DS1305
TRICKLE CHARGER RESISTOR SELECT Table 2
RS BITS
RESISITORS
TYPICAL VALUE
00
01
10
11
None
R1
None
2 kΩ
4 kΩ
8 kΩ
R2
R3
If RS is 00, the trickle charger is disabled independent of TCS.
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 5 volts is applied to VCC1 and a super cap is
connected to VCC2 . Also assume that the trickle charger has been enabled with 1 diode and resister R1
between VCC1 and VCC2. The maximum current I MAX would therefore be calculated as follows:
I MAX
=
~
~
(5.0V - diode drop)/R1
(5.0V - 0.7V)/2 kΩ
2.2 mA
Obviously, as the super cap charges, the voltage drop between VCC1 and VCC2 will decrease and therefore
the charge current will decrease.
POWER CONTROL
Power is provided through the VCC1, VCC2 and VBAT pins. Three different power supply configurations
are illustrated in Figure 5. Configuration 1 shows the DS1305 being backed up by a non-rechargeable
energy source such as a lithium battery. In this configuration, the system power supply is connected to
VCC1 and VCC2 is grounded. The DS1305 will be write protected if VCC1 is less than VBAT .
Configuration 2 illustrates the DS1305 being backed up by a rechargeable energy source. In this case, the
VBAT pin is grounded, VCC1 is connected to the primary power supply and VCC2 is connected to the
secondary supply (the rechargeable energy source). The DS1305 will operate from the larger of VCC1 or
VCC2. When VCC1 is greater than VCC2 + 0.2 volt (typical), VCC1 will power the DS1305. When VCC1 is
less than VCC2, VCC2 will power the DS1305. The DS1305 does not write protect itself in this
configuration.
Configuration 3 shows the DS1305 in battery operate mode where the device is powered only by a single
battery. In this case, the VCC1 and VBAT pins are grounded and the battery is connected to the VCC2 pin.
Only these three configurations are allowed. Unused supply pins must be grounded.
SERIAL INTERFACE
The DS1305 offers the flexibility to choose between two serial interface modes. The DS1305 can
communicate with the SPI interface or with a standard 3-wire inter-face. The interface method used is
determined by the SERMODE pin. When this pin is connected to VCC, SPI communication is selected.
When this pin is connected to ground, standard 3-wire communication is selected.
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DALLAS [ DALLAS SEMICONDUCTOR ]
