Datasheet
DS1672
10 of 15
Trickle Charger
The trickle charger is controlled by the trickle charge register. The simplified schematic of Figure 5
shows the basic components of the trickle charger. The trickle charge select (TCS) bit (bits 4–7) controls
the selection of the trickle charger. In order to prevent accidental enabling, only a pattern on 1010 will
enable the trickle charger. All other patterns will disable the trickle charger. The DS1672 powers up with
the trickle charger disabled. The diode select (DS) bits (bits 2, 3) select whether or not a diode is
connected between V
CC
and V
BACKUP
. If DS is 01, no diode is selected or if DS is 10, a diode is selected.
The RS bits (bits 0, 1) select whether a resistor is connected between V
CC
and V
BACKUP
and
what the value
of the resistor is. The resistor selected by the resistor select (RS) bits and the diode selected by the diode
select (DS) bits are as follows:
TCS TCS TCS TCS DS DS RS RS FUNCTION
X
X
X
X
0
0
X
X
Disabled
X
X
X
X
1
1
X
X
Disabled
X
X
X
X
X
X
0
0
Disabled
1
0
1
0
0
1
0
1
No diode, 250Ω resistor
1
0
1
0
1
0
0
1
One diode, 250Ω resistor
1
0
1
0
0
1
1
0
No diode, 2kΩ resistor
1
0
1
0
1
0
1
0
One diode, 2kΩ resistor
1
0
1
0
0
1
1
1
No diode, 4kΩ resistor
1
0
1
0
1
0
1
1
One diode, 4kΩ resistor
0
0
0
0
0
0
0
0
Initial default value--disabled
Warning: The resistor value of 250Ω must not be selected whenever V
CC
is greater
than 3.63V.
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 3V is applied to V
CC
and a super cap is
connected to V
BACKUP
. Also assume that the trickle charger has been enabled with a diode and resistor R2
between V
CC
and V
BACKUP
. The
maximum current I
MAX
would, therefore, be calculated as follows:
I
MAX
= (5.0V - diode drop) / R1 ≈ (5.0V - 0.6V) / 2kΩ ≈ 2.2mA
As the super cap changes, the voltage drop between V
CC
and V
BACKUP
will decrease and, therefore, the
charge current will decrease.