Analog Integrated Circuit Device Data
Freescale Semiconductor
105
PC34708
Functional Block Requirements and Behaviors
32.768 kHz Crystal Oscillator RTC Block Description and Application Information
7.9.3
Coin Cell Battery Backup
The LICELL pin provides a connection for a coin cell backup battery or supercap. If the main battery is deeply discharged,
removed, or contact-bounced (i.e., during a power cut), the RTC system and coin cell maintained logic will switch over to the
LICELL for backup power. This switch over occurs for a BP below 1.8 V threshold with LICELL greater than BP. A small capacitor
should be placed from LICELL to ground under all circumstances.
Upon initial insertion of the coin cell, it is not immediately connected to the on chip circuitry. The cell gets connected when the IC
powers on, or after enabling the coin cell charger when the IC was already on.
The coin cell charger circuit will function as a current-limited voltage source, resulting in the CC/CV taper characteristic typically
used for rechargeable Lithium-Ion batteries. The coin cell charger is enabled via the COINCHEN bit. The coin cell voltage is
programmable through the VCOIN[2:0] bits. The coin cell charger voltage is programmable in the ON state where the charge
current is fixed at ICOINHI.
If COINCHEN=1 when the system goes into Off or User Off state, the coin cell charger will continue to charge to the predefined
voltage setting but at a lower maximum current ICOINLO. This compensates for self discharge of the coin cell and ensures that
if/when the main cell gets depleted, that the coin cell will be topped off for maximum RTC retention. The coin cell charging will
be stopped for the BP below UVDET. The bit COINCHEN itself is only cleared when an RTCPORB occurs.
Table 109. Coin cell Voltage Specifications
VCOIN[2:0]
Output Voltage
000
2.50
001
2.70
010
2.80
011
2.90
100
3.00
101
3.10
110
3.20
111
3.30
Table 110. Coincell Electrical Specifications
Characteristics noted under conditions BP = 3.6 V, VBUS = 5.0 V, -40°C ≤ TA ≤ 85°C, unless otherwise noted. Typical values at
BP = 3.6 V and TA = 25°C under nominal conditions, unless otherwise noted.
Symbol
Characteristic
Min
Typ
Max
Unit
Notes
COIN CELL CHARGER
VLICELLACC Voltage Accuracy
-
100
-
mV
ILICELLON
Coin Cell Charge Current in On and Watchdog modes ICOINHI
-
60
-
μA
ILICELLOFF Coin Cell Charge Current in Off, cold start/warm start, and Low Power Off
modes (User Off / Memory Hold) ICOINLO
-
10
-
μA
ILICELACC
Current Accuracy
-
30
-
%
COLICELL
LICELL Bypass Capacitor
-
100
-
nF
LICELL Bypass Capacitor as coin cell replacement
-
4.7
-
μF