DS1305 datasheet(9/22 Pages) MAXIM

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DS1305

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Table 3. TRICKLE CHARGER RESISTOR AND DIODE SELECT

TCS

Bit 7

TCS

Bit 6

TCS

Bit 5

TCS

Bit 4

Bit 3

Bit 2

Bit 1

Bit 0

FUNCTION

Disabled

1 Diode, 2kΩ

1 Diode, 4kΩ

1 Diode, 8kΩ

2 Diodes, 2kΩ

2 Diodes, 4kΩ

2 Diodes, 8kΩ

Initial power-on state

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 5V 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 IMAX would, therefore, be calculated as follows:

IMAX = (5.0V - diode drop) / R1  (5.0V - 0.7V) / 2k 2.2mA

As the super cap charges, the voltage drop between VCC1 and VCC2 decreases and, therefore, the charge

current decreases.

POWER CONTROL

Power is provided through the VCC1, VCC2, and VBAT pins. Three different power-supply configurations

are illustrated in Figure 4. Configuration 1 shows the DS1305 being backed up by a nonrechargeable

energy source such as a lithium battery. In this configuration, the system power supply is connected to

VCC1 and VCC2 is grounded. The DS1305 is write-protected if VCC1 is less than VBAT. The DS1305 is fully

accessible when VCC1 is greater than VBAT + 0.2V.

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 operates from the larger of VCC1 or

VCC2. When VCC1 is greater than VCC2 + 0.2V (typical), VCC1 powers the DS1305. When VCC1 is less than

VCC2, VCC2 powers 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.

DS1305 Datasheet(PDF) 9 Page - Maxim Integrated Products

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