Rainbow Electronics DS1306 - Manual (Page 9)

DS1306
9 of 21
Table 2. 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 0 0 Disabled
X X X X 0 0 X X Disabled
X X X X 1 1 X X 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
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.