table 4-13. Backup Battery Charger Characteristics
Parameter
Test Conditions
Min
Typ
Max
Unit
VBACKUP-to-MADC input attenuation
VBACKUP from 1.8 to 3.3 V
0.33
V/V
Copyright
2008–2011, Texas Instruments Incorporated
Power Module
45