XR18910 Datasheet, PDF (16/19 Pages) Exar Corporation – 8:1 Sensor Interface Analog Front End

XR18910
Application Information (Continued)
LDO Enable / Select (Power to External Bridge Sensors)
The XR18910 includes an on-board LDO that provides a
regulated voltage that can be used to power external input
bridge sensors. Two voltage options are available, 3V and
2.65V. The LDO voltage is selected via the I²C compatible
two-wire serial interface.
When the XR18910 is powered-up, the default LDO voltage
is 3V.
When the XR18910 is active (not in sleep mode), the LDO
is always on. If the LDO voltage is not used, the LDO output
can be left floating. The LDO can either stay on or shut
down while the XR18910 is in Sleep Mode.
■■ Set LDO to shut down while XR18910 is in Sleep Mode to
save power
■■ Set LDO to stay on while XR18910 is in Sleep Mode to
improve wake-up time
The LDO voltage and disable setting are selected via I²C
using the register address 0x07 followed by another byte
of data to select the voltage and disable setting. Refer to
the Register List in Table 1 and the example below for more
information.
Example: The example below illustrates how to select an
LDO voltage of 2.65V and keep the LDO enabled during
Sleep Mode.
To start communication with the XR18910, repeat steps 1-3 as
shown in the Inputs and Input Selection section on page 14.
Step 4
Master sends address of register to access
76543210
00000111
LDO Settings register address
= 0x07
Step 5
9
XR18910 sends acknowledge
A
Since the LDO Settings register was accessed, the
XR18910 is expecting another byte of data from the master
to complete the command. Refer to the “Byte of Parameter”
column in the Register List (Table 1). D0 and D1 are used to
select the LDO voltage and enable/disable the LDO during
Sleep Mode. Bit 0 (D0) controls the LDO voltage (0: 3V;
1: 2.65V). Bit 1 (D1) is only applicable in Sleep Mode. Bit
1 controls whether the LDO shuts down or stays on during
sleep mode (0: Enable; 1: Disable). When the XR18910 is
active, the LDO is always on.
Step 6
Master sends code to select LDO
voltage of 2.65V and Enable LDO
during Sleep Mode
765432
1
0
000000
0
0
0 = Enable 1 = 2.65V
Step 7
9
XR18910 sends acknowledge
A
Step 8
0
Master sends stop condition
P
NOTES:
White Block = host to XR18910, Orange Block = XR18910 to host, Grey Block =
Notes.
Current Sense Mode (Monitoring the LDO Current)
Current Sense Mode is activated via I²C using the register
address 0x08. When activated, the LDO current is sensed
and a proportional voltage is present at the output of the
XR18910 (ILDO = VOUT/RL). Current Sense Mode stays
active until the XR18910 receives any input select command
0x10, 0x12, 0x14, 0x15, 0x18, 0x1A, 0x1C, or 0x1E).
Current sense mode can be used to monitor the change
over time of the bridge impedance.
Sleep Mode (Analog Power Down)
Sleep mode is activated via I²C using the register address
0x05. When activated, the XR18910 will enter sleep mode.
During sleep mode, the analog portion of the XR18910
is disabled. All register settings are retained during sleep
mode.
During sleep mode, the nominal supply current will drop
below 70µA (with LDO on) and below 45µA (with LDO off).
During sleep mode, the master can read the value in any
register that saves a value during sleep mode. The only
I²C commands that can be received or processed are the
SLEEP_OUT (wake up) command (0x04) and the LDO
on/off and voltage command (0x07). All other register
addresses will be ignored.
Register address 0x04 is used to return to normal operation
(exit Sleep Mode).
By default, the XR18910 is active.
REV1A
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