MAX769 データシートの表示（PDF） - Maxim Integrated

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MAX769

IC PAGER SYSTEM 28-QSOP

Maxim Integrated 

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

Two-Way Pager System IC

A faster A/D shortcut can be used for battery measure-

ments when the goal is a “go, no go” determination. For

this type of test, the CH DAC can simply be set to the

desired limit, and CH_ OUT supplies the result on the

next serial-write operation. One instance in which this

shortcut saves time is during a battery-impedance

check. The unloaded battery voltage can first be mea-

sured, if time allows, using one of the techniques

described in the previous paragraph. Then the magni-

tude of the loaded voltage drop can be quickly

checked with a single comparison to see if it is within

the desired limit.

The A/D circuitry can be invoked in both Run and Coast

Modes.

Open-Drain Drivers

Two open-drain drivers (DR1 and DR2) are activated

via the serial interface. DR1 and DR2 are grounded

1.8Ω (typical) NFETs that can sink up to 120mA. The

maximum sink current is limited by on-resistance and

package dissipation to about 240mA total sink current

for both switches. Note that DR1 and DR2 are designed

to sink current only from the main battery (BATT) and

cannot be pulled above BATT.

DR2 is controlled by an external input (DR2IN) as well as

a serial input bit. DR2IN is ANDed with the DR2ON serial-

control bit, allowing DR2 to drive an audio beeper. The

audio-frequency clock is applied to DR2IN, and ON/OFF

gating is applied to DR2ON. Both DR2IN (pin 18) and

DR2ON (serial bit) must be high for DR2 to switch on.

Coast Mode/Voltage Selection

Reduce the operating current by setting the RUN/COAST

bit low via the serial input. This shifts the DC-DC boost

converter from low-noise PWM operation (Run Mode) to

a very low operating current mode (Coast Mode) in

which switching pulses are only provided as needed to

satisfy the load. To further reduce operating current in

Coast Mode, lower VOUT using the OV0–OV4 serial bits.

The MAX769 starts up in Coast Mode. Select Run Mode

with the serial interface after power-up.

Various circuit functions can be disabled as follows:

Functions that always remain on in Coast Mode are:

• Serial I/O

• Reference (REF)

• OUT

• REG1

• LBI, RSIN (and LBO, RSO)

Functions that can be programmed on or off in Coast

Mode 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 pres-

ent)

• 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 MAX769 has an internal POR circuit (VOUT < 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 2.

Note that the MAX769 always comes out of reset in

Coast Mode; consequently, it cannot supply full power

until Run Mode is selected by serial 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 1). The current source can

charge a small 3-cell NiCd or NiMH battery (typically a

coin cell) or a 1-cell lithium battery. The charge current

can be set to either 15mA or 1mA. OUT sets the maxi-

mum charge (or float) voltage. When charging is imple-

mented, VOUT must also be set high enough to allow

sufficient headroom for the charger current source. The

VOUT - VNICD difference should normally be between

0.2V and 0.5V. Charger current vs. NICD voltage is

graphed in the Typical Operating Characteristics. Note

also that charging current reduces the OUT current

available for other loads.

Backup Linear Regulator

The BACKUP serial input bit turns on the backup regu-

lator, which sources current from NICD to OUT. This

regulator backs up OUT by using the rechargeable bat-

tery (at NICD) when the main battery (at BATT) is

depleted or removed. The backup regulator pass

device’s resistance is typically 5Ω, so it can typically

supply 20mA with only 100mV of dropout.

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