DS1305 Datasheet, PDF (9/22 Pages) Dallas Semiconductor – Serial Alarm Real Time Clock RTC

Table 3. TRICKLE CHARGER RESISTOR AND DIODE SELECT
TCS
Bit 7
X
X
X
1
1
1
1
1
1
0
TCS
Bit 6
X
X
X
0
0
0
0
0
0
1
TCS
Bit 5
X
X
X
1
1
1
1
1
1
0
TCS
Bit 4
X
X
X
0
0
0
0
0
0
1
DS
Bit 3
X
0
1
0
0
0
1
1
1
1
DS
Bit 2
X
0
1
1
1
1
0
0
0
1
RS
Bit 1
0
X
X
0
1
1
0
1
1
0
RS
Bit 0
0
X
X
1
0
1
1
0
1
0
FUNCTION
Disabled
Disabled
Disabled
1 Diode, 2kΩ
1 Diode, 4kΩ
1 Diode, 8kΩ
2 Diodes, 2kΩ
2 Diodes, 4kΩ
2 Diodes, 8kΩ
Initial power-on state
DS1305
The user determines diode and resistor selection 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 1 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) / 2k 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 DS1305 being backed up by a nonrechargeable
energy source such as a lithium battery. In this configuration, the system power supply is connected to
V_CC1 and V_CC2 is grounded. The DS1305 is write-protected if V_CC1 is less than V_BAT. The DS1305 is fully
accessible when V_CC1 is greater than V_BAT + 0.2V.
Configuration 2 illustrates the DS1305 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 DS1305 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 DS1305. When V_CC1 is less than
V_CC2, V_CC2 powers the DS1305. The DS1305 does not write-protect itself in this configuration.
Configuration 3 shows the DS1305 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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