FN6667 datasheet(28/34 Pages) RENESAS

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ISL12020M

FN6667 Rev 6.00

Page 28 of 34

January 9, 2015

Application Section

Power Supply Considerations

The ISL12022M contains programmed EEPROM registers which

are recalled to volatile RAM registers during initial power-up.

These registers contain DC voltage, frequency and temperature

calibration settings. Initial power-up can be either application of

VBAT or VDD power, whichever is first. It is important that the

initial power-up meet the power supply slew rate specification to

avoid faulty EEPROM power-up recall. Also, any glitches or low

voltage DC pauses should be avoided, as these may activate

recall at a low voltage and load erroneous data into the

calibration registers. Note that a very slow VDD ramp rate

(outside data sheet limits) will almost always trigger erroneous

recall and should be avoided entirely.

Battery-Backup Details

The ISL12020M has automatic switchover to battery-backup

when the VDD drops below the VBAT mode threshold. A wide

variety of backup sources can be used, including standard and

rechargeable lithium, Super Capacitors, or regulated secondary

sources. The serial interface is disabled in battery-backup, while

the oscillator and RTC registers are operational. The SRAM

The input voltage range for VBAT is 1.8V to 5.5V, but keep in

mind the temperature compensation only operates for

VBAT > 2.7V. Note that the device is not guaranteed to operate

with a VBAT < 1.8V, so the battery should be changed before

discharging to that level. It is strongly advised to monitor the low

battery indicators in the status registers and take action to

replace discharged batteries.

If a Super Capacitor is used, it is possible that it may discharge to

below 1.8V during prolonged power-down. Once powered up, the

device may lose serial bus communications until both VDD and

VBAT are powered down together. To avoid that situation,

including situations where a battery may discharge deeply, the

circuit in Figure 21 can be used.

The diode, DBAT, will add a small drop to the battery voltage but

will protect the circuit should battery voltage drop below 1.8V.

The jumper is added as a safeguard should the battery ever need

to be disconnect from the circuit.

The VDD negative slew rate should be limited to below the data

sheet spec (10V/ms) otherwise battery switchover can be

delayed, resulting in SRAM contents corruption and oscillator

operation interruption.

Some applications will require separate supplies for the RTC VDD

and the I2C pullups. This is not advised, as it may compromise

the operation of the I2C bus. For applications that do require

serial bus communication with the RTC VDD powered down, the

SDA pin must be pulled low during the time the RTC VDD ramps

down to 0V. Otherwise, the device may lose serial bus

communications once VDD is powered up and will return to

normal operation ONLY once VDD and VBAT are both powered

down together.

Layout Considerations

The ISL12020M contains a quarts crystal and requires special

handling during PC board assembly. Excessive shock and vibrations

should be avoided. Ultrasound cleaning is not advisable. See Note 7

on page 6 in the electrical specifications table pertaining to solder

reflow effects on oscillator accuracy.

The crystal pins X1 and X2 have a very high impedance and

oscillator circuits operating at low frequencies (such as

32.768kHz) are known to pick up noise very easily if layout

precautions are not followed. Most instances of erratic clocking

or large accuracy errors can be traced to the susceptibility of the

oscillator circuit to interference from adjacent high speed clock

or data lines. Careful layout of the RTC circuit will avoid noise

pickup and insure accurate clocking.

FIGURE 20. READ SEQUENCE (CSR SLAVE ADDRESS SHOWN)

SIGNALS

FROM THE

MASTER

SIGNALS FROM

THE SLAVE

SIGNAL AT

SDA

IDENTIFICATION

BYTE WITH

R/W = 0

ADDRESS

BYTE

IDENTIFICATION

BYTE WITH

R/W = 1

LAST READ

DATA BYTE

FIRST READ

DATA BYTE

1111

11 11

FIGURE 21. SUGGESTED BATTERY-BACKUP CIRCUIT

DBAT

CBAT

CIN

BAT43W

0.1µF

VDD = 2.7V

TO 5.5V

VBAT = 1.8V

TO 3.2V

JBAT

ISL12020M

VDD

GND

VBAT

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