DS1306_05 Datasheet, PDF (9/21 Pages) Dallas Semiconductor – Serial Alarm Real-Time Clock

Table 2. TRICKLE CHARGER RESISTOR AND DIODE SELECT
TCS TCS TCS TCS DS DS RS RS
Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0
FUNCTION
X
X
XXXX
0
0
Disabled
X
X
XX
0
0
XX
Disabled
X
X
XX
1
1
XX
Disabled
1
0
1
0
0
1
0
1
1 Diode, 2kΩ
1
0
1
0
0
1
1
0
1 Diode, 4kΩ
1
0
1
0
0
1
1
1
1 Diode, 8kΩ
1
0
1
0
1
0
0
1
2 Diodes, 2kΩ
1
0
1
0
1
0
1
0
2 Diodes, 4kΩ
1
0
1
0
1
0
1
1
2 Diodes, 8kΩ
0
1
0
1
1
1
0
0 Initial power-on state
DS1306
If RS is 00, the trickle charger is disabled independently of TCS.
Diode and resistor selection is determined by the user according to the maximum current desired for
battery or super cap charging. The maximum charging current can be calculated as illustrated in the
following example. Assume that a system power supply of 5V is applied to V_CC1 and a super cap is
connected to V_CC2. Also assume that the trickle charger has been enabled with one diode and resister R1
between V_CC1 and V_CC2. The maximum current I_MAX would, therefore, be calculated as follows:
I_MAX = (5.0V - diode drop) / R1 » (5.0V - 0.7V) / 2kΩ » 2.2mA
As the super cap charges, the voltage drop between V_CC1 and V_CC2 decreases and, therefore, the charge
current decreases.
POWER CONTROL
Power is provided through the V_CC1, V_CC2, and V_BAT pins. Three different power supply configurations
are illustrated in Figure 4. Configuration 1 shows the DS1306 being backed up by a non-rechargeable
energy source such as a lithium battery. In this configuration, the system power supply is connected to
V_CC1 and V_CC2 is grounded. When V_CC falls below V_BAT the device switches into a low-current battery
backup mode. Upon power-up, the device switches from V_BAT to V_CC when V_CC is greater than
V_BAT + 0.2V. The device is write-protected whenever it is switched to V_BAT.
Configuration 2 illustrates the DS1306 being backed up by a rechargeable energy source. In this case, the
V_BAT pin is grounded, V_CC1 is connected to the primary power supply, and V_CC2 is connected to the
secondary supply (the rechargeable energy source). The DS1306 operates from the larger of V_CC1 or
V_CC2. When V_CC1 is greater than V_CC2 + 0.2V (typical), V_CC1 powers the DS1306. When V_CC1 is less than
V_CC2, V_CC2 powers the DS1306. The DS1306 does not write-protect itself in this configuration.
Configuration 3 shows the DS1306 in battery-operate mode, where the device is powered only by a single
battery. In this case, the V_CC1 and V_BAT pins are grounded and the battery is connected to the V_CC2 pin.
Only these three configurations are allowed. Unused supply pins must be grounded.
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