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21-0036

I2C Serial Real-Time Clock

Maxim Integrated 

DS1339 I2C Serial Real-Time Clock

TRICKLE CHARGER REGISTER (10h)

The simplified schematic in Figure 5 shows the basic components of the trickle charger. The trickle-charge select

(TCS) bits (bits 4 to 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 and 3) select whether or not a diode is connected between

VCC and VBACKUP. The ROUT bits (bits 0 and 1) select the value of the resistor connected between VCC and VBACKUP.

Table 6 shows the bit values.

Table 6. Trickle Charger Register (10h)

BIT 7

TCS3

X

X

X

1

1

1

1

1

1

0

BIT 6

TCS2

X

X

X

0

0

0

0

0

0

0

BIT 5

TCS1

X

X

X

1

1

1

1

1

1

0

BIT 4

TCS0

X

X

X

0

0

0

0

0

0

0

BIT 3

DS1

0

1

X

0

1

0

1

0

1

0

BIT 2

DS0

0

1

X

1

0

1

0

1

0

0

BIT 1

ROUT1

X

X

0

0

0

1

1

1

1

0

BIT 0

ROUT0

X

X

0

1

1

0

0

1

1

0

FUNCTION

Disabled

Disabled

Disabled

No diode, 250Ω resistor

One diode, 250Ω resistor

No diode, 2kΩ resistor

One diode, 2kΩ resistor

No diode, 4kΩ resistor

One diode, 4kΩ resistor

Initial power-up values

Warning: The ROUT value of 250Ω must not be selected whenever VCC is greater than 3.63V.

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 3.3V system power supply is applied to VCC and a super cap is connected to VBACKUP. Also assume that the

trickle charger has been enabled with a diode and resistor R2 between VCC and VBACKUP. The maximum current IMAX

would therefore be calculated as follows:

IMAX = (3.3V - diode drop) / R2 ≈ (3.3V - 0.7V) / 2kΩ ≈ 1.3mA

As the super cap or battery charges, the voltage drop between VCC and VBACKUP decreases and therefore the

charge current decreases.

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