Rainbow Electronics MAX769 - Manual (Page 11)

MAX769
2 or 3-Cell, Step-Up/Down,
Two-Way Pager System IC
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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
1.8
Ω
(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 V
OUT 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 (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 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, 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. 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 5
Ω
, so it can typically
supply 20mA with only 100mV of dropout.