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DS1672S-2 Datasheet(PDF) 10 Page - Maxim Integrated Products |
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DS1672S-2 Datasheet(HTML) 10 Page - Maxim Integrated Products |
10 / 15 page DS1672 10 of 15 Microprocessor Monitor A temperature-compensated comparator circuit monitors the level of VCC. When VCC falls to the power- fail trip point, the RST signal (open drain) is pulled active and read/write access is inhibited. When VCC returns to nominal levels, the RST signal is kept in the active state for tRPU (typically) to allow the power supply and microprocessor to stabilize. Note, however, that if the EOSC bit is set to a logic 1 (to disable the oscillator during write protection), the reset signal will be kept in an active state for tRPU plus the startup time of the oscillator. Trickle Charger The trickle charger is controlled by the trickle charge register. The simplified schematic of Figure 5 shows the basic components of the trickle charger. The trickle charge select (TCS) bit (bits 4–7) controls the selection of the trickle charger. In order to prevent accidental enabling, only a pattern on 1010 will enable the trickle charger. All other patterns will disable the trickle charger. The DS1672 powers up with the trickle charger disabled. The diode select (DS) bits (bits 2, 3) select whether or not a diode is connected between VCC and VBACKUP. If DS is 01, no diode is selected or if DS is 10, a diode is selected. The RS bits (bits 0, 1) select whether a resistor is connected between VCC and VBACKUP and what the value of the resistor is. The resistor selected by the resistor select (RS) bits and the diode selected by the diode select (DS) bits are as follows: TCS TCS TCS TCS DS DS RS RS 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 Ω resistor 1 0 1 0 1 0 0 1 One diode, 250 Ω resistor 1 0 1 0 0 1 1 0 No diode, 2k Ω resistor 1 0 1 0 1 0 1 0 One diode, 2k Ω resistor 1 0 1 0 0 1 1 1 No diode, 4k Ω resistor 1 0 1 0 1 0 1 1 One diode, 4k Ω resistor 0 0 0 0 0 0 0 0 Initial default value—disabled 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 3V 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 = (5.0V - diode drop) / R1 ≈ (5.0V - 0.7V) / 2kΩ ≈ 2.2mA As the super cap changes, the voltage drop between VCC and VBACKUP will decrease and, therefore, the charge current will decrease. |
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