MAX1242 Maxim, MAX1242 Datasheet - Page 8

• Manufacturer Part Number: MAX1242
• Description: +2.7V to %.25V / Low-Power / 10-Bit Serial ADCs in SO-8
• Manufacturer: Maxim

+2.7V to +5.25V, Low-Power, 10-Bit
Serial ADCs in SO-8
When power is first applied, and if SHDN is not pulled
low, it takes the fully discharged 4.7µF reference
bypass capacitor up to 20ms to provide adequate
charge for specified accuracy. With an external refer-
ence, the internal reset time is 10µs after the power
supplies have stabilized. No conversions should be
performed during these times.
To start a conversion, pull CS low. At CS’s falling edge,
the T/H enters its hold mode and a conversion is initiat-
ed. After an internally timed conversion period, the end
of conversion is signaled by DOUT pulling high. Data
can then be shifted out serially with the external clock.
Figure 5. Shutdown Sequence
Figure 6. Average Supply Current vs. Conversion Rate
8
_______________________________________________________________________________________
10,000
1000
SHDN
DOUT
100
10
Initialization after Power-Up and
1
CS
0.1
V
R
CODE = 0101010100
MAX1242
V
DD =
LOAD
DD
1
= 3V
V
= , C
REF
MAX1242
V
DD
CONVERSIONS/SEC
10
LOAD
= 5V
Starting a Conversion
= 50pF
POWERED UP
100
Serial Interface
CONVERSION 0
1k
MAX1243
V
DD
= 3V
10k
COMPLETE CONVERSION SEQUENCE
100k
POWERED DOWN
Power consumption can be reduced significantly by
shutting down the MAX1242/MAX1243 between con-
versions. Figure 6 shows a plot of average supply cur-
rent vs. conversion rate. Because the MAX1243 uses
an external reference voltage (assumed to be present
continuously), it “wakes up” from shutdown more quick-
ly, providing lower average supply currents. The wake-
up time, t
the time when a conversion may be initiated (Figure 5).
For the MAX1242, this time depends on the time in
shutdown (Figure 7) because the external 4.7µF refer-
ence bypass capacitor loses charge slowly during
shutdown. The MAX1243’s wake-up time is largely
dependent on the external reference’s power-up time. If
the external reference is not shut down, the wake-up
time is approximately 4µs.
Figure 7. Typical Reference-Buffer Power-Up Delay vs. Time in
Shutdown
Using
WAKE
0.8
0.6
0.4
0.2
0.0
t
1.0
WAKE
0.001
S S H H D D N N to Reduce Supply Current
, is the time from SHDN deasserted to
POWERED UP
0.01
CONVERSION 1
TIME IN SHUTDOWN (sec)
0.1
1
10