DS1340_06 Datasheet, PDF (9/14 Pages) Maxim Integrated Products – I2C RTC with Trickle Charger

I²C RTC with Trickle Charger
Control Register (07h)
Bit 7: Output Control (OUT). This bit controls the out-
put level of the FT/OUT pin when the FT bit is set to 0. If
FT = 0, the logic level on the FT/OUT pin is 1 if OUT = 1
and 0 if OUT = 0. The initial power-up OUT value is 1.
Bit 6: Frequency Test (FT). When this bit is 1, the
FT/OUT pin toggles at a 512Hz rate. When FT is written
to 0, the OUT bit controls the state of the FT/OUT pin.
The initial power-up value of FT is 0.
Bit 5: Calibration Sign Bit (S). A logic 1 in this bit indi-
cates positive calibration for the RTC. A 0 indicates
negative calibration for the clock. See the Clock
Calibration section for a detailed description of the bit
operation. The initial power-up value of S is 0.
Bits 4 to 0: Calibration Bits (CAL4 to CAL0). These
bits can be set to any value between 0 and 31 in binary
form. See the Clock Calibration section for a detailed
description of the bit operation. The initial power-up
value of CAL0–CAL4 is 0.
Trickle-Charger Register (08h)
The simplified schematic in 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. 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, 3) select whether or not a diode
is connected between V_CC and V_BACKUP. If DS is 01,
no diode is selected; if DS is 10, a diode is selected.
The ROUT bits (bits 0, 1) select the value of the resistor
connected between V_CC and V_BACKUP. Table 3 shows
the resistor selected by the resistor select (ROUT) bits
and the diode selected by the diode select (DS) bits.
Warning: The ROUT value of 250Ω must not be select-
ed whenever V_CC is greater than 3.63V.
The user determines diode and resistor selection
according to the maximum current desired for battery
or super cap charging (Table 4). The maximum charg-
ing current can be calculated as illustrated in the fol-
lowing example.
Assume that a 3.3V system power supply 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 calculat-
ed as follows:
I_MAX = (3.3V - diode drop) / R2 ≈ (3.3V - 0.7V) /
2kΩ ≈ 1.3mA
As the super cap charges, the voltage drop between
V_CC and V_BACKUP decreases and therefore the charge
current decreases.
BIT 7 BIT 6 BIT 5 BIT 4 BIT 3 BIT 2 BIT 1 BIT 0
TCS3 TCS2 TCS1 TCS0 DS1 DS0 ROUT1 ROUT0
1 OF 16 SELECT
NOTE: ONLY 1010b
ENABLES CHARGER
1 OF 2
SELECT
1 OF 3
SELECT
V_CC
TCS_0-3 = TRICKLE-CHARGER SELECT
DS_0-1 = DIODE SELECT
TOUT_0-1 = RESISTOR SELECT
R1
250Ω
R2
2kΩ
R3
4kΩ
V_BACKUP
Figure 6. Trickle Charger Functional Diagram
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