Max847, Cell, step-up two-way pager system ic, Table 2. serial-bit assignments – Rainbow Electronics MAX847 User Manual

Page 12: Table 3. serial-bit power-on-reset (por) states

MAX847

pulse width modulation (PWM), as well as synchroniza-
tion, to minimize noise.

Some MAX847 functions are confined to Run Mode
while others remain active in both Run and Coast.
These are outlined as follows.

Various circuit functions can be disabled as follows:

Functions that

always remain on are:

• Serial I/O

• Reference (REF)

• OUT

• REG1

• LBI, RSIN (and LBO, RSO)

Functions that can be

programmed on or off

are (Table 1):

• DR1 and DR2

• REG2 and REG3

• NICD charger (Note: This may overload OUT if

turned on in Coast Mode when other loads are
present)

• NICD backup regulator

• CH0, CH1, CH2 and CH DAC

Functions that

always turn off in Coast Mode

are:

• SYNC and PLL circuits

• DC-DC PWM control circuits

Power-On Reset

The MAX847 has an internal POR circuit (V

OUT

< 1.6V)

to ensure an orderly power-up when a battery is first
applied. This feature is separate from the RSO com-
parator; however, if RSO goes low during operation, all
serial registers are set to the same predetermined
states as on power-up. The POR states for each regis-
ter are listed in Table 3.

Note that the MAX847 always comes out of reset in
Coast Mode; consequently, it cannot supply full power
until Run Mode is selected by either the RUN pin or ser-
ial command. System software cannot exercise full load
current until Run Mode is enabled.

Charger Circuit

A charger current source from OUT to NICD is activat-
ed via a serial bit (Table 2). The current source can
charge a small 3-cell NICD or NIMH battery (typically
coin cell) or a 1-cell lithium battery. The charge current
can be set to either 15mA or 1mA. When both 15mA
and 1mA are set, the charger runs at 15mA. OUT sets
the maximum charge (or float) voltage. When charging
is implemented, V

OUT

must also be set high enough to

allow sufficient headroom for the charger current
source. The V

OUT

- V

NICD

difference should normally

be between 0.2V and 0.5V. Charger current vs. output
voltage is graphed in the

Typical Operating

Characteristics

. Note also that charging current

reduces the OUT current available for other loads.

1-Cell, Step-Up
Two-Way Pager System IC

______________________________________________________________________________________

R2 (MSB)

DR1_ON

REG2_ON

LBO_Sets_

BACKUP

15mA_CHG

OV3

OV1

Table 2. Serial-Bit Assignments

DR2_ON

OV4

RUN/

COAST

1mA_CHG

OV0

REG3_ON

BACKUP

OV2

DAC5

DAC3

DAC1

DAC6

DAC4

DAC0

DAC2

Table 3. Serial-Bit Power-On-Reset (POR) States

POR = 0

POR = 1

POR = 0

POR = 1

POR = 0