ISL12030 Datasheet, PDF(15/17 Page) Intersil Corporation – Low Power RTC with 50/60 Cycle AC Input, Alarms and Daylight Savings Correction

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ISL12030 Datasheet, PDF (15/17 Pages) Intersil Corporation – Low Power RTC with 50/60 Cycle AC Input, Alarms and Daylight Savings Correction

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ISL12030

Application Section

AC Input Circuits

The AC input ideally will have a 2.5VP-P sine wave at the

input, so this is the target for any signal conditioning circuitry

for the 50/60Hz waveform. Note that the peak-to-peak

amplitude can range from 1VP-P up to VDD, although it is

best to keep the max signal level just below VDD. The AC

input provides DC offset so AC coupling with a series

capacitor is advised.

If the AC power supply has a transformer, the secondary

output can be used for clocking with a resistor divider and

series AC coupling capacitor. A sample circuit is shown in

Figure 8. Values for R1/R2 are chosen depending on the

peak-to-peak range on the secondary voltage in order to

match the input of the ISL12030. CIN can be sized to pass

up to 300Hz or so, and in most cases, 0.47ÂµF should be the

selected value for a Â±20% tolerance device.

The AC input to the ISL12030 can be damaged if subjected

to a normal AC waveform when VDD is powered down. This

can happen in circuits where there is a local LDO or power

switch for placing circuitry in standby, while the AC main is

still switched ON. Figure 8 shows a modified version of the

Figure 9 circuit, which uses an emitter follower to essentially

turn off the AC input waveform if the VDD supply goes down.

Adding a Super Capacitor Backup

Since any loss of VDD power will reset the SRAM memory

including control and RTC register sections, then having

some form of VDD backup is a good idea. Figure 10 shows

connections for a super capacitor backup using VDD for the

normal source and a signal diode for charging. Be careful

not to use a normal Schottky diode as the leakage will

greatly reduce the backup life of the super capacitor.

This form of backup should yield at least one full day of

backup time, assuming the SCL/SDA pins and their pull-ups

are pulled to ground on powerdown.

VIN (AC) = 1.5VP-P TO 5VP-P

CIN

120VAC

50/60Hz

ISL12030

120VAC

50/60Hz

FIGURE 8. AC INPUT USING A TRANSFORMER SECONDARY

VDD

VIN (AC) = 1.5VP-P TO VDD (MAX)

CIN

ISL12030

FIGURE 9. USING THE VDD SUPPLY TO GATE THE AC INPUT

FN6617.1

January 15, 2008

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