Trickle charge register (10h), Figure 5. trickle charge register (10h) bit values – Rainbow Electronics DS1339 User Manual
Page 9
DS1339
9 of 18
is cleared when written to logic 0. This bit can only be written to logic 0. Attempting to write to logic 1
leaves the value unchanged.
A2F (Alarm 2 Flag) – A logic 1 in the A2F bit indicates that the time matched the Alarm 2 registers. If
the A2IE bit is a logic 1 and the INTCN bit is set to a logic 1, the SQW/
INT
pin is also asserted. A2F is
cleared when written to logic 0. This bit can only be written to logic 0. Attempting to write to logic 1
leaves the value unchanged.
TRICKLE CHARGE REGISTER (10h)
The simplified schematic of Figure 6 shows the basic components of the trickle charger. The trickle-
charge select (TCS) bits (bits 4–7) control the selection of the trickle charger. In order 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 V
BACKUP
. The ROUT bits (bits
0, 1) select the value of the resistor connected between V
CC
and V
BACKUP
. Bit values are shown in Figure
5.
Figure 5. TRICKLE CHARGE REGISTER (10h) BIT VALUES
BIT 7 BIT 6 BIT 5 BIT 4 BIT 3 BIT 2
BIT 1
BIT 0
TCS3 TCS2 TCS1 TCS0
DS1
DS0
ROUT1
ROUT0
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, 250
W resistor
1
0
1
0
1
0
0
1
One diode, 250
W resistor
1
0
1
0
0
1
1
0
No diode, 2k
W resistor
1
0
1
0
1
0
1
0
One diode, 2k
W resistor
1
0
1
0
0
1
1
1
No diode, 4k
W resistor
1
0
1
0
1
0
1
1
One diode, 4k
W resistor
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 3.3V 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
= (3.3V - diode drop) / R2
» (3.3V - 0.7V) / 2kW » 1.3mA
As the super cap or battery charges, the voltage drop between V
CC
and V
BACKUP
decreases and, therefore,
the charge current decreases.