DS1305
Table 3. TRICKLE CHARGER RESISTOR AND DIODE SELECT
TCS
Bit 7
TCS
Bit 6
TCS
Bit 5
TCS
Bit 4
DS
Bit 3
DS
Bit 2
RS
Bit 1
RS
Bit 0
FUNCTION
X
X
X
X
X
X
X
X
X
0
X
0
0
0
Disabled
Disabled
X
X
X
1
1
1
1
1
1
0
X
0
0
0
0
0
0
1
X
1
1
1
1
1
1
0
X
0
0
0
0
0
0
1
1
0
0
0
1
1
1
1
1
1
1
1
0
0
0
1
X
0
1
1
0
1
1
0
X
1
0
1
1
0
1
0
Disabled
1 Diode, 2kꢀ
1 Diode, 4kꢀ
1 Diode, 8kꢀ
2 Diodes, 2kꢀ
2 Diodes, 4kꢀ
2 Diodes, 8kꢀ
Initial power-on state
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 VCC1 and a super cap is connected to
VCC2. Also assume that the trickle charger has been enabled with 1 diode and resister R1 between VCC1
and VCC2. The maximum current IMAX would, therefore, be calculated as follows:
IMAX = (5.0V - diode drop) / R1 (5.0V - 0.7V) / 2k 2.2mA
As the super cap charges, the voltage drop between VCC1 and VCC2 decreases and, therefore, the charge
current decreases.
POWER CONTROL
Power is provided through the VCC1, VCC2, and VBAT 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
VCC1 and VCC2 is grounded. The DS1305 is write-protected if VCC1 is less than VBAT. The DS1305 is fully
accessible when VCC1 is greater than VBAT + 0.2V.
Configuration 2 illustrates the DS1305 being backed up by a rechargeable energy source. In this case, the
VBAT pin is grounded, VCC1 is connected to the primary power supply, and VCC2 is connected to the
secondary supply (the rechargeable energy source). The DS1305 operates from the larger of VCC1 or
VCC2. When VCC1 is greater than VCC2 + 0.2V (typical), VCC1 powers the DS1305. When VCC1 is less than
VCC2, VCC2 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 VCC1 and VBAT pins are grounded and the battery is connected to the VCC2 pin.
Only these three configurations are allowed. Unused supply pins must be grounded.
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