2 or 3-Cell, Step-Up/Down,

Two-Way Pager System IC

______________________________________________________________________________________

13

All DC-DC converter and charging circuitry is disabled

when the backup regulator is turned on, but all other

functions remain active. Activate BACKUP manually or by

serial command, or set it to trigger automatically via LBO.

Automatic Backup

Setting the LBO_Sets_BACKUP serial bit (Table 1) pro-

grams the IC so that when LBO goes low, the backup

regulator automatically turns on without instructions from

the microprocessor (µP). When the LBO_Sets_BACKUP

bit is 0, the backup regulator is turned on only by setting

the BACKUP bit. The BACKUP bit also overrides the

LBO_Sets_BACKUP bit. Figure 3 shows the logic for this

function.

If the main battery is depleted and the NiCd battery is

drained during backup, RSO goes low while the back-

up regulator is supplying OUT (if RSI is used to monitor

OUT or REG1). When RSO falls, the serial registers

reset to their POR states (with the DC-DC converter on

in Coast Mode and the backup regulator off, see

Tables 1, 2, and 3). This prevents the IC from getting

hung up with the DC-DC converter off when a new main

battery is inserted. This sequence is required because

if the MAX769 did not default to “DC-DC converter on”

when coming out of reset, the µP (still reset by RSO)

would not be able to provide the device with serial

instructions to turn on.

Serial Interface

The MAX769 has an SPI-compatible serial interface.

The serial-interface lines are Chip Select (CS), Serial

Clock (SCL), Serial Data In (SDI), and Serial Data Out

(SDO). Serial input data is arranged in 8-bit bytes. Most

bytes contain a 3-bit address pointer (R2, R1, R0)

along with 5 bits of input data (D4–D0). For common

operations such as selecting Run or Coast Mode, acti-

vating REG2 or REG3, or turning on DR1 or DR2, only

the 000 (R2, R1, R0) address register needs to be writ-

ten. The serial input data format for all MAX769 opera-

tions is outlined in Tables 1, 2, and 3.

15mA_CHG

1mA_CHG

TO

CHARGER

CONTROL

TO

BACKUP

REGULATOR

BACKUP

LBO_SETS_BACKUP

LBO

Figure 3. Logic for Charger Control and BACKUP and for

LBO_Sets_BACKUP Serial Input Bits

2.5

1

0

1

1

0

4.9

1

1

1

1

1

4.8

1

1

0

1

1

4.7

0

1

1

1

1

4.6

0

1

0

1

1

4.5

1

1

1

0

1

4.4

1

1

0

0

1

4.3

0

1

1

0

1

4.2

0

1

0

0

1

4.1

1

0

1

1

1

4.0

1

0

0

1

1

3.9

0

0

1

1

1

3.8

0

0

0

1

1

3.7

1

0

1

0

1

3.6

1

0

0

0

1

3.5

0

0

1

0

1

3.4

0

0

0

0

1

3.3

1

1

1

1

0

3.2

1

1

0

1

0

3.1

0

1

1

1

0

3.0

0

1

0

1

0

2.9

1

1

1

0

0

2.8

1

1

0

0

0

2.7

0

1

1

0

0

2.6

0

1

0

0

0

2.4

1

0

0

1.9

1.8

1

0

(V)

2.3

0

0

0

0

OV1

1

1

0

2.2

0

0

0

OV3

0

0

1

0

2.1

1

0

1

0

0

2.0

1

0

0

0

0

0

OV0

OV2

OV4

1

0

0

0

0

SERIAL-DATA BIT