
DS1672
5 of 12
TRICKLE CHARGER
The trickle charger is controlled by the trickle charge register. The simplified schematic of Figure 3
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:
TC
S
X
X
X
1
1
1
1
1
1
TCS
TCS
TCS
DS
DS
RS
RS
Function
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
0
1
X
0
1
0
1
0
1
0
1
X
1
0
1
0
1
0
X
X
0
0
0
1
1
1
1
X
X
0
1
1
0
0
1
1
Disabled
Disabled
Disabled
No diode, 250 resistor
One diode, 250 resistor
No diode, 2k resistor
One diode, 2k resistor
No diode, 4k resistor
One diode, 4k resistor
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 - 0.7V) / 2k
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.