DS1339 I2C Serial Real-Time Clock
TRICKLE CHARGER REGISTER (10h)
The simplified schematic in Figure 7 shows the basic components of the trickle charger. The trickle-charge select
(TCS) bits (bits 4 to 7) control the selection of the trickle charger. To prevent accidental enabling, only a pattern on
1010 enables the trickle charger. All other patterns disable the trickle charger. The trickle charger is disabled when
power is first applied. The diode-select (DS) bits (bits 2 and 3) select whether or not a diode is connected between
V
CC and VBACKUP. The ROUT bits (bits 0 and 1) select the value of the resistor connected between VCC and VBACKUP.
Table 3 shows the bit values.
Table 3. Trickle Charger Register (10h)
BIT 7
BIT 6
BIT 5
BIT 4
BIT 3
BIT 2
BIT 1
BIT 0
FUNCTION
TCS3
TCS2 TCS1
TCS0
DS1
0
DS0
0
ROUT1 ROUT0
X
X
X
1
1
1
1
1
1
X
X
X
0
0
0
0
0
0
X
X
X
1
1
1
1
1
1
X
X
X
0
0
0
0
0
0
X
X
0
0
0
1
1
1
1
X
X
0
1
1
0
0
1
1
Disabled
Disabled
Disabled
1
1
X
0
X
1
No diode, 250ꢁ resistor
One diode, 250ꢁ resistor
No diode, 2kꢁ resistor
One diode, 2kꢁ resistor
No diode, 4kꢁ resistor
One diode, 4kꢁ resistor
Initial power-up values
1
0
0
1
1
0
0
1
1
0
0
0
0
0
0
0
0
0
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 3.3V system power supply is applied to VCC and a super cap is connected to VBACKUP. Also assume that the
trickle charger has been enabled with a diode and resistor R2 between VCC and VBACKUP. The maximum current IMAX
would therefore be calculated as follows:
IMAX = (3.3V - diode drop) / R2 W (3.3V - 0.7V) / 2kꢁ W 1.3mA
As the super cap or battery charges, the voltage drop between VCC and VBACKUP decreases and therefore the
charge current decreases.
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DALLAS [ DALLAS SEMICONDUCTOR ]
